UOFP- Unit operation in Food Processing

Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 1 of 91
UNIT OPERATIONS IN FOOD
PROCESSING
Notes for Diploma in Food Technology
[Prepared BY:- Mohit Jindal]
2020
Food Technology Department
[Government Polytechnic, Mandi Adampur, HIsar-125052]
9/3/2020

UNIT OPERATIONS IN FOOD
PROCESSING
DETAILED CONTENTS
1. Preliminary Unit operation
Cleaning, sorting & Grading - aims, methods and applications
2. Size Reduction and Sieve Analysis
Theory of comminution; Calculation of energy required during size reduction. Crushing efficiency; Size
reduction equipment; Size reduction of fibrous, dry and liquid foods; effects of size reduction on
sensory characteristics and nutritive value of food
Sieving: Separation based on size (mesh size); types of screens; effectiveness of screens
3. Mixing
Mixing, Agitating, kneading, blending, homogenization and related equipment
4. Separation Processes
Principles of Filtration, Sedimentation, Crystallization and Distillation and equipment used
LIST OF PRACTICALS
1. Analysis of sampled foods for physical characteristics
2. Determination of critical speed of ball-mill
3. Size reduction and particle size distribution using hammer-mill
4. Steam distillation of herbs
5. Concentration by crystallization
6. Clarification of apple juice using filter press
7. Visit to a public distribution system (PDS) showing storage facilities, warehouse, cold storage,
refrigeration system and slaughter house etc
8. Visit to various food industries for demonstration of various unit operations
RECOMMENDED BOOKS
1. Handling, Transportation and Storage of Fruits and Vegetables by A Lloyd, Ryall Penizer (AVI
Publications)
2. Proceedings of Regional Workshop on Warehouse Management of Stored Food Grains by Girish and
Ashok Kumar (UNDP)
3. Modern Potato and Vegetable Storage by Volkind and Roslov (Amerind)
4. Controlled Atmospheric Storage of Fruits by Mettel Skilv
5. Food Grains in Tropical and Sub Tropical Areas by Hall
6. Food Storage Part of a system by Sinha and Muir (AVI)
7. Post Harvest Technology of Fruits and Vegetables – Handling, Processing, Fermentation and Waste
Management by LR Verma and VK Joshi; Indus Publishing com., New Delhi
8. Drying and Storage of Grains and Oilseeds by Brooker & Hall, CBS

A physical entity, which can be observed and/or measured, is defined qualitatively by a dimension.
For example, time, length, area, volume, mass, force, temperature, and energy are all considered
dimensions like unit of length may be measured as a meter, centimeter, or millimeter.
Primary dimensions, such as length, time, temperature, and mass, express a physical entity.
Secondary dimensions involve a combination of primary dimensions (e.g., volume is length cubed;
velocity is distance divided by time).
Physical quantities are measured by variety of unit systems. The most common systems
include the Imperial (English) system; the centimeter, gram, second (cgs) system; and the meter,
kilogram, second (mks) system. International organizations have attempted to standardize unit
systems, symbols, and their quantities. As a result of international agreements, the Systeme
International d’Unites, or the SI units have emerged. The SI units consist of seven base units, two
supplementary units, and a series of derived units.
Base Units
The SI system is based on a choice of seven well-defined units, which by convention are regarded as
dimensionally independent. The definitions of these seven base units are as follows:
1. Unit of length (meter): The meter (m) is the length equal to 1,650,763.73 wavelengths in
vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the
krypton-86 atom.
2. Unit of mass (kilogram): The kilogram (kg) is equal to the mass of the international prototype
of the kilogram. (The international prototype of the kilogram is a particular cylinder of
platinum-iridium alloy, which is preserved in a vault at Sèvres, France, by the International
Bureau of Weights and Measures.)
3. Unit of time (second): The second (s) is the duration of 9,192,631,770 periods of radiation
corresponding to the transition between the two hyperfine levels of the ground state of the
cesium-133 atom.
4. Unit of electric current (ampere): The ampere (A) is the constant current that, if maintained
in two straight parallel conductors of infinite length, of negligible circular cross-section, and
placed 1 m apart in vacuum, would produce between those conductors a force equal to 2*107
newton per meter length.
5. Unit of thermodynamic temperature (Kelvin): The Kelvin (K) is the fraction 1/273.16 of the
thermodynamic temperature of the triple point of water.
6. Unit of amount of substance (mole): The mole (mol) is the amount of substance of a system
that contains as many elementary entities as there are atoms in 0.012 kg of carbon 12.
7. Unit of luminous intensity (candela): The candela (cd) is the luminous intensity, in the
perpendicular direction, of a surface of 1/600,000 m 2 of a blackbody at the temperature of
freezing platinum under a pressure of101, 325 newton/m2
.
Derived Units

Derived units are algebraic combinations of base units expressed by means of multiplication and
division. For simplicity, derived units often carry special names and symbols that may be used to
obtain other derived units. Definitions of some commonly used derived units are as follows:
1. Newton (N): The newton is the force that gives to a mass of 1 kg an acceleration of 1 m/s2
.
2. Joule (J): The joule is the work done when due to force of 1 N the point of application is
displaced by a distance of 1 m in the direction of the force.
3. Watt (W): The watt is the power that gives rise to the production of energy at the rate of 1 J/s.
4. Volt (V): The volt is the difference of electric potential between two points of a conducting
wire carrying a constant current of 1 A, when the power dissipated between these points is
equal to 1 W.
5. Ohm ( Ω): The ohm is the electric resistance between two points of a conductor when a
constant difference of potential of 1 V, applied between these two points, produces in this
conductor a current of 1 A, when this conductor is not being the source of any electromotive
force.
6. Coulomb (C): The coulomb is the quantity of electricity transported in 1 s by a current of 1 A.
7. Farad (F): The farad is the capacitance of a capacitor, between the plates of which there
appears a difference of potential of 1 V when it is charged by a quantity of electricity equal to
1 C.
8. Henry (H): The henry is the inductance of a closed circuit in which an electromotive force of 1
V is produced when the electric current in the circuit varies uniformly at a rate of 1 A/s.
9. Weber (Wb): The weber is the magnetic flux that, linking a circuit of one turn, produces in it
an electromotive force of 1 V as it is reduced to zero at a uniform rate in 1 s.
10. Lumen (lm): The lumen is the luminous flux emitted in a point solid angle of 1 steradian by a
uniform point source having an intensity of 1 cd.
Supplementary Units
This class of units contains two purely geometric units, which may be regarded either as base units or
as derived units.
1. Unit of plane angle (radian): The radian (rad) is the plane angle between two radii of a circle
that cut off on the circumference an arc equal in length to the radius.
2. Unit of solid angle (steradian): The steradian (sr) is the solid angle that, having its vertex in
the center of a sphere, cuts off an area of the surface of the sphere equal to that of a square with
sides of length equal to the radius of the sphere

 Physical properties
Food engineering is related to the analysis of equipment and systems used to process food on a
commercial production scale. Design of food equipment and processes to insure food quality and
safety we should know the response of the food materials to physical and chemical treatments. Raw
food materials are biological in nature and as such have certain unique characteristics which
distinguish them from other manufactured products. Because food materials are mainly of biological
origin they have
(a) Irregular shapes commonly found in naturally occurring raw materials;
(b) Properties with a non-normal frequency distribution;
(c) Heterogeneous composition;
(d) Composition that varies with variety, growing conditions, maturity and other factors; and they are
(e) Affected by chemical changes, moisture, respiration, and enzymatic activity.
 Rheological properties
The majority of industrial food processes involve fluid movement. Liquid foods such as milk and
juices have to be pumped through processing equipment or from one container to another. A number
of important unit operations such as filtration, pressing and mixing are, particular applications of fluid
flow. The mechanism and rate of energy and mass transfer are strongly dependent on flow
characteristics. The flow properties and deformation properties of fluids are the science called
‘rheology’ or the relationship between stress and strain is the subject matter of the science known as
rheology
 Mechanical Properties
Mechanical properties are those properties that determine the behavior of food materials when
subjected to external forces. Mechanical properties are important in processing (conveying, size
reduction) and consumption (texture, mouth feel).
The forces acting on the material are usually expressed as stress, i.e. intensity of the force per unit area
(N.m2
or Pa.). The dimensions and units of stress are like those of pressure.
The response of materials to stress is deformation, expressed as strain. Strain is usually expressed as a
dimensionless ratio, such as the elongation as a percentage of the original length.
We define three ideal types of deformation:
 Elastic deformation: deformation appears instantly with the application of stress and
disappears instantly with the removal of stress.
 Plastic deformation: deformation does not occur as long as the stress is below a limit value
known as yield stress. Deformation is permanent, i.e. the body does not return to its original
size and shape when the stress is removed.
 Viscous deformation: deformation (flow) occurs instantly with the application of stress and it is
permanent. The rate of strain is proportional to the stress
 Thermal Properties
In the food industry every process involves thermal effects such as heating, cooling or phase
transition. The thermal properties of foods are important in food process engineering. The following
properties are of particular importance: thermal conductivity, thermal diffusivity, specific heat, latent
heat of phase transition and emissivity.
 Electrical Properties
The electrical properties of foods are particularly relevant to microwave and ohmic heating of
foods and to the effect of electrostatic forces on the behavior of powders. The most important
properties are electrical conductivity and the dielectric properties. Ohmic heating is a
technique whereby a material is heated by passing an electric current through it.
Size and Shape
The size and shape of a raw food material can vary widely. The variation in shape of a product
may require additional parameters to define its size. The size of spherical particles like peas or
cantaloupes is easily defined by a single characteristic such as its diameter. The size of non-spherical
objects like wheat kernels, bananas, pears, or potatoes may be described by multiple length
measurements.

Particle size is used in sieve separation of foreign materials or grading (i.e., grouping into size
categories). Particle size is particularly important in grinding operations to determine the condition of
the final product and determines the required power to reduce the particle’s size.
Various types of cleaning, grading and equipments are designed on the basic physical
properties such as size, shape, specific gravity and colures. The shape of product is the important
parameter which effect covering characteristics of solid materials. The shape is also procedure in
calculation of various cooling and heating of food material.
Size is actually related or correlated to the property weight.
Shape affects the grade given to fresh fruit. To make the highest grade a fruit or vegetable must have
the commonly recognized expected shape of that particular fruit/vegetable.
Roundness, as defined as, “is a measure of the sharpness of the corners of the solid.”
where R in this case is the mean radius of the object and r is the radius of curvature of the sharpest
corner.
where: Di = diameter of largest inscribed circle
Dc = diameter of smallest circumscribed circle
Colour
Color is an important quality parameter because colour and colour uniformity are vital
components of visual quality of fresh foods and play a major role in consumer choice. Automatic
measurement of color is essential in many process control applications, such as sorting of fruits and
vegetables in packing houses, control of roasting of coffee and nuts, control of frying of potato chips,
oven toasting of breakfast cereals, browning of baked goods etc. However, it may be less important in
raw materials for processing. For low temperature processes such as chilling, freezing or freeze-
drying, the colour changes little during processing, and thus the colour of the raw material is a good
guide to suitability for processing. Any color within the visible range can be represented with the help
of three dimensional coordinates (or three-dimensional color space) L,a,b.
The axis L represents ‘luminosity’ with 0=black and 100=white.
The ‘a ’axis gives the position of the measured color between the two opponent colors red and green,
with red at the positive and green at the negative end.
The ‘b’ axis reflects the position of the color in the yellow (positive) – blue (negative) channel.

With the help of the L*a*b space system, any color is represented by a simple equation containing the
three parameters. Color measurement instruments (colorimeters) are photoelectric cell-based devices,
capable of reading the L,a,b values and ‘ calculating ’ the color perceived.
Density:-
Density is defined as objects mass per unit volume. Mass is a property. The symbol most
often used for density is ρ (the lower case Greek letter rho). Mathematically, density is defined as mass
divided by volume. It is an indication of how matter is composed in the body material with more
compact density has higher density
The density can be expressed as
where
ρ = density (kg/m3
)
m = mass (kg)
V = volume (m3
)
The SI units for density are kg/m3
. The imperial (U.S.) units are lb/ft3
(slugs/ft3
). While people
often use pounds per cubic foot as a measure of density in the U.S.1 gram/cm3
= 1000 kg/m3
= 62.4
lb/ft3
The density of a material is equal to its mass divided by its volume and has SI units of kg m-3
.
The density of materials is not constant and changes with temperature and pressure. Increasing the
pressure always increases the density of a material. Increasing the temperature generally decreases the
density. Knowledge of the density of foods is important in separation processes and differences in
density can have important effects on the operation of size reduction and mixing equipment. Product
density influences the amount and strength of packaging material. Breakfast cereal boxes contain a
required weight of cereal. More weight of material can be placed into a box if the cereal density is
greater. Also, food density influences its texture or mouth feel. Processing can affect product density
by introducing more air, such as is done in the manufacture of butter or ice cream.
Bulk Density:-
It is the weight of the food material in a unit volume. It is of importance in the packaging,
handling and other operations.
Bulk density is defined as the mass of many particles of the material divided by the
total volume they occupy.
Or
The weight of a material (including solid particles and any contained water) per unit volume including
voids.
Or
Bulk density is overall mass of the material divided by the volume occupied by the material
The total volume includes particle volume, inter-particle void volume, and internal pore
volume.
Bulk density is not an intrinsic property of a material; it can change depending on how the material is
handled. For example, a powder poured into a cylinder will have a particular bulk density; if the
cylinder is disturbed, the powder particles will move and usually settle closer together, resulting in a
higher bulk density. For this reason, the bulk density of powders is usually reported both as "freely
settled" (or "poured" density) and "tapped" density (where the tapped density refers to the bulk density
of the powder after a specified compaction process, usually involving vibration of the container.)
Oil, water and air occupy voids in the soil, called pore spaces.
Bulk density = Oven dry soil weight / volume of soil solids and pores
Particle density is the volumetric mass of the solid soil. It differs from bulk density because the
volume used does not include pore spaces.
Particle density = oven-dry soil weight / volume of soil solids
Porosity:-

The void space can be describing the porosity which is expressed as volume not occupied
as good material. Porosity is the percentage of air between the particles compared to a unit volume of
particles.
Porosity is that portion of the material volume occupied by pore spaces. This property does not have
to be measured directly since it can be calculated using values determined for bulk density and particle
density. Finding the ratio of bulk density to particle density and multiplying by 100 calculates the
percent solid space. If subtracting % solid space from 100 gives the % of soil volume that is pore
space.
% solid space = (bulk density / particle density) x 100
% porosity = 100 - (% solid space)
Sample Calculation of Porosity:
A 260 cm3 cylindrical container was used to collect an undisturbed soil sample. The container and soil
weighed 413 g when dried. When empty the container weighed 75 g. What is the bulk density and
porosity of the soil?
To determine bulk density:
Sample Volume = 260 cm3;
Sample Weight = 413 - 75 = 338 g;
Bulk density = 338 g/260 cm3= 1.3 g /cm3
To determine porosity:
Bulk density = 1.3 g /cm3;
Particle density = 2.65 g /cm3;
Porosity = 100 - (1.3/2.65 x 100) = 51%
Specific gravity.
The Specific Gravity - SG - is a dimensionless unit defined as the ratio of density of the
substance to the density of water at a specified temperature. Apparent specific gravity is the ratio of
the weight of a volume of the substance to the weight of an equal volume of the reference substance.
Specific Gravity can be expressed
SG = ρsubstance / ρH2O
where
SG = Specific Gravity of the substance
ρsubstance = density of the fluid or substance (kg/m3
)
ρH2O = density of water - normally at temperature 4 o
C (kg/m3
)
It is common to use the density of water at 4 o
C because at this point the density of water is at
the highest - 1000 kg/m3
or 62.4 lb/ft3
. Specific gravity can also be calculated from the following
expression:
Specific gravity varies with temperature. The reference substance is nearly always water for
liquids or air for gases. Temperature and pressure must be specified for both the sample and the
reference. Pressure is nearly always 1 atm equal to 101.325 kPa. Temperatures for both sample and
reference vary from industry to industry. The density and specific gravity value as a stain and other
communities are used in design of solid storage separation of desired materials cleaning and grading,
texture and softness of food quality, the concentration of solutions of various materials such as brines,
hydrocarbons, sugar solutions (syrups, juices, honeys, brewers wort, must etc.) and acids.
Specific gravity can be measured in a number of ways.
1. Pycnometer
2. Digital density meters

Thermal Conductivity:-
Thermal conductivity is a measure of the ability of a material to transfer heat. It may be define
as the rate of heat flow through unit thickness of material per unit area normal to direction of heat flow
and per unit time per unit temperature difference is called thermal conductivity.
Or
The thermal conductivity is the heat energy transferred per unit time and per unit surface area,
divided by the temperature difference.
Thermal conductivity, k (also denoted as λ or κ), is the property of a material's ability
to conduct heat. It appears primarily in Fourier's Law for heat conduction. Heat flows at a higher rate
across materials of high thermal conductivity than across materials of low thermal conductivity.
Materials of high thermal conductivity are widely used in heat sink applications and materials of low
thermal conductivity are used as thermal insulation. Thermal conductivity of materials is temperature
dependent. Thermal energy always moves from that of higher concentration to lower concentration--
that is, from hot to cold.
In the following equation, thermal conductivity is the proportionality factor k. The distance of heat
transfer is defined as ∆x, which is perpendicular to area A. The rate of heat transferred through the
material is Q, from temperature T1 to temperature T2, when T1>T2. SI units for thermal conductivity
watt per meter kelvin W/ (m K), m kg s-3
K-1
Viscosity:-
Viscosity is a resistance of a fluid which is
being deformed by either shear stress or tensile stress. In the other word we can say viscosity is the
property of fluid by virtue of which is opposing its flow.
Or
Viscosity is resistance to flow
Or
Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of
fluid friction.
Viscosity is an important characteristic of liquid foods in many areas of food processing. For
example the characteristic mouthfeel of food products such as tomato ketchup, cream, syrup and
yoghurt depends on their viscosity (or 'consistency'). The viscosity of many liquids changes during
Material
Thermal
conductivity
(W/m K)*
Diamond 1000
Silver 406.0
Copper 385.0
Gold 314
Brass 109.0
Aluminum 205.0
Iron 79.5
Steel 50.2
Fiberglass 0.04
Polystyrene
(styrofoam)
0.033

heating/cooling or concentration and this has important effects on, for example, the power needed to
pump these products.
A liquid having a series of layers and when it flows over a surface, the uppermost layer flows
fastest and drags the next layer along at a slightly lower velocity, and so on through the layers. The
force that moves the liquid is known as the shearing force or 'shear stress' and the velocity gradient is
known as the 'shear rate'. If shear stress is plotted against shear rate, most simple liquids and gases
show a linear relationship and these are termed 'Newtonian' fluids. Examples include water, most
oils, gases, and simple solutions of sugars and salts. Where the relationship is non-linear the fluids are
termed 'non-Newtonian'. For all liquids, viscosity decreases with an increase in temperature but for
most gases it increases with temperature.(Lewis 1990).
In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction".
Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. All
real fluids have some resistance to stress and therefore are viscous. A fluid which has no resistance to
shear stress is known as an ideal fluid or in viscid fluid. Zero viscosity is observed only at very low
temperatures, in super fluids.
The word "viscosity" is derived from the Latin "viscum", meaning mistletoe and also a
viscous glue (birdlime) made from mistletoe berries. Viscosity represented by the symbol η "eta".
Viscosity is the ratio of the tangential frictional force per unit area. The SI unit of viscosity is
the pascal second [Pa s]. The pascal second is rarely used today the most common unit of viscosity is
the dyne second per square centimeter [dyne s/cm2
], which is given the name poise [P] after the
French physiologist Jean Poiseuille (1799–1869). Ten poise equal one pascal second [Pa s] making
the centipoise [cP] and millipascal second [mPa s] identical.
1 pascal second = 10 poise
1 pascal second = 1,000 millipascal second
1 centipoise = 1 millipascal second
The other quantity called kinematic viscosity (represented by the symbol ν "nu") is the ratio of
the viscosity of a fluid to its density. The SI unit of kinematic viscosity is the square meter per
second [m2
/s]. A more common unit of kinematic viscosity is the square centimeter per
second [cm2
/s], which is given the name stokes [St] after the Irish mathematician and
physicist George Stokes (1819–1903).
1 m2
/s = 10,000 cm2
/s [stokes]
1 m2
/s = 1,000,000 mm2
/s [centistokes]
1 cm2
/s 1 stokes
1 mm2
/s = 1 centistokes
Thermal Diffusivity:-
It is defined as the ratio of thermal conductivity to the ‘volumetric heat capacity’ of the
material. Volumetric heat capacity is obtained by multiplying the mass specific heat c p by the density
ρ.
or
It may be calculated by dividing thermal conductivity with the specific heat and density. In heat
transfer analysis, thermal diffusivity usually denoted α but a, κ, k, and D are also used. It has the SI
unit of m²/s. The formula is:
where
is thermal conductivity (W/(m·K))

is density (kg/m³)
is specific heat capacity (J/(kg·K))
Thermal conductivity is a property that determines HOW MUCH heat will flow in a material, while
thermal diffusivity determines HOW RAPIDLY heat will flow within it. In a substance with high
thermal diffusivity, heat moves rapidly through because the substance conducts heat quickly relative to
its volumetric heat capacity or 'thermal bulk'. The substance generally does not require much energy
transfer to or from its surroundings to reach thermal equilibriumIt is important to determine heat
transfer rate in solid food material of any shape. It shows capacity of food material to store heat.
Heat
In physics, heat is energy in transfer other than as work or by transfer of matter. When there is
a suitable physical pathway, heat flows from a hotter body to a colder one.
Or
A form of energy associated with the motion of atoms or molecules and capable ofbeing trans
mitted through solid and fluid media by conduction, through fluid media byconvection, and through e
mpty space by radiation.
Or
The transfer of energy from one body to another as a result of a difference intemperature or a c
hange in phase.
Specific Heat:-
The specific heat is the amount of heat per unit mass required to raise the temperature by one degree
Celsius without change in surface.
Or
It may be defined as amount of heat that must be added or removed from 1 kg of substance by 1º C
without change in surface. The relationship does not apply if a phase change is encountered, because
the heat added or removed during a phase change does not change the temperature
cp = Q / (mΔT)
where
cp is the specific heat (kJ/kg o
, kJ/kg o
C)
Q is the heat added(kJ)
m is the mass(kg)
T is the change in temperature (K, o
C)
It is denoted by Cp. SI unit of heat capacity is kJ/(kg K)..Because of the high specific heat of
water relative to other materials, water will change its temperature less when it absorbs or loses a
given amount of heat. The reason you can burn your finger by touching the metal handle of a pot on
the stove when the water in the pot is still lukewarm is that the specific heat of water is ten times
greater than that of iron.
For example, the specific heat of water is around 4180 Joules per kilogram, so it takes 4180J of
energy to raise the temperature of 1kg of water by 1 degree Celsius.
Specific heat of weight agriculture materials is the sum of dry materials and moisture content.
It is an essential part of thermal analysis of food processing or equipment used for heating. Specific
heat can be thought of as a measure of how well a substance resists changing its temperature when it
absorbs or releases heat.
Latent heat:-
The quantity of heat absorbed or released by a substance undergoing a
change of state, such as
ice changing to water or water to steam, at constant temperature and pressure.
OR

Latent is the energy released or absorbed by a body or a thermodynamic system during a
constant-temperature process.
Or
Heat absorbed or released as the result of a phase change is called latent heat. There is no temperature
change during a phase change, thus there is no change in the kinetic energy of the particles in the
material.
The term was introduced around 1762 by Scottish chemist Joseph Black. It is derived from the
Latin latere (to lie hidden). The SI unit for specific latent heat is J/kg. Two of the more common forms
of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting) and latent
heat of vaporization (boiling). These names describe the direction of energy flow when changing
from one phase to the next: from solid to liquid, and liquid to gas.
A specific latent heat (L) expresses the amount of energy in the form of heat (Q) required to
completely effect a phase change of a unit of mass (m), usually 1kg, of a substance as an intensive
property:
where:
Q is the amount of energy released or absorbed during the change of phase of the
substance (in kJ),
m is the mass of the substance (in kg), and
L is the specific latent heat for a particular substance (kJ-kgm
−1
), either Lf for fusion, or Lv for
vaporization
The energy released comes from the potential energy stored in the bonds between the particles.
 exothermic (warming processes)
o condensation
o freezing
o deposition
 endothermic (cooling processes)
o evaporation/boiling
o melting
o sublimation
Endothermic meaning that the system absorbs energy on going from solid to liquid to gas. The change
is exothermic (the process releases energy) for the opposite direction.
Sensible heat
When an object is heated, its temperature rises as heat is added. The increase in heat is called
sensible heat. Similarly, when heat is removed from an object and its temperature falls, the heat
removed is also called sensible heat.
Sensible heat is heat exchanged by a body or thermodynamic system that changes the
temperature, and some macroscopic variables of the body, but leaves unchanged certain other
macroscopic variables, such as volume or pressure.
The terms sensible heat and latent heat are not special forms of energy; instead they
characterize the same form of energy, heat, in terms of their effect on a material or a thermodynamic
system. A good way to remember the distinction is that a change in sensible heat may be ″sensed″
with a thermometer, and a change in latent heat is invisible to a thermometer – the temperature
reading doesn't change. For example, during a phase change such as the melting of ice, the
temperature of the system containing the ice and the liquid is constant until all ice has melted. The
terms latent and sensible are correlative. Heat that causes a change in temperature in an object is called
sensible heat.

Enthalpy
Enthalpythermodynamic function of a system, equivalent to the sum of the internalenergy of t
he system plus the product of its volume multiplied by the pressure exerted on it by its surroundings.
Or
Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system. It is
equal to the internal energy of the system plus the product of pressure and volume
+H indicates that heat is being absorbed in the reaction (it gets cold) and  H indicates that heat
is being given off in the reaction (it gets hot). Enthalpy is a defined thermodynamic potential,
designated by the letter "H", that consists of the internal energy of the system (U) plus the product of
pressure (p) and volume (V) of the system. The unit of measurement for enthalpy in the International
System of Units (SI) is the joule, but other historical, conventional units are still in use, such as the
British thermal unit and the calorie. The enthalpy is an extensive property. The enthalpy of a
homogeneous system is defined as:
H=U+pV
where
H is the enthalpy of the system
U is the internal energy of the system
p is the pressure of the system
V is the volume of the system.

Preliminary Unit operation- Cleaning, sorting & Grading - aims, methods and
applications
PREPARATIVE OPERATIONS IN FOOD INDUSTRY
The preliminary preparative operations in food processing include cleaning, sorting and
grading of food raw material. These may be considered as separation operation. Cleaning involves the
separation of contaminants from the desired raw materials. Sorting involves the separation of
the raw materials into different categories based on their physical characteristics such as size,
shape and colour. Grading involves the separation of the raw materials into categories based on
the differences in their overall quality.
CLEANING OF FOOD RAW MATERIALS
Cleaning is an essential preliminary operation in any food industry. The ultimate quality of the
finished product, storage stability, organoleptic properties, safety from health hazards, and consumer
acceptance depend on cleaning process. The methods adopted depend on the type of raw material, type
and extent of contamination, the degree of cleaning to be achieved and the type of finished product.
Different food raw materials are associated with different types of contaminants. These include
 Mineral contaminants- soil, sand, stone metallic particles, grease and oil.
 plant part- stalks, pits, husks and rope,
 Animal parts and contaminants—excreta, hair, insects eggs and body part
 Chemical contamination- sprayed residues of pesticides, insecticides and fertilizers
 Microbial contaminants—microorganisms and their metabolites.
The chosen cleaning process must satisfy the following requirements in order to achieve the aforesaid
objective:-
1. The separation efficiency of the process must be high and consistent and should produce
minimum wastage of good material
2. Damage of cleaned raw material must be avoided.
3. Recontamination of the cleaned food should be avoided by complete removal of the
contaminants.
4. The design of the process equipment should be such that recontamination of the cleaned food
due to flying dust or wash water is prevented.
5. The cleaning process must leave the cleaned surface in acceptable condition,
6. The volume and concentration of liquid effluents must be kept be minimum and the effluents
should be disposed off effectively Complete cleaning of a raw material is not possible and in
practice, a balanced approach, considering the economic aspects of cleaning and the need to
produce good quality food, is usually adopted,
Cleaning Methods
The cleaning methods can be classified into two groups, namely
 Dry cleaning methods which include
screening, brushing, aspiration, abrasion and
magnetic separation
 Wet cleaning methods which include
soaking, spraying, flotation, ultrasonic
cleaning, filtration and settling.
Dry cleaning methods
These methods are relatively cheap and convenient
as the cleaned surface is dry However, a major
drawback is the spread of dust.

Screening-Screens are primarily size separators or sorting machines but may be used as
cleaning equipment for removing contaminants of different size from that of the raw material. These
machines are useful in cleaning fine materials such as flour and ground spices but must be frequently
cleaned to remove oversized contaminants which may otherwise get pulverized due to abrasion and
spread contamination of the raw material.
Abrasion cleaning- Abrasion between food particles or between the food and moving parts of
cleaning machinery is used to loosen and remove adhering contaminants. Tumblers, vibrators,
abrasive discs and rotating brushes are used for this purpose.
Aspiration cleaning- Aspiration (or
winnowing) is based on the differences in the
aerodynamic properties of materials. The raw
material to be cleaned is fed into a stream of air
flowing at controlled velocity to separate the raw
materials into two or more streams (e.g. light and
heavy streams). The cleaned products are usually
discharged as the middle stream leaving the heavy
debris (stones, pieces of metal or wood) behind
while floating off the light debris such as stalks,
husks and hairs. This method is used in cleaning
cereals, nuts, beans, onions, melon, eggs and other
foods which are not amenable to wetting. The
method cannot be used with oxidation-sensitive
materials.
Magnetic cleaning- This type of
cleaning involves where the food
contaminated with high amount of
metallic material. Magnetic separators
used for this type of cleaning include
rotating or stationary magnetic drums,
magnetized belts, magnets located over
belts carrying the food or staggered
magnetized grids through which the food
is passed. A magnetic separator is a piece
of equipment that magnetically attracts
and removes foreign metal pieces from
other materials. The process of magnetic
separation is utilized in many industries,
some of which include:
 Food and beverages,
 Pharmaceuticals,
 Recycling,
 Mining,
 Coal,
 Aggregate,
 Plastic,
 Rubber,
 Chemicals,
 Packaging, and
 Textiles
Miscellaneous dry cleaning methods- Such cleaning methods include:
1. Electrostatic cleaning
2. radio isotope separation

171
3. X-ray separation.
Electrostatic cleaning- Electrostatic cleaning can be used in a limited number of cases where
the surface charge on raw materials differs from contaminating particles. The principle can be used to
distinguish grains from other seeds of similar geometry but differences in electrostatic charging of
materials under controlled humidity conditions, charged particles being removed by oppositely
charged or earthed rollers, grids, etc. and it has also been described for cleaning tea. The feed is
conveyed on a charged belt and charged particles are attracted to an oppositely charged electrode
according to their surface charge.
Radio isotope separation- Clods of earths and stones may be separated from the potatoes.
X-ray separation- Stones, gloss and metal fragments in foods such as confectionery can be
separated by this method.
Wet cleaning methods-
Wet cleaning has the advantage of removing firmly adherent soils and owing the use of
detergents and sanitizers. However, wet methods have a number of disadvantages such as the use of
large amounts of high quality water and generation of large volume of effluent (about 15,000 liters per
ton of canned food). Wet cleaning methods include soaking, spray washing, flotation washing and
ultrasonic cleaning methods.
Soaking- This is the simplest method and is often used as preliminary stage in the cleaning of
heavily contaminated root vegetables and other foods. Soaking softens adhering soil and also
facilitates the removal of sand, stone, and ether abrasive material. The use of warm water and
detergents increase the efficiency but the use of chemicals may affect the texture of the food, e.g,
sodium hexametaphosphate softens peas while some metal ions toughen peas and peaches destined
for canning, Chlorination is used to decrease bacterial load of water in the soak tank.
Spray washing. This is the most widely used method for wet cleaning of fruits and
vegetables. The surface of the food is subjected to water sprays, The efficiency of spray washing
depends on several parameters such as water pressure, volume of water, temperature, the distance
of the food from jets, the time of spraying and number of spray jets used. A small volume of water at
high pressure is the most effective combination. High pressure sprays may be used to cut out parts
of peaches and tomatoes and to remove adherent soil and black moulds on citrus fruits. It may
damage ripe fruits and vegetables such as straw berries and tomatoes and delicate vegetables such
as asparagus.

The washer is equipped with a central spray rod which is fitted with jets for spraying water.
A rubber disc cleaner requires less amount of water for cleaning. It uses soft rubber discs
spinning axially at about 500 rpm. The soil is collected into the base of the channel. The disc
cleaner uses only about 20 liters of water per ton of fruit while other washers use 1500-5000
litres.
Flotation washing- The method depends on the differences in buoyancy of the desired and
undesired parts of the food raw material to be cleaned. For example, bruised or rotten apples
sink in water and can be removed at the base of tank and the good fruit can be collected as
overflow. The flotation washer effectively removes stones, dirt and plant debris from peas,
beans, dried fruits and similar materials. Water requirement is about 4,000-10,000 liters per ton
of raw material to be cleaned.
Froth flotation has been used to separate peas from weed seeds by immersing the peas in
dilute mineral oil-detergent emulsion through which air is blown, the contaminants float on
foam and are removed. The cleaned peas are given a final wash to remove the emulsion.
Dewatering- Wet cleaning results in a cleaned product that may have some excess water
adhering to it. Dewatering may be effected by passing the food over vibratory screens or
specially designed rotary screens. In the case of cleaned peas for freezing, or washed wheat for
milling, centrifuges may be used. Occasionally it may be necessary to resort to drying
procedures, as in the case of cereals or fruits, which arc to be stored or sold as fresh.
The two main objectives of cleaning food raw materials are
1. Removal of contaminants which constitute a health hazard or which are aesthetically
unacceptable

2. Control of microbiological loads and biochemical reactions which impair subsequent process
effectiveness and product quality.
SORTING OF FOODS
Sorting and grading are terms which are frequently used interchangeably in the food
processing industry, but strictly speaking they are distinct operations.
Sorting is a separation based cm a single measurable property of raw material
units, while grading is the assessment of the overall quality of a food using a number of
attributes". Sorting may be regarded as a separation operation based on the differences in
physical properties of the food raw materials or products such as colour, size, shape or weights
of the food raw material.
Sorting is an important operation in controlling the effectiveness of many processes in
food industry. For example, sorted vegetables and fruits are better suited for mechanized
operations of peeling, pitting and coring or blanching. Similarly, food materials of uniform size
or shape are better suited for efficient heat transfer during sterilization, pasteurization,
dehydration or freezing.
Sorting and grading can both damage the food raw material or product because of
improper handling by human operators (operator damage), dumping (dumping damage) or
dropping of material (drop damage). Such damages can be eliminated or minimized by
choosing effective food process.
Sorting Methods
Sorting methods include weight sorting, shape sorting, size sorting and photometric or colour
sorting.
Weight sorting- Weight is usually the most precise method of sorting. The weight of a
food unit is proportional to the cube of its characteristic dimension and hence weight sorting is
more precise compared to dimensional sorting. Meat cuts, fish fillets, fruits such as apples,
pears and citrus fruits,
vegetables such as
potatoes, carrots and
onions and eggs are sorted
by weight using spring-
loaded, strain gauge, or
electronic weighing
devices incorporated into
conveying systems. An alternative system is to use the "catapult' principle where units are
thrown into different collecting section, depending on their weight. A disadvantage of weight
sorting is the relatively long time required per unit and other methods are more appropriate
with smaller items such as legumes or cereals, or if faster throughput is required.
Size sorting- Different types of screens are used for size separation of foods,

The screen designs commonly used in food
industry may be grouped into two types: (i) variable aperture screens using cable, belt, roller or
screw sorters and (ii) fixed aperture screens using stationary, vibratory, rotary, gyratory or
reciprocating screens. Fixed aperture screens of flat-bed type are used in preliminary sorting of
potatoes, carrots and turnips. Multi-deck screens are used in size sorting of cereals, nuts and
also partly processed and finished foods such as flour, sugar, salt, ground spices and herbs.
Drum screens are used for sorting peas, beans and other similar foods capable of withstanding
tumbling action in a rotating drum screen. Variable aperture screens with continuously variable
apertures of roller, belt or screw type find use in size sorting of fruits and vegetables.
Shape sorting- Shape sorting
is adopted when food raw
materials contain undesirable
material even after size or
weight sorting and cleaning.
For example, cleaned and size
or weight sorted wheat may
still contain weed seeds of
similar size and weight
compared to wheat. Shape
sorting on the basis of a
combination of length and diameter is useful under such circumstances. A disc sorter is used
for shape sorting wheat, rice, oats and barley. The principle is that disks or cylinders with
accurately shaped indentations will pick up seeds of the correct shape when rotated through the
stock, while other shapes will remain in the feed.

Photometric/Color sorting- Photometric sorting uses optical properties of foods to
effect separation of desired material from contaminants. The goal is the separation of items that
are discolored, toxic, not as ripe as required, or still with hull. The color separator separates the
fruits, vegetables or grains due to difference in color or brightness. The color separators are
generally used for larger crop seeds like peas and beans. These seeds differ in color because of
varietal differences and also due to immaturity or disease. Color sorters are also used for color
sorting harvested foodstuffs, such as coffee, nuts, rice, and other cereals such as wheat or rye
and pulses.
Two photocells are fixed at a particular angle, which direct both beams to one point of
the parabolic trajectory of the grains. A needle is placed on the other side, which is connected
to a high voltage source. When a beam falls on a dark object through photoelectric cells,
current is generated on the needle. The needle end receives a charge and imparts it to the dark
seeds. The grains are then passed between two electrodes with a high potential difference
between them. The seed is compared with a selected background or color range, and is
separated into two fractions according to difference in color. Since this machine views each
produce individually, the capacity is low.
Reflectance properties are used to indicate:
1. Raw material maturity (e.g. color of fruit, vegetables and meat indicates ripeness
and freshness characterize ;)
2. the presence of surface defects (e.g. worm holed cereals or nuts and bruised
fruits)
3. The extent of heat processing (e.g. in manufacture of bread and potato chips or
crisps).
Other sorting methods- Sorting on the basis of surface roughness or stickiness may be used
for separating seeds. In Surface Texture/Roughness Separator the mixture to be separated is fed over
the centre of an inclined draper belt moving in upward direction. The round and smooth grains roll or
slide down the draper at faster rate than the upward motion of the belt, and these are discharged in a

hopper. The flat shape or rough surfaced particles are carried to the top of the inclined draper and
dropped off into another hopper.
GRADING OF FOODS
Grading is quality separation on the basis of an overall assessment of those properties, which affect the
acceptance of the food raw material for processing, and finished food product for consumer
acceptance and safety. The grading factors which determine the quality of the food include:
1. Process suitability
2. consumer safety
3. Consumer acceptance.
The grading parameters commonly used in food industry include the following:
 size and shape as functional and acceptability factors,
 maturity to describe the freshness of eggs, ripeness of fruits and aging of meat,
 texture to grade the crumb structure in bread and cakes, crispness in apples and viscosity of
creams
 flavour and aroma as indicators of ripeness of fruits as well as effectiveness of processing
conditions,
 colour as indicator for consumer acceptability and effectiveness of process,
 Blemishes such as cloudy yolk, blood spot and shell cracks in eggs, bruises in fruits and
insect holes in coffee beans and cereals to indicate their defect and impurity.
Contaminants and undesired parts such as rodent hair and insect parts in flour, soil and spray residues
on fruits and vegetables, microorganisms and their metabolites on meat, toxic metals in shell fish,
hone fragments in meat products, pod residues in peas and beans and stalks and stones in fruits all
these are the adverse qualities of the raw food materials.
Grading Methods
Grading methods may be classified into two types:
 Quality control procedures in which the quality of the food is determined by laboratory tests on
samples drawn statistically from a batch of food.
 Procedures in which the total quantity of food is subjected to physical separation in quality
categories. This grading may be carried out manually or by specialized machines.
For proper grading, the food unit must be presented singly before the human grader or machine for
assessment. These devices may be roller or vibratory tables or rotating wheels equipped peripherally
with pneumatic devices which pick up food pieces, rotate them for viewing and then release them at a
given signal.
Manual grading is done by trained operators who are able to assess a number of grading
parameters simultaneously. For example, eggs are graded manually by candling.
. Machine grading is only feasible where quality of a food is linked to a single physical property,
and hence a sorting operation leads to different grades of material. But can be carried out by
combining a group of sorting operations so as to separate the food units on quill it basis. Thus wheat of
a particular variety may be graded by a combination of cleaning and sorting operations. Sometimes a
single property may be helpful in grading the food. Thus peas of small size are recognized to be most
tender and of highest quality so that size sorting of cleaned peas results in quality grading. Peas may
also be graded on the basis of their density using flotation in brines of varying densities. Similarly,
potatoes or high density, desirable for manufacturing French fries, potato crisps and dehydrated
mashed potato, may be graded using Rotation in brines. Mechanical grading is cost effective and
efficient.

Size Reduction - Theory of Comminution; Calculation of energy required during size
reduction. Crushing efficiency; Size reduction equipment; Size reduction of fibrous, dry and
liquid foods; effects of size reduction on sensory characteristics and nutritive value of food
Size reduction
Size reduction is a process of reducing large solid unit masses-vegetables or chemical substances
into small unit masses, coarse particles or fine particles.
The term size reduction is applied to all the ways in which particles of the food materials is reduced
into smaller size. The size reduction is done for different purpose and by different methods. Crushing,
grinding, hammering and cutt9ng are the main methods of size reduction for food material.
In the case of liquids and semisolids, size reduction operations include mashing, atomizing,
homogenizing etc. The following are some important applications size reduction in the food industry:
 Milling of cereal grains to obtain flour
 Fine grinding of chocolate mass
 Flaking of soybeans prior to solvent extraction
 Cutting of vegetables and fruits to desired shapes (Cubes, strips, slices…)
 Fine mashing of baby food
 Homogenization of milk and cream
Theory of Comminution: It is the process of size reduction. So that the surface area of the produce
increases and solvent can easily interact with the produce. Most of the natural produce is to be dried.
Drying can be done in sun or shade or in the protected area depending upon the type of the
constituents. It is preferred that drying should be slow at low temperature. The dried material is to be
crushed or broken into small parts before extraction/ distillation. During crushing/grinding temperature
of the produce should not be increased. Some of the volatiles get evaporated even at 45o
C. The
homogeneity of the ground particle shows the efficacy of the extraction of active ingredient.
Calculation of energy required during size reduction. Crushing efficiency;
Energy and Power for Size reduction:
The cost of power is the major expense in crushing and grinding operation. Thus, accurate
estimation of the energy required is important in the design and selection of size reduction equipment.
During size reduction, the solid particles are first distorted and strained. By applying additional
force, the stressed particles are distorted beyond their ultimate strength and suddenly rupture into
fragments. Thus, new surface is generated. The energy of stress in excess of the new surface energy
created appears as heat.
It is not possible to estimate accurately the power requirement of crushing and grinding
equipment to effect the size reduction of a given material, but a number of empirical laws have been
put forward e.g. Rittinger's law, Kick's law and Bond's law.
Kick's law: Kick’s law states that the work required for crushing a given mass of material is constant
for the same reduction ratio, that is, the ratio of the initial particle size to the final particle size. Kick
assume that the energy required to reduce a material in size was directly proportional to the size
reduction ratio dL/L.
E = KKfc loge (L1/L2)
Where,
Kk is called Kick’s constant
fc is called the crushing strength of the material

Equation of Kick's Law implies that the specific energy required to crush a material, for
example from 10 cm down to 5 cm, is the same as the energy required to crush the same material from
5 mm to 2.5 mm.
Rittinger's theory: Von Rittinger proposed a theory stating that the energy consumed in comminution
is proportional to new surface produced. Rittinger on the other hand, assumed that the energy
required for size reduction is directly proportional, not to the change in length dimensions, but to the
change in surface area.
E = KRfc(1/L2– 1/L1)
where,
KR is called Rittinger's constant
fc is called the crushing strength of the material
Equation of Rittinger's Law means that energy required to reduce L for a mass of particles from 10 cm
to 5 cm would be the same as that required to reduce, for example, the same mass of 5 mm particles
down to 4.7 mm. This is a very much smaller reduction, in terms of energy per unit mass for the
smaller particles, than that predicted by Kick's Law.
Bond's theory: Bond's so called third theory of comminution states that the energy required is
proportional to the length of crack initiating breakage
E = Ei (100/L2)1/2
[1 - (1/q1/2)
]
Where,
Ei is the amount of energy to reduce unit mass of the material from an infinitely large particle size
down to a particle size of 100 mm.
q=L1/L2
It appears that Kick's results apply better to coarser particles, Rittinger's to fine ones with Bond's being
intermediate.
Crushing efficiency:
It is defined as the ratio of the surface energy created by crushing to the energy absorbed by the solid
Ƞc = es (Ab-Aa)
Wn
Where
Ƞc = crushing efficiency
Wn = energy absorbed by material, J/kg
es = surface energy per unit area, J/m2
Ab = area of product, m2
Aa = area of feed, m2
The energy created by fracture is very small as compared to the energy stored in the material at
the time of rupture, and most of the mechanical energy stored in the material is converted into heat.
Crushing efficiencies are thus low.

The energy absorbed by the solid (Wn) is less than the energy supplied to the machine (W).Part
of the total energy input to the machine is utilized to overcome the friction in the bearings and other
moving parts and the remaining part is available for crushing. The mechanical efficiency is the ratio of
the energy absorbed to the energy input.
The minimum energy required for crushing is the energy required for creating fresh surface. In
addition, energy is absorbed by the particulate material due to deformation, friction, etc., which results
in an increase of the material temperature.
Advantages of size reduction
1. Size reduction increases the digestion of the food.
2. smaller particles are easy to pack
3. Facilitating separation of different parts of a material (milling wheat to obtain flour and bran
separately).
4. Accelerating heat and mass transfer (atomization of milk as a fine spray into hot air in spray
drying)
5. Size reduction also increase the reactivity of solids
6. Size reduction make the food eatable
7. Facilitating mixing and dispersion
8. Size reduction also reduces the bulk of fibre material.
9. Obtaining pieces and particles of defined shapes.
10. Easy to handle and pack
11. To improve blending efficiency of formulations, composites e.g insecticides, dyes, paints
Disadvantages of size reduction
The destruction of cells resulting increased in surface are and promotes oxidation deterioration.
Due to this high microbiological deterioration and increased the enzyme activity which effect the
quality aroma and texture.
Criteria for size reduction
An ideal crusher would (1) have a large capacity; (2) require a small power input per unit of product;
and (3) yield a product of the single size distribution desired.

Principles of size reduction
Most size reduction machines are based on the following principles:
1. Compression
2. Impact
3. Attrition or rubbing
4. Cutting
1. Compression/Crushing: - When an external force is applied on a material in excess of its
strength, the material fails because of its rupture in many directions. The particles produced
after crushing are irregular in shape and size. The type of material and method of force
application affects the characteristics of new surfaces and particles. Food grain flour, grits and
meal, ground feed for livestock are made by crushing process. Crushing is also used to extract
oil from oilseeds and juice from sugarcane.
2. Impact: - When a material is subjected to sudden blow of force in excess of its strength, it
fails, like cracking of nut with the help of a hammer. Operation of hammer mill is an example
of dynamic force application by impact method.
3. Attrition/ Shearing: - It is a process of size reduction which combines cutting and crushing.
4. Cutting: In this method, size reduction is accomplished by forcing a sharp and thin knife
through the material. In the process minimum deformation and rupture of the material results
and the new surface created is more or less undamaged. An ideal cutting device is a knife of
excellent sharpness and it should be as thin as practicable. The size of vegetables and fruits are
reduced by cutting.
SIZE REDUCTION EQUIPMENT
Size-reduction equipment is divided into crushers, grinders, ultrafine grinders, and cutting
machines.
Crushers do the heavy work of breaking large pieces of solid material into small lumps. A primary
crusher breaking it into 150 to 250 mm lumps. A secondary crusher reduces these lumps to particles
perhaps 6 mm in size. Example: -
1. Jaw crushers
2. Gyratory crushers
3. Crushing rolls
4. Cone Crushers
Intermediate Crusher: - Feed size is about 50mm to 5 mm and final product size may be 5 to 0.1mm
1. Hammer mills; impactors
2. Rolling-compression mills
3. Granulator
Fine Crusher/ Grinders reduce crushed feed to powder. The product from an intermediate grinder
might pass a 4O mesh screen; most of the product from a fine grinder would pass a 200-mesh screen
and the feed size may be in the range of 5-2mm.
1. Attrition mills
4. Tumbling mills
a) Rod mills
b) Ball mills; pebble mills
c) Tube mills; compartment mills
Ultrafine grinder accepts feed particles no larger than 6 mm; the product size is typically 1 to 50 μm.
1. Hammer mills (Fine Impact Mill)
2. Fluid-energy mills

Cutters give particles of definite size and shape, 2 to 10 mm in length.
1. Knife cutters; dicers; slitters
These machines do their work in distinctly different ways. Compression is the characteristic action of
crushers. Grinders employ impact and attrition, sometimes combined with compression; ultrafine
grinders operate principally by attrition. A cutting action is of course characteristic of cutters, dicers,
and slitters.
Crushers
Crushers are slow-speed machines for coarse reduction of large quantities of solids. The main
types are jaw crushers, gyratory crushers, smooth-roll crushers, and toothed-roll crushers.
The first three operate by compression and can break large lumps of very hard materials, as in
the primary and secondary reduction.
Toothed-roll crushers tear the feed apart as
well as crushing it; they handle softer feeds
like coal, bone, and soft shale.
Jaw crushers: In a jaw crusher feed is
admitted between two jaws, set to form a V
open at the top. A jaw crusher consists of a vertical
fixed jaw and another swinging jaw moving in the
horizontal plane. The two jaws make 20-30o
angle
between them. The jaw faces may be flat or slightly bulged. Feed is admitted between the jaws. Large
lumps caught between the upper parts of the jaws are broken; drop into the narrower space below. It is
crushed several times between the jaws before it is discharged at the bottom opening. After sufficient
reduction they drop out the bottom of the machine. The jaws open and close 250 to 400 times per
minute. A jaw crusher is a primary crusher which produces a course product

Gyratory crushers: A gyratory crusher is
similar in basic concept to a jaw crusher.
This type of crushers consist a concave
surface and a conical head and both surfaces
are rough and typically lined. The inner cone
has a slight circular movement, but does not
rotate. An eccentric drives the bottom end of
the shaft. A gyratory crusher is one of the
main types of primary crushers in a mine or
ore processing plant.
The crushing action is caused by the
closing of the gap between the movable
center surface and main frame of the
crusher. The gap is opened and closed by an
eccentric on the bottom of the spindle that
causes the central vertical spindle to gyrate.
The speed of the crushing head is typically 125 to 425 gyrations per minute. Less maintenance is
required than with a jaw crusher; and the power requirement per ton of material crushed is smaller.
Crushing rollers
a. Smooth-roll crushers
b. Toothed-roll crushers
Smooth-roll crushers: In this type of crushers two heavy smooth-faced metal rolls are present, which
are mounted horizontally. The size of the rollers may be from a few centimeters to meters to
diameters. Generally, one of the rollers is driven directly, while the second one runs freely. The
material to be crushed is feed form the hopper into the gap between the two rollers. Due to rotation of
these rollers the material is crushed. Typical rolls are 600 mm to 2000 mm in diameter. Rollers speed
range from 50 to 300 r/min. Smooth-roll crushers are secondary crushers, with feeds 12 to 75 mm in
size and products 12 mm to about 1 mm. The limiting size Dp.max. of particles that can be nipped by
the rolls depends on the coefficient of friction between the particle and the roll surface, but in most
cases, it can be estimated from the simple relation.

Dp.max. = 0.04R + d5.1 (5.1)
Where R = roll radius
d = half the width of the gap between the
rolls.
The maximum size of the
product is approximately equal to 2d. The particle
size of the product depends on the spacing between
the rolls, as does the capacity of a given machine.
Smooth-roll crushers give few fines and virtually no
oversize. To avoid damaging machine, at least one
roll must be spring mounted.
Toothed Roll Crusher: -Toothed roll
crusher is widely used in coal, metallurgy, mining,
chemical industry, building materials and other
industries, and it is more suitable to crush coal in
large coal mines or coal preparation plant so that it also can be called coal crusher. Toothed roll
crusher has high crushing capacity. The distance between the rollers can be adjusted by hydraulic
pressure. Such crushers may contain two rolls, as in smooth-roll crushers. A single-roll toothed
crusher is also used. Some crushing rolls for coarse feeds
carry heavy pyramidal teeth.
Toothed-roll crushers are much more versatile than
smooth-roll crushers, within the limitation that they cannot
handle very hard solids. They operate by compression,
impact, and shear, not by compression alone, as do
smooth-roll machines. Some heavy-duty toothed double-
roll crushers are used for the primary reduction of coal and
similar materials. The particle size of the feed to these
machines may be as great as 500 mm; their capacity
ranges up to 500 tons/h.
Grinders
The term grinder describes a variety of size-reduction machines for intermediate duty. The product
from a crusher is often fed to a grinder, in which it is reduced to powder. The chief types of
commercial grinders described in this section are hammer mills and impactors, rolling-compression
machines, attrition mills, and tumbling mills.
1. Hammer mills:
Principle: - It operates on the principle of impact between rapidly moving hammers mounted on rotor
and the stationary powder material.
These mills all contain a high-speed rotor turning inside a cylindrical casing. The shaft is
usually horizontal Feed dropped into the top of the casing is broken and falls out through a bottom
opening. In a hammer mill the particles are broken by sets of swing hammers attached to a rotor disk.
A particle of feed entering the grinding and shatters into pieces by hammers, then the material pushed
through a screen that covers the discharge opening.
Several rotor disks of 150 to 450 mm diameter carrying four to eight swing hammers are often
mounted on the same shaft.. Intermediate hammer mills yield a product 25 mm to 20-mesh in particle
size. Hammer mills grind almost anything tough fibrous solids like bark or leather, steel turnings, soft
wet pastes, sticky clay, hard rock. For fine reduction they are limited to the softer materials.

Hammer mills
Particles are broken by impact without rubbing action in a hammer mill.
2. Rolling-compression machines:
Principle
Material is compressed by application of stress and attrition. Stress is applied by rotating heavy
wheels, Muller or Rollers.
Roller mills are similar to roller crushers, but they have smooth or finely fluted rolls,
and rotate at differential speeds. They are used very widely to grind flour. Because of their
simple geometry, the maximum size of the particle that can pass between the rolls can be
regulated. If the friction coefficient between the rolls and the feed material is known, the
largest particle that will be nipped between the rolls can be calculated, knowing the geometry
of the particles. In this kind of mill the solid particles are caught and crushed between a rolling
member and the face of a ring or casing. The most common types are rolling-ring pulverizers,
bowl mills, and roller mills. They pulverize up to 50 ton/h. When classification is used, the
product may be as fine as 99 percent through a 200-mesh screen.
3. Attrition mills: In attrition mill particles of soft solids are rubbed between the grooved flat
faces of rotating circular disks. The axis of the disks is usually horizontal, sometimes vertical.
In a single-runner mill one disk is stationary and one rotates; in a double-runner machine both
disks are driven at high speed in opposite directions. Feed enters through an opening in the hub
of one of the disks; it passes outward through the narrow gap between, the disks and discharges
from the periphery into a stationary casing. The width of the gap, within limits, is adjustable.
Mills with different patterns of grooves, corrugations, or teeth on the disks perform a variety of
operations, including grinding, cracking, granulating, and shredding, and even some operations
not related to size reduction at all, such as blending. There are two type of mills Single-runner
mills and double run mill. The disks of a single-runner mill are 250 to 1400 mm in diameter;
turning at 350 to 700 r/min. Disks in double-runner mills turn faster, at 1200 to 7000 r/min.
The disks may be cooled with water or refrigerated brine. Cooling is essential with heat-

sensitive solids like spices, rubber which would otherwise be destroyed. Attrition mills grind
from 1 to 8 ton/h to products that will pass a 200-mesh screen.
USAGE EXAMPLES
Attrition mills are used for fine grinding operations in the production of spices (pepper, cinnamon, and
paprika), food (peanuts, grain, cereal), fibers (chips, cork, cellulose) and blending (face powders,
insecticides). The pictures below show pepper and cinnamon, finished products from attrition milling.
ADVANTAGES DISADVANTAGES
 Finely ground products.
 Large range of sizes available.
 Energy consuming.
 Needs specific input size.
4. Tumbling/Ball mills: tumbling mills is basically of three types
a) Rod mills
b) Ball mills; pebble mills
c) Tube mills; compartment mills
Principle
It operates on the principle of impact and attrition.
A cylindrical shell slowly turning about a horizontal axis and filled to about half its volume with a
solid grinding medium forms a tumbling mill. The shell is usually steel, lined with high-carbon steel
plate, porcelain, silica rock, or rubber. The grinding medium is metal rods in a rod mill, lengths of
chain or balls of metal, rubber, or wood in a ball mill, flint pebbles or porcelain or zircon spheres in a
pebble mill. Tumbling mills may be continuous or batch. In a batch machine solid to be ground is
loaded into the mill through an opening in the shell. The opening is then closed and the mill turned on
for several hours; it is then stopped and the product is discharged. In a continuous mill the solid flows
steadily through the revolving shell, entering at one end through a hollow turn-on and leaving at the
other end through the turn-on or through peripheral openings in the shell. In all tumbling mills the
grinding rods are usually steel, 25 to 125 mm in diameter.

In a ball mill or pebble mill most of the reduction is done by impact as the balls or pebbles drop
from near the top of the shell. In a large ball mill the shell might be 3 m in diameter and 4.25 m long.
The balls are 25 to 125 mm in diameter; the pebbles in a pebble mill are 50 to 175 mm in size.
A tube mill is a continuous mill with a long cylindrical shell, in which material is ground for 2 to 5
times as long as in the shorter ball mill. Tube mills are excellent for grinding to very fine powders in a
single pass where the amount of energy consumed is not of primary importance. Putting slotted
transverse partitions in a tube mill converts it into a compartment mill.
One compartment may contain large balls, other small balls, and a third pebbles. The amount of
energy expended is suited to the difficulty of the breaking operation, increasing the efficiency of the
mill.
USAGE EXAMPLES
Vertical spindle mills are used in the mineral industry to grind materials such as phosphate, limestone,
magnesite, and bauxite.
 Easily cleaned.
 Dust-free operation.
 High capacity.
 Automatic operation.
 Rings and rollers wear easily.
Ultrafine Grinders
Many commercial powders must contain particles averaging 1 to 20 μm in size, with substantially all
particles passing a standard 325-mesh screen that has openings 44 μm wide. Mills that reduce solids to
such fine particles are called ultra fine grinders. Ultrafine grinding of dry powder is done by grinders,
such as high-speed hammer mills, provided with internal or external classification, and by fluid-energy
or jet mills. Ultrafine wet grinding is done in agitated mills.
1. Hammer mills: As like given above
2. Fluid energy mills:
Principle: It operates on the principle of impact and attrition.

In these mills the particles are suspended in a high - velocity gas stream. The reduction occurs
when the particles strike or rub against the walls of the confining chamber, but most of the reduction is
believed to be caused by interparticle attrition. Internal classification keeps the larger particles in the
mill until they are reduced to the desired size. Gas is usually compressed air or superheated steam,
admitted at a pressure of 7 atm through energizing nozzles.
The grinding chamber is an oval loop of pipe 25 to 200 mm in diameter and 1.2 to 2.4 m high.
Feed enters near the bottom of the loop through a venturi injector. Classification of the ground
particles takes place at the upper bend of the loop. As the gas stream flows around this bend at high
speed, the coarser particles are thrown outward against the outer wall while the fines congregate at the
inner wall. A discharge opening in the inner wall at this point leads to a cyclone separator and a bag
collector for the product. They reduce up to 1 ton/h of non sticky solid to particles averaging! to 10
11m in diameter, using 1 to 4 kg of steam or 6 to 9 kg of air per kilogram of product. Loop mills can
process up to 6000 kg/h.
USAGE EXAMPLES
Pulverizers are commonly used for chemicals, pigments and food processing. The
microscale air impact pulverizer is used in laboratories, where small samples are needed.
ADVANTAGES
 Air needed is free.
 Large range of sizes available.
 Homogeneous blend.
DISADVANTAGES
 Energy consuming
3. Agitated mills: For some ultrafine grinding operations, small batch non rotary mills containing
a solid grinding medium are available. The medium consists of hard solid elements such as
balls, pellets, or sand grains. These mills are vertical vessels 4 to 1200 L in capacity, filled with
liquid in which the grinding medium is suspended. In some designs the charge is agitated with
a multiarmed impeller; in others, used especially for grinding hard materials (such as silica or
titanium dioxide), a reciprocating central column “vibrates” the vessel contents at about 20 Hz.
Concentrated feed slurry is admitted at the top, and product (with some liquid) is withdrawn
through a screen at the bottom. Agitated mills are especially useful in producing particles 1 /lm
in size or finer.

4. Colloid mills: In a colloid mill,
intense fluid shear in a high-
velocity stream is used to disperse
particles or liquid droplets to form
a stable suspension or emulsion.
The final size of the particles or
droplets is usually less than 5 /lm..
Syrups, milk, purees, ointments,
paints, and greases are typical
products processed in this way. In
most colloid mills the feed liquid is
pumped between closely spaced
surfaces one of which is moving
relative to the other at speeds of 50
m/s or more. In the mill the liquid
passes through the narrow spaces
between the disk-shaped rotor and
the casing. The clearances are adjustable down to 25 /lm. Often cooling is required to remove
the heat generated. The capacities of colloid mills are relatively low, ranging from 2 or 3 L/min
for small mills up to 440 L/min for the largest units.
USAGE EXAMPLES
Colloid mills are used largely in asphalt production and grease manufacturing. They are also used in a
wide variety of industries, such as paints, pigments, food and cosmetics, such as in the production of
the lipstick. In the food processing industry, colloid mills are used in the production of mayonnaise,
peanut butter, salad dressings, buttered syrups, and chocolate toppings.Pin mills are commonly used to
produce talc, clays, resins, flour and starch.
 Self-cleaning.
 Rugged and durable.
 Wide variety of uses.
 In colloid mills, the feed must be in a
pumpable slurry.
 Pins in pin mills wear easily.
CONE MILLS
GENERAL INFORMATION
Unlike most types of mills, cone mills can be used for hard to grind products while using less energy
than other types of mills. Cone mills are preferred in some industries because they produce less noise,
dust, and heat than traditional milling equipment.
EQUIPMENT DESIGN
Material is fed into the conical chamber by gravity or conveying it. Inside the chamber is a rotor that
spins at a low velocity and forces the material against the wall. The rotor has two paddles that pass
over the material on the wall, inducing a shear force on it. This shear force breaks apart the material
and when the particles are small enough they pass through the holes in the wall and fall into a
collection container. Since the rotor is spinning at a low velocity the particles that pass through the

wall tend to have a uniform size and the rotor generates little heat. This system is completely enclosed
so that little noise and dust is generated.
USAGE EXAMPLES
Cone milling is used in the pharmaceutical, food, and chemical industries. It is widely used in
pharmaceuticals for wet and dry granulation. In the food industry it is many used for grinding of foods
such as sugar, candy, and chocolate.
 High efficiency
 Low heat generation
 Low noise and dust emissions
 Can mill sticky materials
 Easy to clean
 Small volume
Cutting Machines
In some size-reduction problems the feed
stocks are too tenacious or too resilient to be
broken by compression, impact, or attrition. In
other problems the feed must be reduced to
particles of fixed dimensions. These requirements
are met by devices that cut, chop, or tear the feed
into a product with the desired characteristics.
The saw-toothed crushers mentioned above do
much of their work in this way. True cutting
machines include rotary knife cutters and
granulators. These devices find application in a
variety of processes but are especially well
adapted to size reduction problems in the
manufacture of rubber and plastics.
Principle of rotary cutters
Size Reduction involves successive cutting / Shearing the feed material with help of sharp knife
Knife cutters: A rotary knife cutter contains a horizontal rotor turning at 200 to 900 r/min in a
cylindrical chamber. On the rotor are 2 to 12 flying knives with edges of tempered steel or satellite
passing with close clearance over 1 to 7 stationary bed knives. Feed particles entering the chamber
from above are cut several hundred times per minute and emerge at the bottom through a screen with 5
to 8 mm openings. Sometimes the flying knives are parallel with the bed knives; sometimes,
depending on the properties of the feed, they cut at an angle. Rotary cutters and granulators are similar
in design. A granulator yields more or less irregular pieces; a cutter may yield cubes, thin squares, or
diamonds.
Size reduction of fibrous foods
Most fruits and vegetables fall into the general category of ‘fibrous’ foods. Fruits and vegetables have
an inherently firmer texture and are cut at ambient or chill temperatures. There are five main types of
size reduction equipment, classified in order of decreasing particle size, as follows.
1. Slicing equipment consists of rotating or reciprocating blades which cut the food as it passes
beneath. In some designs food (Figure 6.1) is held against the blades by centrifugal force. In other (for
slicing meats) the food is held on a carriage as it travels across the blade. Harder fruits such as apples

are simultaneously sliced and de-cored as they are forced over stationary knives fitted inside a tube. In
a similar design (the hydro cutter) foods are conveyed by water at high speed over fixed blades.
Slicing equipment
2. Dicing equipment is for vegetables, fruits and meats. The food is first sliced and then cut into strips
by rotating blades. The strips are fed to a second set a rotating knives which operate at right angles to
the first set and cut the strips into cubes (Figure 6.2).
Dicing equipment
3. Flaking equipment for flaked nuts, fish or meat is similar to slicing equipment. Adjustment of the
blade type and spacing is used to produce the flakes.
4. Shredding equipment. Typical equipment is a modified hammer mill in which knives are used
instead of hammers to produce a flailing or cutting action. A second type of shredder is known as the
squirrel cage disintegrator. Here two concentric cylindrical cages inside a casing are fitted with knife
blades along their length. The two cages rotate in opposite directions and food is subjected to powerful
shearing and cutting forces as it passes between them.
5. Pulping equipment is used for juice extraction from fruits or vegetables and for pureed and pulped
meats. A combination of compression and shearing forces is used in each type of equipment. A rotary
grape crusher consists of a cylindrical metal screen fitted internally with high-speed rotating brushes
or paddles. Grapes are heated if necessary to soften the tissues, and pulp is forced through the
perforations of the screen by the brushes. The size of the perforations determines the fineness of the
pulp. Skins, stalks and seeds discarded from the end of the screen. Other types of pulper, including
roller presses and screw presses are used for juice expression.
A bowl chopper is used to chop meat and harder fruits and vegetables into a coarse pulp (for
example for sausage meat or mincemeat preserve). A horizontal, slowly rotating bowl moves the
ingredients beneath a set of high-speed rotating blades. Food may be passed several times beneath the
knives until required degree of size reduction and mixing has been achieved.
Size reduction of Liquid Foods (Emulsification and Homogenization)

The terms emulsifiers and homogenizers are often used interchangeably for equipment used to
produce emulsions. Emulsification is the formation of a stable emulsion by the intimate mixing of two
or more immiscible liquids, so that one (the dispersed phase) is dispersed in the form of very small
droplets within the second (the continuous phase). Homogenization is the reduction in size (to 0.5-
3μm) and increase in number of solid or liquid particles of the dispersed phase, by the application of
intense shearing forces, to increase the intimacy and stability of the two substances. Homogenization
is therefore a more severe operation than emulsification. Both operations are used to change the
functional properties or eating quality of foods. They have little or no effect on nutritional value or
shelf life.
The four main types of homogenizer are as follows:
1. High-speed mixers;
2. Pressure homogenizers;
3. Colloid mills;
4. Ultrasonic homogenizers.
1. High-speed mixers
Turbine or propeller-type high-speed mixers are used to pre-mix emulsions of low-viscosity
liquids. They operate by shearing action on the food at the edges and tips of the blades.
2. Pressure homogenizers
These consist of a high-pressure pump, operating at 10,000-70,000kPa, which is fitted with a
homogenizing valve on the discharge side. When liquid is pumped through the small adjustable gap
(300μm) between the valve and the valve seat, the high-pressure results in a high liquid velocity (8400
ms−1). There is then an almost instantaneous drop in velocity as the liquid emerges from the valve.
These extreme conditions of turbulence produce powerful shearing force. In some foods (for example
milk products) there may be inadequate distribution of the emulsifying agent over the newly formed
surfaces, which causes fat globules to clump together. Pressure homogenizers are widely used before
pasteurization and ultrahigh temperature sterilization of milk, and in the production of salad creams,
ice cream and some sauces.
3. Colloidal mills
These homogenizers are essentially disc mills. The small (0.05-1.3 mm) gap between a vertical
disc which rotates at 3000-15000 rev min−1 and a similar sized stationary disc creates high shearing
forces. Size reduction takes place between a stationary part (stator) and a rotating cone (rotor). The
premix is feed into the area between the rotor and stator by centrifugal force. With the high peripheral
speed, the rotor generates high shear fields within the fluid in the working area. They are more

effective than pressure homogenizers for high-viscosity liquids, but with intermediate viscosity liquids
they tend to produce larger droplet sizes than pressure homogenizers do. Numerous designs of disc,
including flat, corrugated and conical shapes, are available for different applications.
For highly viscous foods (for example peanut butter, meat or fish pastes) the discs may be
mounted horizontally (the paste mill). The greater friction created in viscous foods may require these
mills to be cooled by recirculating water.
4. Ultrasonic homogenizers
High-frequency sound waves (18-30 kHz) cause alternate cycles of compression and tension in
low-viscosity liquids and capitation of air bubbles, to form an emulsion with droplet sizes of 1-2μm.
In. operation, the dispersed phase of an emulsion is added to the continuous phase and both are
pumped through the homogenizers at pressures of 340-1400kPa. The ultrasonic energy is produced by
a metal blade, which vibrates at its resonant frequency. Vibration is produced either electrically or by
the liquid movement (Figure 6.8). The frequency is controlled by adjusting the clamping position of
the blade. This type of homogenizer is used for the production of salad creams, ice cream, synthetic
creams and essential oil emulsions. It is also used for dispersing powders in liquids
Size reduction of dry foods
There are a large number of mills available for application to specific types of food.
1. Ball mills
This type of mill consists of a slowly rotating, horizontal steel cylinder which is half filled with
steel balls 2.5-15cm in diameter. At low speeds or when small balls are used, shearing forces
predominate. With larger balls or at higher speeds, impact forces become more important. A
modification of the ball mill named the rod mill has rod instead of balls to overcome problems
associated with the balls sticking in adhesive foods.
2. Disc mills
A disc mill is a type of
crusher can be used to grind,
cut, shear, crack, rub, curl,
twist, hull, blend or refine. It
works in a similar manner to
the ancient Burhstone mill in
that the feedstock is fed
between opposing discs or
plates. The disc may be
grooved, or spiked. There are
a large number of designs of
disc mill. Each type employs
shearing forces for fine
grinding or shearing and impact forces for coarser grinding. For example,
1. single-disc mills in which food passes through an adjustable gap between a stationary
casing and a grooved disc which rotates at high speed,
2. double-disc mills in which two discs rotate in opposite directions to produce greater
shearing forces,

3. Hammer mills
A horizontal cylindrical chamber is lined with a toughened steel breaker plate. A high-speed
rotor inside the chamber is fitted with hammers along its length. In operation, food is disintegrated
mainly by impact as the hammers drive it against the breaker plate. In some designs the exit from the
mill is restricted by a screen and food remains in the mill until the particles are sufficiently small do
pass through the screen apertures. Under these ‘choke’ conditions; shearing forces play a larger part in
the size reduction. Two or more steel rollers revolve towards each other and pull particles of food
through the ‘nip’ (the space between the rollers) (Figure 6.6). The main force is compression but, if the
rollers are rotated at different speeds, or if the rollers are fluted (shallow ridges along the length of the
roller), there is an additional shearing force exerted on the food. The size of the nip is adjustable for
different foods and overload springs protect against accidental damage from metal or stones.
4. Roller mills
Two or more steel rollers revolve towards each other and pull particles of food through the ‘nip’ (the
space between the rollers). The main force is compression but, if the rollers are rotated at different
speeds, or if the rollers are fluted (shallow ridges along the length of the roller), there is an additional
shearing force exerted on the food. The size of the nip is adjustable for different foods and overload
springs protect against accidental damage from metal or stones.
Effect on the sensory characteristics
Size reduction is used in processing to control the textural or theological
properties of foods and to improve the efficiency of mixing and heat transfer,
the texture of many foods (for example bread, hamburgers and juices) is
controlled by the condition used during size reduction of the ingredients. There
is also art indirect effect on the aroma and flavour of some foods. The disruption of
cells and resulting increase in surface area promotes oxidative deterioration and
higher rates of microbiological and enzymic activity. Oxidation of carotenes
bleaches colours and flavour and reduce the nutritive value. There is a less of
volatile compounds form spices and some nuts. That may be due to the
expose of new surface or due to rise in temperature during milling.
Size reduction therefore has little or no preservative effect there may be
small change in sensory characteristics during size reduction.
In most of food the destruction of cell allows enzymes and
substrate to be come more thoroughly mixed which cause increase in
deterioration aroma and flavour. Additionally, the release of cellular material
provides a substance for the microbial growth and this can also result in the
development of off flavours.
The texture of food is greatly changed by sized reduction both by
physical reduction in the size of tissues and also by the release by the
hydrolytic enzyme.
The speed and duration of size reduction and gap between
completion of size reduction and after their processing are closely controlled
to achieved the desire texture
Nutritive value
The increase in surface area of foods during size reduction causes loss of nutritional value due
to oxidation of fatty acids and carotenes. Losses of vitamin C and thiamin in chopped or sliced fruits
and vegetables are substantial. Losses during storage depend on the temperature and moisture content
of the food and on the concentration of oxygen.
Factors related to nature of raw materials affecting size reduction

 Hardness- It is easier to break soft material than hard materials. Ex: For iodine hammer mill is used.
 Fibrous- These are tough in nature. A soft, tough material has more difficulty than a hard, brittle
substance. Ex: Ginger. Here cutters can be used.
 Friable-These tend to fracture along well-defined planes. Brittle substances can be easily converted into
fine particles. Ex: Sucrose. Mechanism used is attrition, impact and pressure.
 Elastic/ Sticky-Become soft during milling. Ex: synthetic gums, waxes, resins. Low melting substances
should be chilled before milling. These are milled using hammer, colloid or fluid energy mill.
 Melting point- Waxy substances, fats and oils are softened during size reduction due to heat generated.
This is avoided by cooling the mill and the substance.
 Hygroscopic- Certain substances absorb moisture content rapidly. This wet mass hampers the milling
process. Ex: Potassium carbonate. Closed system such as porcelain ball mill is used
 Thermoability- Certain Substances are degraded by hydrolysis and oxidation, due to moisture and
atmospheric oxygen. Heat produced on milling enhances these reactions. Closed system is used here
with an inert atmosphere of CO2 and N. Vitamins and antibiotics are milled using fluid energy and ball
mills.
Other Factors affecting size reduction
 Purity required- The size reduction of such hard substances leads to the abrasive wear of milling parts,
causing contamination. Such mills are to be avoided. The mills should be thoroughly cleansed between
different batches.
 Flammability- Under certain conditions fine dust such as dextrin, starch, sulphur are potential explosive
mixtures. All electrical switches should be explosive proof and mill should be well grounded
 Particle size- The feed should be of proper size and enter the equipment at a uniform rate to get a fine
powder. Several stages are carried out in size reduction process. Pretreatment of fibrous materials with
pressure rollers and cutters facilitates further Comminution.
 Moisture content- Presence of more than 5% moisture influences hardness, toughness, stickiness of
substance. In general, materials with moisture content below 5% are suitable for dry grinding and above
50 % for wet grinding.

Sieving: Separation based on size (mesh size); types of screens;
effectiveness of screens
SCREENS
The basic purpose of any screen is to separate a mixture of particles / items of different sizes
into two distinct fractions. These fractions are,
(1) the underflow, the particles that pass through the screen,
(2) the overflow or oversize, the materials that are retained over the screen.
A screen can be termed as ideal screen that separates the mixture in such a way that the largest
particle of underflow is just smaller than screen opening, while the smallest particle of overflow is just
larger than the screen opening. But in practice a given screen does not gives perfect separation as
stated above, and is called actual screen. The underflow may contain material coarser than screen size,
whereas the overflow may contain particles smaller than screen size. In most screens the grain/ seed
drops through the screen opening by gravity. Coarse grains drop quickly and easily through large
opening in a stationary surface. With finer particles, the screening surface must be agitated in some
way.
The common ways are
(1) revolving a cylindrical screen about a horizontal axis
(2) shaking, gyrating or vibrating the flat screens.
Screen showing how a feed is separated into two products, the oversize (overflow) and the undersize
(underflow or fines).
To get the maximum, minimum and other particle sizes, you would need to pass the material through a
series of screens. The amount retained in each screen, that is size fraction, is weighed and its
percentage calculated from the total mass of sample. This operation is called screen analysis.
Industrial screening can be done using structures made up of any of the following
 spaced metal bars
 perforated or slotted plates
 woven wire or fabric screens

When metal screening material is to be used, selection must be based on how compatible that metal is
with the material being screened and also on the strength of screen required For example, if you are
screening very heavy feed, you will need a strong screen. Commonly used metals include steel,
stainless steel, bronze, copper and nickel.
Screening can be done wet or dry. Screen structures include: metal bars, perforated or slotted
plates, woven wire or fabric. Metals used include steel, stainless steel, bronze, copper, nickel.
Selection of material is based on compatibility with materials being screen and strength required.
1. Woven screen sizes
Woven screens are commonly used in industry. To
describe the size of woven screen material, two
terminologies used:
Aperture: This is the minimum clear space in mm or
mm between the edges of the opening.
Mesh: This is the number of apertures per linear inch,
i.e. number of apertures in 25.4mm along the wire. If
you count the number of openings from one wire along the inch perpendicular to the wire, the number
you get is the mesh of that screen. Screen analysis data is given in either mesh or aperture sizes.
2. Perforated Metal Screens
 Round openings: The round openings in a perforated sheet metal screen are
measured by the diameter (mm or in.) of the openings. For example, 1/18
screen has round perforation of 1/18 in. in diameter or 2 mm.
 Oblong openings: The oblong or slotted openings in a perforated sheet metal
screen are designated by two dimensions; the width and length of the opening.
While mentioning oblong openings the dimension of width is listed first then
the length as 1.8 x 20 mm. Generally, the direction of the oblong opening is
kept in the direction of the grain flow over the screen.
 Triangular openings: There are two different systems used to measure
triangular perforations. The most commonly used system is to mention
the length of each side of the triangle in mm, it means, 9 mm triangle
has 3 equal sides each 9 mm long. The second system is to mention
openings according to the diameter in mm that can be inscribed inside
the triangle. This system is identified by the letter Vas 9V, l0V etc.
3. Wire mesh Screens
 Square mesh: The square openings in wire mesh are measured by
the number of openings per inch in each direction. A 9×9 screen has 9
openings per inch.
 Rectangular mesh: the rectangular openings in wire mesh screens
are measured in the same way as square wire mesh screen. A 3×6
rectangular wire mesh screen will have 3 openings per inch in one
direction and 6 openings per inch in the other direction. The rectangles
formed by the wire mesh are parallel to the direction of grain flow.

Type of screen equipment
1. Grizzly
The grizzly is a simple device consisting of a grid made up of metal bars, usually built on a
slope, across which the material is passed. The path of material flow is parallel to the length of bars.
The bars are usually so shaped that the top is wider than the bottom. The grizzly is often constructed in
the form of a short endless belt so that the oversize is dumped over the end while the sized material
passes through. In this case bar length is transverse to the path of materials. The grizzly is used for
coarsest and rough separations.
2. Revolving Screen/Cylinder Sorter
Revolving screen is a cylinder that rotates about its
longitudinal axis. The wall of the cylinder is made of
perforated steel plate or sometime the cloth wire on a frame,
through which the material falls as the screen rotates. The
axis of cylinder is inclined along with the feed end to the
discharge end. Sizing is achieved by having smallest opening
screen at the feed end with progressively larger opening
screens towards the discharge end. This type of sorter is
simple and compact with no vibration problem. But the
capacity of cylinder sorter is lesser than the vibrating screen
of same size.
The speed of operation and the inclination of cylinder can change the capacity, bed depth and
efficiency of these screens. Effective screening area (not the total surface of cylinder) is calculated by
multiplying the length of cylinder by 1/3 of the diameter.
3. Shaking Screen
Like the vibrating screen, shaker is a rectangular surface over
which material moves down on an inclined plane. Motion of the screen
is back and forth in a straight line. Although in some cases vibration is
also given to the screen. Unlike the vibrating screen, the shaker does not
tumble or turn material enroute except that some shaking screens have a
step-off between surfaces having different size openings, so that there
may be two or three tumbles over the full length of the screen. The
shaker is widely used as combined screen and conveyor for many types
of bulk material.
4. Rotary Screen
Rotary and gyratory screens are either circular or rectangular decked. Their motion is almost
circular and affects sifting action. These are capable of accurate and complete separation of very fine
sizes but their capacity is limited.
These screens are further classified into two categories.

a. Gyratory Screens
Gyratory equipment, used in mechanical screening
and sieving is based on a circular motion of the machine.
Unlike other methods, gyratory screen operates in a gentler
manner and is more suited to handle fragile products, enabling
it to produce finer products. A distinct difference to other
techniques is that the gyratory motion applied here depends on
eccentric weights instead of vibrations, which can be varied
based on individual process requirement.
Industry Applications
Process Processing ceramics, pulp and paper mill, paints, sand, starch slurry
Food
Screening of refined table salt, papaya cubes, turmeric pigment; clarification of alkaline
extracts
Chemical
Screening hydrate lime, effluent overflow from hydrocyclone; classification of polyester
beads, anhydrous aluminium chloride
b. Circular Screens
These are also rotary screens but their motion in horizontal plane is circular over the entire
surface. Similar to the gyratory screens, the screening
surface of circular screens is also little bit tilted for
allowing the material to move over them.
5. Vibratory Screen
The vibratory screens are agitated/ rapidly
vibrated by an eccentric unit and keep the material
moving and prevent binding as for us possible. Vibrating
screens are commonly used in industry where large
capacity and high efficiency are desired. When materials
to be separated are put on a vibratory screen, because of
its vibration, materials are also agitated and separated
during their transit over the screen. These screens are
classified as mechanically vibrated screens and
electrically vibrated screens. The vibrations can be produced either mechanically or electrically with
frequency of 1800 to 3600 or even more per minute. The eccentricity is usually of two types
(1) a shaft to which off centre weights are attached, and
(2) a shaft that itself is eccentric or off centered.
In the later case the eccentricity is balanced by a fly wheel for providing uniform vibration. Most
vibrating screens are inclined downward from the feed end. Vibration is provided to the screen
assembly only, and the body and other surrounding structure are isolated from vibration. Generally,
upto three decks are used in vibrating screens. The capacity of vibrating screen is higher than any
other similar sized screen and is very popular for cleaning and grading of granular agricultural
products.
6. Horizontal Screen
Horizontal screens are special case of vibrating screen. These are designed for operation with low
head room. They operate absolutely flat without the aid of gravity. All sorting, stratification and
material transportation' take place on the strength of a sharp forward thrust which imparts motion to
particles with a missile like trajectory, while the return stroke pulls the deck out from underneath the

bed. Effectiveness of these screens is higher because material is kept on the screen for a longer period
in comparison to inclined screens.
7. Other Screens
Various other types of screens used for cleaning and separation are listed below:
1. Rotex screens
2. Hummer screens
3. Circular vibrators
4. Symon's rod deck screens
5. Resonant vibrant screens
6. Centrifugal screens
Screening effectiveness- Screen effectiveness is the measure of success in closely separating
overflows A from underflow B. Screen capacity is the mass of feed per unit time per unit surface area.
e.g. tons hr-1
.m-2
. Capacity and effectiveness are opposing factors which need reasonable balance.
When capacity is increased, screen effectiveness drops. Particles which can pass through the screen
are hindered from doing so as a result of high capacity. The overall chance for particle to pass through
screen is a function of the number of times particle hits the screen and the probability of passage
during a single hit. A particle has the greatest chance of passing through the screen if.

Blinding of screen: It refers to the phenomenon wherein elongated, sticky, etc. particles become
wedged into the openings during screening and thus prevent the other particles from passing
through it. Thus, binding of screen is plugging of screen with solid particles. Binding reduces both
screen capacity and effectiveness.
To solve this problem and therefore increase screening effectiveness, industrial screen are operated
in either of the following modes:
1. Shaking: This a vertical up-down motion of the screen
2. Vibration: This is a sideways motion on a horizontal plane
3. Gyration: This is combined horizontal and vertical motion around an Axis.
4. Brushing: A brush is used to sweep through the screen surface remove blocking particles from
screen surface
Factors affecting efficiency
The probability of passage of a particle through a given screen mainly depends on
 the fraction of the total surface represented by openings
 the ratio of the diameter of the particle to the width of an opening in the screen,
 the number of contacts between the particle and the screen surface.
 nature and the shape of the particles,
 frequency and the amplitude of the shaking,
 methods used to prevent sticking or bridging of particles in the apertures of the sieve
 tension and physical nature of the sieve material.

Mixing: - Mixing (or blending) is a unit operation in which a uniform mixture is obtained from
two or more components, by dispersing one within the other(s) or
It is a combination of two or more components to form a single uniform mixture.
Mixing is an important, oven fundamental, operation in nearly all food and chemical processes. In the
mixing of solid particles, three mechanisms may be involved:
Mixing heavy pastes, plastic solids and rubber is more of an art than a science. It is not
possible to achieve a completely uniform mixture of dry powders or particulate solids. The degree of
mixing that is achieved depends on:
 the relative particle size, shape and density of each component
 the moisture content, surface characteristics and flow characteristics of each component
 the tendency of the materials to aggregate
 the efficiency of a particular mixer for those components.
In general, materials that are similar in size, shape and density are able to form a more uniform
mixture than are dissimilar materials. During a mixing operation, differences in these properties also
cause unmixing (or separation) of the component parts. In some mixtures, uniformity is achieved after
a given period and then unmixing begins. It is therefore important in such cases to time the mixing
operation accurately. The uniformity of the final product depends on the equilibrium achieved between
the mechanisms of mixing and unmixing, which in turn is related to the type of mixer, the operating
conditions and the component foods
Mixing equipments:-
Many forms or mixers have been produced from time to time. The easiest way in which to classify
mixers is to divide them according to whether they mix liquids, dry powders, or thick pastes.
 Mixers for dry powders: - During mixing of dry powders do not change their properties, so
lighten machine are required for mixing of dry powders. Example-Ribbon blender, internal
screw mixer, tumbling mixer, Impact Wheel
 Mixers for Liquid material: - Formixingofliquids,the propeller mixer is most common
satisfactory machine. In using propeller mixers, it is Important to avoid regular flow patterns
such as swirl round a cylindrical tank which may accomplish very little mixing. To break up
these streamline patterns, baffles are often fitted or the propellermayfittedasymmetrically.
 Mixers for high-viscosity liquids and pastes: - Dough and pastes are mixed in machines which
have, to be heavy and powerful. Because of the large power requirements. The most commonly
used mixer for these very heavy materials is the kneader which employs two contra-rotating arms of
special shape, which fold and shear the material.
Mixers are classified into types that are suitable for:
1. dry powders or particulate solids
2. low- or medium-viscosity liquids
3. high-viscosity liquids and pastes
4. Dispersion of powders in liquids.
1. Equipment for Solids Mixing(dry powders or particulate solids)
A huge variety of devices for the mixing of solids is available.
 Ribbon Blender:- Ribbon mixers have two or more thin narrow metal blades formed on a
shaft into helices, one blade being
right-handed and the other left-
handed. As the shaft rotates sections
of the powder move in opposite
directions and so particles are
vigorously displaced relative to each

other which counter-rotate in a closed hemispherical trough. This type of mixer is used for dry
ingredients and small-particulate foods. The ribbon blender consists of a trough in which
rotates. It is usually operated in batch mode with mixer volumes up to about 15 m, but
continuous operation is possible with feed rates up to 10 t/h. Helical ribbon mixers can be used
for slightly cohesive solids, for very thin pastes or for the addition of liquids to solids.
Application- Finely divided solids Wet solid mass Sticky and plastic solids Also used for solid- Liquid and
solid- semisolid mixing.
Advantages-Headroom requirement is less Rapid break down of agglomerates Minimum dead spots
Disadvantages-It requires high power. Produces size reduction for materials. Not suitable for fragile
crystals
Tumbling mixers / Cone Mixers: - Tumbling mixers/blender is a powder/solid mixing equipment
that consists of a closed metallic vessel that rotates about an axis either manually or with the help of a

motor at an optimum speed. Powder particles, unlike fluids, must first be set in motion by some
external action to achieve a proper mix. Diffusion is the main mechanism of mixing in tumbling
mixer. The powdered materials to be blended are loaded into the blender container and the movement
of the powdered particles occurs by tilting the material beyond the angle of repose using gravity to
impel flow. Different shapes of the diffusion mixer result in the movement of the material in various
planes, which is necessary for rapid mixing.
CONSTRUCTION OF TUMBLING MILL: • Basic parts used are metallic vessels in which powder is mixed, an
electric motor for the rotation of the vessel, and also baffles which helps in the mixing of the powders. • A
tumbler consists of a metallic enclosed vessel rotated on its axis which causes the particles to mix of tumble
over each other onto the mixture surface. • The mixing is done through the vessel by the baffles which are
present in the metallic vessel. • To achieve the fast blending the ingredients are loaded top to bottom instead
of side to side. It is simple and very reliable. it is made up of carbon steel or stainless steel. • The mixed
powder is discharged through butterfly or swivel gate or pinch valve. • The mixing of the powders can be
achieved by slow rotation either manually or with the help of the electric motor.
The degree of mixing/blending achieved by using tumbling mixer in carrying out a mixing operation is
dependent on
1. The fill-up volume (should not be more than 50-60% of the total blender volume)
2. The residence time.
3. The rotation speed (increasing the speed above the optimum speed causes adhesion of the
powder on the walls of the mixer)
4. The charging method used in charging the powder.
5. Inclination angle of the mixer.
Different type of mixer includes the horizontal drum, double-cone, V-cone, Y-cone, and cube. The
above figures show double-cone mixer and Y-cone mixer.
Advantages of tumbling mixers: i. Suitable for mixing friable materials because they produce mild forces
causing gentle mixing. ii. A perfect method for charging the powder into the mixer by adding the
components together side by side. iii. Can handle large volumes. iv. Easy to clean, which allows for greater
production flexibility. v. Little wear on equipment. vi. Gentle mixing for delicate particles vii. High quality
control is possible Limitations i. It cannot handle highly cohesive mixtures. ii. This cannot be adapted to a
continuous blending process.

 Vertical Screw Mixers- In vertical screw mixers, a rotating vertical screw is located in a cylindrical
or cone shaped vessel. The screw may be
mounted centrally in the vessel or may rotate or
orbit around the central axis of the vessel near
the wall. Materials are lifted from the bottom to
the top of the hopper and are then exchanged
with materials on the way up. Such mixers are
schematically shown in Fig. 17.4. A vertical
screw blender (Fig. 17.4a) may be desired for
larger batches handled in a small space, while
the orbiting screw mixer (Fig. 17.4b) is used for
difficult mixes. The latter arrangement is more
effective and stagnant layers near the wall are
eliminated. Vertical screw mixers are quick,
efficient, and particularly useful for mixing small
quantities of additives into large masses of
material. Specialized atmospheres as well as
normal temperatures and pressures are accessible
for multipurpose operations.
APPLICATION OF INTERNAL SCREW MIXERS :
Mixing and homogenization of powders, pastes and slurries • Granulation or agglomeration of powders • Addition or injection of liquids
into dry powders • Reaction under vacuum or pressure conditions • Processing of powders under inert conditions • Heating and cooling
of powders • Storage of non free flowing powders • De aeration or densification of powders • Homogenization of particle size and color
 Impact Wheel- Fine, light powders such as insecticides may be blended continuously by
spreading them out in a thin layer under centrifugal action. A premix of the several dry
ingredients is fed continuously near the center of a high-speed spiraling disk 10 to 27 inches in
diameter, which throws it outward into a stationary casing. The intense shearing forces acting
on the powders during their travel over the disk surface thoroughly blend the various materials.
The attrition mill is an effective mixer of this type.
2. Equipment for liquids Mixing
For the deliberate mixing of liquids, the propeller mixer is the most common and the most satisfactory.
The mixing of liquids is achieved in an agitated tank. A
large number of designs of agitator are used to mix
liquids in unbaffled or baffled vessels. A large number
of different types of impellers are in use; different
impellers impart different flow patterns to the liquid and
they must be matched to the rheology of the liquid and
to the desired shear rate. Mixing vessels usually have
rounded bottoms, rather than flat ones, to prevent the
formation of dead spaces.

When an impeller rotates in a liquid the liquid is likely to swirl in a mass and a vortex will
form. This is undesirable; because the possibility either of unwanted dissolution of air and improper
mixing. Consequently baffles are fitted to the tank. Normally four baffles are used. Baffles minimize
vortex formation, prevent swirling of the liquid, and result in more rapid mixing. Impellers which have
short blades (less than a quarter of the diameter of the vessel) are known as propeller agitators.
Propeller agitators operate at 400–1500 rev
min_1 and are used for blending miscible liquids,
diluting concentrated solutions, preparing syrups or
brines and dissolving other ingredients.
Agitator Types:-Agitators come in many sizes and
shapes. There are two types of agitators –
mechanical and electronically controlled. In the first
article, we will cover mechanical process agitators.
The basic types of mechanical agitators are:
 Paddle Agitators: - Paddle agitators are used where a uniform laminar flow of liquids is
desired.
 Anchor Agitators: - It is mainly used in reactors.
 Radial Propeller Agitators:-Radial agitators consist of propellers that are similar to marine
propellers. Ideal for applications where shear is the primary requirement, or where agitation
close to the bottom of the tank is desired.
 Propeller Agitators:-A propeller agitator is shaped with blades tapering towards the shaft to
minimize centrifugal force and produce maximum axial flow. Propeller agitators are popular
for simple mixing jobs.
 Turbine Agitators: - Turbine agitators can create a turbulent movement of the fluids due to the
combination of centrifugal and rotational motion. Ideal for low-viscosity, high-speed direct
drive mixers.

 Helical Agitators:-These agitators have blades with a twisted mechanism, just like the threads
of a screw. Helical agitators are most useful for mixing viscous liquids.
Choosing an agitator depends upon the specific gravity and viscosity of the products to be mixed.
Agitators need to be designed, engineered and manufactured to suit individual applications. Core
knowledge of fluid mechanics is essential for choosing the right type of agitators.
3. Equipment for high-viscosity liquids and pastes mixing
More viscous liquids, dough and pastes are mixed using different type of mixers like kneaders.
Kneading is a method of mixing used for deformable or plastic solids. There are different types of
twin-shaft horizontal blade mixers are available. According to consistency or viscosity of liquid the
design of blades can be changed like Z-blade (or sigma-blade) mixer. This consists of two heavy-duty
blades which are mounted horizontally in a metal trough. The blades intermesh and rotate towards
each other at either similar or different speeds (14–60 rev min_1) to produce shearing forces between
the two blades and between the blades and the specially designed trough base. Mixing efficiency
should therefore be high to reduce the mixing time. If necessary the walls of the trough are jacketed
for temperature control.
Sigma blade mixing Principle – shear. Inter meshing of sigma blades creates high shear and kneading
action.
Construction and working: • It consists of double tough shaped stationary bowl. • Two sigma shaped
blades are fitted horizontally in each tough of the bowl. • These blades are connected to a fixed speed
drive. • Mixer is loaded from top and unloaded by tilting the entire bowl. • The blades move at
different speeds , one about twice than the other, which allows movement of powder from sides to
centers. • The material also moves top to downwards and gets sheared between the blades and the wall
of the tough resulting cascading action. • Perforated blades can be used to break lumps and aggregates
which creates high shear forces. • The final stage of mix represents an equilibrium state.
Uses of sigma blade mixer: • Used in the wet granulation process in the manufacture of tablets, pill
masses and ointments, • It is primarily used for liquid – solid mixing, although it can be used for solid
– solid mixing. Bakery Industry

Advantages of sigma blade mixer: • Sigma blade mixer creates a minimum dead space during mixing.
• It has close tolerances between the blades and the sidewalls as well as bottom of the mixer shell.
Disadvantages of sigma blade mixer: • Sigma blade mixer works at a fixed speed
PLANETARY ROTARY MOTION MIXER This mixer designed specially for semi-solids, pastes,
ointments, viscous material, pill mass & tablet granulation masses. Principle;- it works on the principle
of shearing & convective in action. Application Single planetary mixer is commonly use for light,
medium viscosity products in the pharmaceutical, cosmetic & food industry. Double planetary mixer
is used in chemicals, rubber & other allied industries. Low speeds are used for dry blending & faster
speeds for the kneading action required in wet granulation. Steam jacketed bowls are used in the
manufacture of sustained release products & ointments. Jacketed construction available for heating &
cooling applications. Available in working capacities of 5 liters to 500 liters .
Advantages speed of rotation can be varied so it is advantageous over sigma blade or ribbon type
blender. Disadvantages It requires high power. It has limited size & is useful for batch work only.

Pug Mill/Paddle Mixer: - Pug mills are machines for mixing materials, usually one of them dry and
the other a liquid. A pug mill or pug mill is a machine in which
clay or other materials are mixed into a plastic state. Industrial
applications are found in bricks, cement and some parts of the
concrete mixing processes. A pugmill may be a fast continuous
mixer. A continuous pugmill can achieve a thoroughly mixed,
homogeneous mixture in a few seconds. A typical pugmill
consists of a horizontal boxlike chamber with a top inlet and a
bottom discharge at the other end, 2 shafts with opposing
paddles, and a drive assembly. Some of the factors affecting
mixing and residence time are the number and the size of the
paddles, paddle swing arc, overlap of left and right swing arc,
size of mixing chamber, length of pugmill floor, and material
being mixed. The paddle tips are adjustable and fairly easily
replaced. The paddle areas are adjusted to ensure there are no
“dead areas” in the pugmill. A “dead area” is a location where
aggregates can accumulate out of reach of the paddles and not
be thoroughly mixed. Dead areas can be avoided by making
sure the clearance between the paddle tips and the liner is less
than one half of the maximum aggregate size. Non-uniform mixing can occur if the pugmill is
overfilled.
Muller Mixer: - it gives different mixing action from that of other machine. This different action is
given white heavy muller wheel. In this particular design the pan is stationery and central vertical shaft
is driven causing the muller wheel roll. In other design of muller mixer the shaft is held stationary and
pan is rotated which cause the wheel to rotate.
Mixing Rolls:- another way of kneading the phase and deformable is to pass them between smooth
metal rod revolving at different speed. 3 to 5 horizontal set of roll one about the other in a vertical step
installed at different angles can complete the kneading thoroughly. The speed of rolls increases
starting from the ist step towards the next.
Agitation-The agitation of a liquid is defined as the establishment of a particular flow pattern within
the liquid, usually a circulatory motion within a container. Mixing is brought about by agitation.

Purposes of agitation Liquids are agitated for a number of purposes, depending on the objectives of
the processing step. These purposes include:
1. Suspending solid particles.
2. Blending miscible liquids, for example, methyl alcohol and water.
3. Dispersing a gas through the liquid in the form of small bubbles.
4. Dispersing a second liquid, immiscible with the first, to form an emulsion or a suspension of
fine drops.
5. Promoting heat transfer between the liquid and a coil or jacket
Kneading: (To mix and work into a uniform mass, as by folding, pressing, and stretching with the
hands or machine) Kneading is a method of mixing used for deformable or plastic solids. It involves
squashing the mass flat folding it over itself and squashing it again and again. This process will be
repeated many times to obtain a uniform product.

Kneading is a process in the making of bread or pasta dough, used to mix the ingredients and add
strength to the final product.
Homogenization
Basic Principle
Homogenization of dispersed systems involves reduction of the size of the dispersed particles.
Such size reduction is achieved by the action of shearing forces. Shear can be applied to the fluid by
mechanical agitation, by forcing the fluid to flow at very high velocity through a narrow passage by
shearing the fluid between two surface move one with respect to the other or by ultrasonic vibrations.
Homogenization application
Homogenization is applied very frequently in food processing, the best known application of
this operation being in the processing of fluid milk, with the objective of preventing the separation of
fat rich cream from the bulk under the effect of gravity.
Other application includes emulsification of salad dressing and sauces, fine mashing of
strained infant food stabilization of tomato concentrates. In the biotechnology high pressure
homogenization is used for cell rupture and release of intracellular material
Homogenizers
The shear forces required for homogenization can be generated in the different ways.
1. High shear mixers
2. colloid mills
3. High pressure homogenizers
4. ultrasonic homogenizers
5. Pressure homogenizers
In this type of homogenizer homogenization is achieved by
forcing the mixture to flow at high velocity through narrow gap. The
homogenizer consists of a high pressure pump and a homogenizing
head. The pump is usually positive displacement pump (piston pump).
The high-pressure in the range of 20 to 70 MPa is required for friction
in the homogenization head. The gap (300μm) between the valve and
the valve seat is different for different requirements. In some foods (for
example milk products) there may be inadequate distribution of the
emulsifying agent over the newly formed surfaces, which causes fat globules to clump together.
Pressure homogenizers are widely used before pasteurization and
ultrahigh temperature sterilization of milk, and in the production of
salad creams, ice cream and some sauces.
6. Colloidal mills
These homogenizers are essentially disc mills. In this type
of homogenizers size reduction is affected due to shearing when the
material is passed between the narrow gap of milling surfaces of
rotor and stator. The rotor rotates at a speed of 3000 to 20000 r.p.m.
the stator have a conical milling surface between which there is an
adjustable clearance between 0.002 to .03 inches. The material is
placed into the hopper of the mill. It is then passed through narrow
gap between the rotor and stator and thus reduced to fine particles.
They are more effective than pressure homogenizers for high-
viscosity liquids, but with intermediate viscosity liquids they tend
to produce larger droplet sizes than pressure homogenizers. For highly viscous foods (for example
peanut butter, meat or fish pastes) the discs may be mounted horizontally (the paste mill). The greater
friction created in viscous foods may require these mills to be cooled by recirculating water
7. Ultrasonic homogenizers

High-frequency sound waves (18-30
kHz) cause alternate cycles of
compression and tension in low-
viscosity liquids and capitation of air
bubbles, to form an emulsion with
droplet sizes of 1-2μm. In. operation,
the dispersed phase of an emulsion is
added to the continuous phase and
both are pumped through the
homogenizers at pressures of 340-1400kPa. The ultrasonic energy is produced by a metal blade, which
vibrates at its resonant frequency. Vibration is produced either electrically or by the liquid movement
(Figure 6.8). The frequency is controlled by adjusting the clamping position of the blade. This type of
homogenizer is used for the production of salad creams, ice cream, synthetic creams and essential oil
emulsions. It is also used for dispersing powders in liquids
Effect of mixing on foods
The action of a mixer has no direct effect on either the nutritional quality or the shelf life of a food but
may have an indirect effect by allowing components of the mixture to react together. The nature and
extent of the reaction depend on the components involved but may be accelerated if significant heat is
generated in the mixer. In general, mixing has a substantial effect on sensory qualities and functional
properties of foods. For example, gluten development is promoted during dough making by the
stretching and folding action which aligns, uncoils and extends protein molecules and develops the
strength of the gluten structure to produce the desired texture in the bread. The main effects are to
increase the uniformity of products by evenly distributing ingredients throughout the bulk.
Important Terms:-
 Baffels-Baffles are fitted to the tank which consists of vertical strips of metal running the full depth of
the inside surface of the tank.
Mixing Index and Rate
Assessing the extent of mixing is of great interest for both equipment manufacturers and food powder
processors. Mixing indices have been proposed to assess the extent of mixing. Mixing indices intend
to provide a measure of the performance of a piece of equipment

Filtration
Filtration is a separation technique that is used to separate a solid that has not dissolved in a
liquid (for example a precipitate).
Filtration is commonly the mechanical
or physical operation which is used for the
separation of solids from fluids (liquids or
gases) by interposing a medium through which
only the fluid can pass.
The fluid that passes through is called the
filtrate. Filtration is also used to describe
some biological processes, especially in water
treatment and sewage treatment in which
undesirable constituents are removed by absorption
into a biological film grown on or in the filter
medium as in slow sand filtration.
The liquid which has passed through the filter is called the filtrate
Filtration differs from sieving. In sieving, separation occurs at a single perforated layer (a
sieve) but in filtration, a multilayer lattice retains those particles. Oversize particles may form a cake
layer on top of the filter and may also block the filter lattice, preventing the fluid phase from crossing
the filter (blinding).
Filtration differs from adsorption, where it is not the physical size of particles that causes
separation but the effects of surface charge. Some adsorption devices containing activated charcoal
and ion exchange resin are commercially called filters, although filtration is not their principal
function.
Filtration differs from removal of magnetic contaminants from fluids with magnets (typically
lubrication oil, coolants and fuel oils), because there is no filter medium.
Applications
 Filtration is used to separate particles and fluid in a suspension, where the fluid can be a liquid, a
gas or a supercritical fluid.
 Filtration, as a physical operation is very important in chemistry for the separation of materials of
different chemical composition.
 Filtration is also important and widely used as one of the unit operations of chemical engineering.
 Commercial devices called "magnetic filters" are sold, but the name reflects their use, not their
mode of operation.
Type of filteration:-
It can be of three type
Surface Filtration:
In surface filtration the medium is used to support the
captured solids which deposit onto the medium (called
septum, membranes, cloth etc) during operation.
Removal of solids is effected by the previously
deposited solids or cake. As the cake builds so does the
resistance to flow. Depending on the force driving the
fluid through the medium the filtrate rate can decrease

and/or the pressure across the filter will increase until the filtering process is terminated, in this case
backwashing is required.
The cake resistance is a function of the porosity and thickness of the deposit which can change with
time. Filter aids, such as, diatomaceous earth
and porous silica particles, are often added as
a layer on the medium for additional support
(called precoat) to reduce compression of the
cake and impart high permeability. Example-
belt filter, rotary vacuum drum filter, cross
flow filter
Depth Filtration:
In depth filtration the suspended particles enter into the porous medium (called grains) and move to
the grain surface for attachment or reentrain into the fluid, repeating this procedure through the filter
column. In this case the medium provides the surface area for attachment and cake growth forms
around the grains. As the cake deposits and fills the open volume inside the column, the porosity, flow
rate, and pressure drop all change with time. A depth filter usually has three to five layers of filtration
media, each of different size and density. Light, coarse material lies at the top of filter bed. The media
become progressively finer and denser in the lower layer and remove small particles of suspended
solids, sand, silts and oxidized iron. (Example: sand filters)
Filter aid filtration
Filter aid filtration is mechanical, not chemical in nature. In this type of filtration irregularly
shaped particles are introduced, called filter aids. The filter aid forms a porous layer on the septum and
becomes the filtering medium that increases the porosity of the cake and reduces resistance of the cake
during filtration.
In filter aid filtration first, a thin protective layer of filter aid, called the precoat, is built up on the filter
septum. After pre-coating, small amounts of filter aid (body feed) are regularly added to the liquid to
be filtered. As filtering progresses, the filter aid, mixed in unfiltered liquid due to this a new filtering
surface is continuously formed.

“Filter Aids” is a group of inert materials that can be used in filtration pretreatment. There are
two objectives related to the addition of filter aids. One is to form a layer of second medium which
protects the basic medium of the system. This is commonly referred to as “precoat”. The second
objective of filter aids is to improve the flow rate by decreasing cake compressibility and increasing
cake permeability. The common filter aids are diatomaceous earth (DE), perlite, cellulose and others.
Diatomaceous earth (DE) is the skeleton of ancient diatoms. Diatomaceous earth and perlite are silica
based minerals. Cellulose can be used for filtration system that cannot tolerate silica.
Membrane filtration
The following processes are
grouped together under
membrane filtration:
 Microfiltration (MF)
 Ultrafiltration (UF)
 Nanofiltration (NF)
 Reverse osmosis (RO)
Membrane filters are
purely mechanical "fine
sieves". Membrane
filtration is a technique used to separate particles from a liquid in order to purify it.
In membrane filtration, a solvent is passed through a semi-permeable membrane. The
membrane's
permeability is determined by the size of the pores in the membrane, and acts as a barrier to particles
which are larger than the pores, while the rest of the solvent can pass freely through the membrane.
The result is a cleaned and filtered fluid on one side of the membrane, with the removed solute on the
other.
Membrane function:-The pore diameters define the membrane type and relate directly to the
separation rates: All substances smaller than the pores can pass through the membrane and larger
substances are held back.
In the context of drinking water treatment, this unselective process is disadvantageous, as both
undesired and desired minerals are partially or totally removed, in particular at nanofiltration and
reverse osmosis.

Types
A differentiation is made between the following, depending on the use and design:
 Hollow-fibre membrane modules
 Spiral modules
 Pipe modules
 Plate modules
 Cushion modules
There are three membrane filtration processes:
1) Microfiltration [MF] - MF is a low pressure [up to 100 psi (7 bar)] process for separating larger
size solutes from aqueous solutions by means of a semi-permeable membrane. This process is
carried out by having a process solution flow along a membrane surface under pressure. Retained
solutes (such as particulate matter) leave with the flowing process stream and do not accumulate
on the membrane surface. Pore ranges from 0.1 - 3 µm (micron meter).
Applications:
Clarification of dark juices for example, in the clarification of wine and dark juices, MF is to
separate the suspended solids from the juice to produce a low turbidity juice while allowing the
passage of color and flavor.
2) Ultra filtration [UF] - UF is a low pressure [up to 150 psi (10 bar)] process for separating solutes
from aqueous solutions by means of a semi-permeable membrane. UF provides an essentially
complete barrier against particles larger than the pore size, bacteria and the much smaller viruses
usually found in the feed water. UF operates by a surface removal mechanism resembling a fine
sieve with a highly uniform pore size. Any particles greater than the pore size are rejected. This
characteristic makes UF membranes ideal for meeting absolute filtration quality requirements.
In addition to high removal efficiency and an absolute removal rating, UF membranes tend to
be more compact, allow higher automation with unattended operation and have lower chemical
usage.
The pore size is approx. 0.02 µm (micron meter).
Applications:
Microbiological contaminants rejection
Reduction of Turbidity [colloids, proteins, large organic molecule]
Wastewater treatment
Treatment of whey in dairy industries
Concentration of biological macromolecules.
Production of ultra pure water for electronics industry
3) Nanofiltration [NF] – NF is a low to moderately high pressure [up to 450 psi (31 bar)] process.

Pore sizes range between UF and RO. Pores have not been observed in NF membranes under any
microscope, however, water can still pass through the membrane and multivalent salts and low
molecular weight organics are rejected. It is difficult to predict the performance of NF membranes
since membrane rejection is influenced by the size, structure and charge of the components in
solution. As a result, piloting is highly recommended for NF applications, even if a detailed feed
water analysis is available.
Applications:
Water softening
Overall reduction of TDS (Total Dissolved Solids)
Color and TOC (Total Organic Carbon).
Separation of organic from inorganic matter (in special food and wastewater applications)
Reverse osmosis
Osmosis is a phenomenon where pure water flows from a dilute solution through a semi
permeable membrane to a higher concentrated solution. Semi permeable means that the membrane
will allow small molecules and ions to pass through it but acts as a barrier to larger molecules or
dissolved substances. Applying an external pressure to reverse the natural flow of pure solvent, thus, is
reverse osmosis. The process is similar to other membrane technology applications.
Reverse osmosis means forcing contaminated water through a membrane (effectively, a very
fine filter) at pressure, so the water passes through but the contaminants remain behind.
Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane
to remove larger particles from drinking water. In reverse osmosis, an applied pressure is used to
overcome osmotic pressure.Reverse osmosis can remove many types of molecules and ions from
solutions, including bacteria, and is used in both industrial processes and the production of potable
water. The result is that the solute is retained on the pressurized side of the membrane and the pure
solvent is allowed to pass to the other side. To be "selective", this membrane should not allow large
molecules or ions through the pores (holes), but should allow smaller components of the solution (such
as the solvent) to pass freely. Reverse Osmosis is capable of removing up to 99%+ of the dissolved
salts (ions), particles, colloids, organics, bacteria and pyrogens from the feed water (although an RO
system should not be relied upon to remove 100% of bacteria and viruses). An RO membrane rejects
contaminants based on their size and charge.

However, key differences are found between
reverse osmosis and filtration. Reverse osmosis also
involves diffusion, making the process dependent on
pressure, flow rate, and other conditions. Reverse
osmosis is most commonly known for its use in
drinking water purification from seawater, removing
the salt and other effluent materials from the water
molecules.
Filtration Equipment
Depth Filters:-
A sand bed filter is a kind of depth filter. Broadly, there are two types of filter for separating
particulate solids from fluids:
 Surface filters, where particulates are captured on a permeable surface
 Depth filters, where particulates are captured within a porous body of material
Slow sand filters are used in water purification for treating raw water to produce a potable product.
They are typically 1 to 2 metres deep, can be rectangular or cylindrical in cross section and are used
primarily to treat surface water. The length and breadth of the tanks are determined by the flow rate
desired by the filters, which typically have a loading rate of 0.1 to 0.2 metres per hour (or cubic metres
per square metre per hour).
Slow sand filters work through the formation of a gelatinous layer (or biofilm) called the
hypogeal layer or Schmutzdecke in the top few millimetres of the fine sand layer. The Schmutzdecke is
formed in the first 10–20 days of operation and consists of bacteria, fungi, protozoa, and a range of
aquatic insect larvae. The surface biofilm is the layer that provides the effective purification in potable
water treatment, the underlying sand providing the support medium for this biological treatment layer.
As water passes through the hypogeal layer, particles of
foreign matter are trapped in the mucilaginous matrix and
soluble organic material is adsorbed. The contaminants are
metabolised by the bacteria, fungi and protozoa. The water
produced from an exemplary slow sand filter is of
excellent quality with 90-99% bacterial cell count
reduction.
Rapid sand filters use relatively coarse sand and other
granular media to remove particles and impurities that
have been trapped in a floc through the use of flocculation

chemicals—typically alum. The unfiltered water flows through the filter medium under gravity or
under pumped pressure and the floc material is trapped in the sand matrix.
Applications for sand filtration:
 Preperation of cooling water
 Treatment of waste water
 Production of drinking water
 Filtration in swimming pools
 Pre filtration for membrane systems
 Filtration of grey or surface water
 Removal of iron
Barrier Filters
 Plate and frame filter press In the plate and frame filter press, a cloth or mesh is spread out
over plates which support the cloth along ridges but at the same time leave a free area, as large
as possible, below the cloth for flow of the filtrate. The plates with their filter cloths may be
horizontal, but they are more usually hung vertically with a number of plates operated in
parallel to give sufficient area.
For filteration slurry is pumped into a corner hole and flows into each frame, allowing solid particles to
accumulate on the filter cloths. The remaining filtered liquid (also known as filtrate) then moves to a drainage port in
the flush plate and into a corner hole that is not being used for feeding the slurry. The filtrate then travels to
discharge piping and is directed to the next step in the process.
 After a period of time, the frames become filled with solids, the slurry feed pump turns off, and the filter press is ready
to open. Each frame should now contain a filter cake, which is the end result of the solids forming on the filter cloths.
The filter cakes are then scraped out of the frames using a spatula, ideally falling into a cake hopper placed below the
press.
Filter cake builds up on the upstream side of the cloth, that is the side away from the plate. In the early
stages of the filtration cycle, the pressure drop across the cloth is small and filtration proceeds at more
or less a constant rate. As the cake increases, the process becomes more and more a constant-pressure
one and this is the case throughout most of the cycle. When the available space between successive
frames is filled with cake, the press has to be dismantled and the cake scraped off and cleaned, after
which a further cycle can be initiated.

The plate and frame filter press is cheap but it is difficult to mechanize to any great extent. Variants of
the plate and frame press have been developed which allow easier discharging of the filter cake. For
example, the plates, which may be rectangular or circular, are supported on a central hollow shaft for
the filtrate and the whole assembly enclosed in a pressure tank containing the slurry. Filtration can be
done under pressure or vacuum.
 Rotary filters
In rotary filters, the flow passes through a
rotating cylindrical cloth from which the
filter cake can be continuously scraped.
Rotary vacuum filter drum consists of a
drum rotating in a tub of liquid to be filtered.
The technique is well suited to slurries, and
liquids with a high solid content, which
could clog other forms of filter. The drum is
pre-coated with a filter aid, typically of
diatomaceous earth (DE) or Perlite. After
pre-coat has been applied, the liquid to be
filtered is sent to the tub below the drum.
The drum rotates through the liquid and the
vacuum sucks liquid and solids onto the
drum pre-coat surface, the liquid portion is
"sucked" by the vacuum through the filter media to the internal portion of the drum, and the filtrate
pumped away. The solids adhere to the outside of the drum, which then passes a knife, cutting off the
solids and a small portion of the filter media to reveal a fresh media surface that will enter the liquid as
the drum rotates. The knife advances automatically as the surface is removed.
Rotary vacuum filters are expensive, but they do provide a considerable degree of mechanization and
convenience.
Advantages and limitations
The advantages and limitations of rotary vacuum drum filter compared to other separation methods
are:
Advantages
 The rotary vacuum drum filter is a continuous and automatic
operation, so the operating cost is low.
 The variation of the drum speed rotating can be used to
control the cake thickness.
 The process can be easily modified (pre-coating filter
process).
 Can produce relatively clean product by adding a showering
device.
Disadvantages
 Due to the structure, the pressure difference is limited up to 1
bar.
 Besides the drum, other accessories, for example, agitators
and vacuum pump, are required.
 The discharge cake contains residual moisture.
 High energy consumption by vacuum pump

Centrifugal filters
Water passes through a micron filter and the filter captures the dirt. Cartridge filters are designed to
run at a lower pressure than sand filters.
Cartridge Filters consists of a filter vessel fitted with one or more cartridges. These cartridge elements
are constructed of paper or polyester cloth, through which the water flows. The filter element holds
particles of dirt only allowing clean water to pass through. The
size of the particles
Air filters
Filters are used quite extensively to remove suspended dust or
particles from air streams. The air or gas moves through a fabric
and the dust is left behind. These filters are particularly useful for
the removal of fine particles. One type of bag filter consists of a
number of vertical cylindrical cloth bags 15-30 cm in diameter,
the air passing through the bags in parallel. Air bearing the dust
enters the bags, usually at the bottom and the air passes out
through the cloth. A familiar example of a bag filter for dust is to
be found in the domestic vacuum cleaner. Some designs of bag
filters provide for the mechanical removal of the accumulated
dust. For removal of particles less than 5 mm diameter in modern
air sterilization units, paper filters and packed tubular filters are
used. These cover the range of sizes of bacterial cells and spores.
Application of filteration in Food Processing
Contamination within a food processing operation can manifest itself in many ways, the most likely
source is from the utilities used in production such as water, air and steam. Filtration can remove these
unwanted contaminants and give added value to the food processing product.
 Water Filtration-provide clarity and microbiological stability for water used in rinsing and
washing applications.
 Pre-Filtration-used to protect and extend the life of the our final membrane filters.
 Gas Filtration- ensure the removal of spoilage organisms from CO2, N2 and air, and can also
be used for sterile venting and blanket transfer.
 Tanker Transfer- provide the assurance of quality during transfer from tank to tank, or tank to
process.
 Bottle Rinsing Filtration-The removal of spoilage organisms such as yeast and bacteria from
glass and PET bottles.
 Steam Filtration- filter can be safely used for sterilisation of process equipment and both
direct and indirect thermal processing of foodstuffs.
Important Terms
 (Diatomaceous Earth (also known as DE, diatomite) is the fossil remains of plankton that died
in the oceans millions of years ago and sank to the bottom to form deposits. Chemically it is
predominantly silica, one of the most abundant minerals on the upper crust of our planet,
earth!)
 Osmosis is a naturally occurring phenomenon and one of the most important processes in
nature. It is a process where a weaker saline solution will tend to migrate to a strong saline

solution. Examples of osmosis are when plant roots absorb water from the soil and our kidneys
absorb water from our blood.
 filter press-a device consisting of a series of cloth filters fixed to frames, used for the large-
scale filtration of liquid under pressure

DISTILLATION, process of heating a liquid until its more volatile constituents pass into the vapor
phase, and then cooling the vapor to recover such constituents in liquid form by condensation
The main purpose of distillation is to separate or to purification of a large variety of materials
mixture by taking advantage of their different volatilities or the separation of volatile materials from
nonvolatile materials.
Seawater, for example, which contains about 4 percent dissolved solids (principally common salt),
may be readily purified by vaporizing the water, condensing the steam thus formed, and collecting the
product, distilled water.
If the boiling points of the constituents of a mixture differ only slightly, complete separation
cannot be achieved in a single distillation.
An important example is the separation of water, which boils at 100° C (212° F), and alcohol, which
boils at 78.5° C (173° F). If a mixture of these two liquids is boiled, the vapor that rises is richer in
alcohol and poorer in water than the liquid from which it came, but it is not pure alcohol.
Distillation is a process of separating the component substances from a liquid mixture by
selective vaporization and condensation.
 Vapor pressure of a pure substance is the pressure exerted by the substance against the
external pressure which is usually atmospheric pressure. Vapor pressure is a measure of the
tendency of a condensed substance to escape the condensed phase. The larger the vapor
pressure, the greater the tendency to escape. When the vapor pressure of a liquid substance
reaches the external pressure, the substance is observed to boil.
 The normal boiling point of a substance is defined as the temperature at which the vapor
pressure of that substance equals atmospheric pressure, 760 mmHg. The normal boiling points
of dichloromethane, water, and d-limonene are, respectively, 40.2°C, 100°C, and 175°C. If the
pressure is less than 760 mmHg, the temperature at which a substance boils will be less than
the normal boiling point.
 Sublimation. If a solid substance is distilled, passing directly into the vapor phase and back
into the solid state without a liquid being formed at any time, the process is called sublimation.
 Relative volatility is a measure of the differences in volatility between 2 components, and
hence their boiling points. It indicates how easy or difficult a particular separation will be.
Application of Distillation in food industry
1. Distillation in flavor industry
 The recovery of volatile components from aromatic plants materials by distillation.

 Fractionalization of essential oils. The process of reclaimating the aroma of fruit products and
specifically fruit juice concentrates by fractional distillation.
 The purification of volatile aromatic chemicals from more or less volatile impurities.
 The recovery of solvents during the process of extraction
 The concentration of natural flavoring materials
2. distillations of whisky
3. In numerous research and analytical technique
4. Separation by membrane distillation on the base of relative volatility of various components in the feed
solution.
5. Distillation of water for labs
6. esters, and other alcohols, are collected as condensate
Relative volatility
Relative volatility is a measure comparing the vapor pressures of the components in a liquid
mixture of chemicals. This quantity is widely used in designing large
industrial distillation processes. In effect, it indicates the ease or difficulty of using distillation to
separate the more volatile components from the less volatile components in a mixture. By convention,
relative volatility is usually denoted as .
Relative volatilities are used in the design of all types of distillation processes as well as other
separation or absorption processes that involve the contacting of vapor and liquid phases in a series
of equilibrium stages.
Definition
For a liquid mixture of two components (called a binary mixture) at a given temperature and pressure,
the relative volatility is defined as
where:
= the relative volatility of the more volatile component to the less volatile component
= the vapor–liquid equilibrium concentration of component in the vapor phase
= the vapor–liquid equilibrium concentration of component in the liquid phase
= the vapor–liquid equilibrium concentration of component in the vapor phase
= the vapor–liquid equilibrium concentration of component in the liquid phase
= commonly called the K value or vapor-liquid distribution ratio of a component
is a unit less quantity. When the volatilities of both key components are equal, = 1 and
separation of the two by distillation would be impossible under the given conditions because the
compositions of the liquid and the vapor phase are the same. As the value of increases above 1,
separation by distillation becomes progressively easier.
 A liquid mixture containing two components is called a binary mixture. When a binary
mixture is distilled, complete separation of the two components is rarely achieved. A liquid
mixture containing many components is called a multi-component mixture. When a multi-
component mixture is distilled, the overhead fraction and the bottoms fraction typically
contain much more than one or two components.
Thus, for the distillation of any multi-component mixture, the relative volatility is often defined as

Large-scale industrial distillation is rarely undertaken if the relative volatility is less than 1.05.
The values of have been correlated empirically or theoretically in terms of temperature,
pressure and phase compositions in the form of equations, tables or graph. values are
widely used in the design of large-scale distillation columns for distilling multi-component
mixtures in oil refineries, petrochemical and chemical plants, natural gas processing plants and
other industries.
Distillation of Mixtures
There are two general types of mixtures to consider mixtures of miscible liquids and mixtures
of immiscible liquids. Their behavior on distillation is very different from one another.
 Miscible liquids which are soluble in each other in all ratios.
 Immiscible liquids do not dissolve in one another to any extent.
Water is immiscible with most organic substances and, for our purposes, will always be one of
the components in a mixture of immiscible liquids. Mixtures obey Dalton's law of partial pressures
which states that vapor pressure above a mixture is equal to the sum of the vapor pressures of the
individual components. For example, for a two component mixture: where PA and PB are the partial
pressures of components A and B respectively.
The difference in the behavior of the two types of mixtures on distillation arises from the differences
in partial pressures. Mixtures of miscible liquids. In a mixture of miscible substances, the partial
pressure of a component depends on the vapor pressure of the pure component and the relative amount
of the component in the mixture.
Raoult's law
If a liquid is placed in an empty, closed container, some molecules at the surface of the liquid
evaporate into the empty space above the liquid. Once vaporized, some of the molecules in the vapor
condense into the liquid in a competing process. As the space above the liquid becomes occupied with
molecules of vaporized liquid, the pressure of the vapor above the liquid rises until it reaches a certain
value. When the pressure stabilizes, the rates of evaporation and condensation are equal. The
pressure of the vapor under these conditions is called the equilibrium vapor pressure.
Raoult's law: the partial pressure of a component in an ideal mixture of miscible
liquids is equal to the mole fraction of the component multiplied by the vapour pressure
of the pure component liquid at the same temp
Pa = xa/ (xa+xb) × pa
0
 pa: partial pressure of A in ideal mixture,
 xa/(xa+xb): mole fraction of A
 pa
0
: vapour pressure of pure liquid A
. Degree of separation in the simplest mixture of two mutually soluble liquids, the volatility of
each is undisturbed by the presence of the other produced by a single distillation would depend only
on the vapor pressure, or the volatility, of the separate components at this temperature. This simple
relationship was first stated by the French chemist François Marie Raoult (1830-1901) and is called
Raoult's la. Raoult's Law only works for ideal mixtures Raoult's law applies only to mixtures of
liquids that are very similar in chemical structure, such as benzene and toluene. In the distillation of 99
percent alcohol produces vapor that has less than 99 percent alcohol. For this reason, alcohol cannot be
concentrated by distillation beyond 97 percent, even by an infinite number of distillations.


In equation form for a mixture of liquids A and B is:
where 'P' is Partial pressure.
In this equation, PA and PB are the partial vapour pressures of the components A and B. In any
mixture of gases, each gas exerts its own pressure. This is called its partial pressure and is independent
of the other gases present. Even if you took all the other gases away, the remaining gas would still be
exerting its own partial pressure.
The total vapour pressure of the mixture is equal to the sum of the individual partial pressures.
The Po values are the vapour pressures of A and B if they were on their own as pure liquids.
xA and xB are the mole fractions of A and B. That is exactly what it says it is - the fraction of the total
number of moles present which is A or B.
You calculate mole fraction using, for example:
Putting all this together in a simple example:
For example, A mixture of 2 moles of methanol and 1 mole of ethanol at a particular temperature.
The vapour pressure of pure methanol at this temperature is 81 kPa, and the vapour pressure of
pure ethanol is 45 kPa.
There are 3 moles in the mixture in total. 2 of these are methanol. The mole fraction of
methanol is 2/3.
Similarly, the mole fraction of ethanol is 1/3. You can easily find the partial vapour pressures using
Raoult's Law - assuming that a mixture of methanol and ethanol is ideal.
First for methanol:
. . . and then for ethanol:
You get the total vapour pressure of the liquid mixture by adding these together.
It follows therefore that
 The "Mixture" boils when the sum of the vapour - pressures equals the atmospheric pressure
P*A + P* B = External pressure; (1 atmosphere)
 The temperature at which the "mixture" boils is lower than that of either of the components

Assume that 'A' has lower B. P. than 'B'. ‘A’ alone boils when it vapour pressure (P*A) equals
the atmospheric pressure. But in the case of the "mixture" we know that both liquid having
different B.P. So the boiling point of the "mixture" is lower than that of A (lower B.P. component).
 The boiling - point of the "mixture" remains constant until one of the liquids has been
completely removed from the still, after that boiling point will rise to that of the remaining
component.
 The composition of the vapour distilling over remains constant until one of the liquids has been
completely removed (till constant B. P. of mixture) from the still.
COMPARISON BETWEEN EVAPORATION AND DISTILLATION
EVAPORATION
(1) Evaporation is from the surface of the liquid.
(2) Evaporation is carried out below, B.P. or at room temperature.
(3) In evaporation the solvent is generally water which is not recovered.
(4) It is a slow process
(5) Evaporation being the operation when the concentrated liquid residue needed.
DISTILLATION
(1) Distillation is from the bulk of the liquid.
(2) Distillation is carried out at B. P.
(3) In distillation the solvent vapors are condensed and are, collected in a receiver.
(4) It is a. fast process.
(5) Distillation being the operation when the condensed vapour is required.
CONDENSERS
In the distillation process, the liquid is heated in a vessel or container is known .as STILL. The
vapours occur in still made to pass through an apparatus called CONDENSER which cools the
vapours. The Condensed or reformed liquid is called the DISTILLATE, and it is collected in. a
suitable vessel called the RECEIVER.
(I) IDEAL PROPERTIES OF CONDENSER:
(1) Easily Cleanable
(2) The cooling surface must be large
(3) Broken part must be replaceable.
(4) Condensing surface should be good conductor of heat, that is metal condensers are preferred than
glass condensers.
(5) The water used for cooling the surface must leave the condenser quickly
(6) The cooling water is arraiged to Move on the counter current principlet
(II) TYPES OF CONDENSERS: The principal classes of condensers are
(A) Liebig condenser
(B) Spiral condenser
(C) Double surface condenser
(D) Ball condenser
(E) Multitubular (LUCAS'S) condenser
(F) Bulb condenser

DISTILLATION PROCESSES
Different types of distillation processes are:
(I) Simple distillation
(II) Fractional distillation
(III) Steam distillation
(IV) Vacuum distillation (distillation under reduced pressure)
(V) Destructive distillation
(VI) Molecular distillation.
 SIMPLE DISTILLATION: (Distillation under atmospheric pressure):
It is most common method of separation in pharmaceutical practice. Simple distillation is used
where sharp separations are not required. It is the process to converting a liquid into its vapour,
transferring the vapour to another place and recovering the liquid by condensing the vapour.
The common features of simple distillation apparatus are:
 A vaporizing chamber, called still.
 Condenser - Heat Exchanger.
Heat exchanger medium is water, air or any other. Liquid after condensation - distillate is
collected in a vessel is called the receiver. For laboratory work (small scale) apparatus made up of
glass is used. The temperature is observed on a thermometer, inserted through a cork, keeping the bulb
below the level of side arm. Bumping, due to superheating is avoided by adding a small chip of
porcelain before distillation.

To separate a volatile constituent, such as alcohol or acetone from a non-volatile' extract, a
steam jacketed still may be used. It has limited heating surface and only used for volatile solvent, but it
is. useless for concentrating watery solutions.
Application of Simple distillation:
1. Simple distillation is used for the purification of organic liquids.
2. Separation of volatile constituents from non-volatile.
3. Recovery of alcohol in the preparation of dry extracts.
4. In the preparation of ether, amyl nitrate and spirit of nitrous ether.
5. In the preparation of distilled water.
 FRACTIONAL DISTILLATION (Rectification)
It is the process .used to separate miscible volatile, liquids having different boiling points. It
differs from simple distillation in that partial vapour is allowed to pass, through fractionating column
before reaching the condenser.
This column enables the contact between ascending vapour from the still with the condensing
vapour returning to the still. Due to this less volatile component (Higher B. P., in case water, a1cohol
mixture) converted from the vapour to the liquid phase, while more volatile moment (Alcohol, lower
B. P.) converted to vapour phase. So that vapour becoming richer in more volatile component
(Alcohol) and the liquid richer in low volatile component (water)
It consists of:
1. A still
2. Rectifying column
3. Condenser.
4. Reflux divider
FRACTIONATING COLUMNS:
A fractionating column is a device which increases the process of fractional distillation by
condensing most of vapours of less volatile component of a mixture and return into still, whereas the
vapours of more volatile components of the liquid are allowed to pass to the condenser.
TYPES OF COLUMN: It may be divided into following two groups. :
 Packed columns
Which contain packing material such as
1. Raschig rings
2. Lessing rings
 Plate columns
The common type of column is:
1. Bubble cap plate
2. Turbo grid plate
3. Sieve - plate.

Applications (Uses):
1. Preparation of 95% Alcohol from fermented liquid which contain 10-14% of alcohol by
coffey's still.
2. It is used in the separation of two miscible liquids.
The most common type of fractional distillation (large Scale)
 STEAM DISTILLATION
Principle:
It is used for the distillation of water - immiscible liquids of high boiling points e.g. turpentine,
aniline; benzene by bubbling steam through the liquid (Immiscible), at the boiling point of water and
condensation of the mixed (both) vapour produced, which is separated by separating funnel.
When two miscible (Fractional distillation) liquids are mixed, each may be considered as a
solution of the one in the other and vapour-pressure of each is lower than that of the pure one. The
vapour pressure exerted by each component in a mixture is termed as partial pressure (PA, PB)

Steam distillation apparatus consist of steam generator fitted with two holed rubber stopper.
Through one passes a bent tube leading the steam to the flask containing non-aqueous liquid, it must
reach to bottom of the flask. There is another tube passing through the other hole and reaching to the
bottom of the steam generator. This tube act as safety tube so excess pressure relieve by safety tube.
The non-aqueous liquid is placed in the flask. Bent tube carries the steam from steam generator and
heating the non-aqueous liquid. Mixture is also heated by slow burner. The vapours of the mixture are
allowed to pass through the condenser and condensed liquids are collected in receiver. Two layers are
separated by separating funnel.
Advantages:
 Steam distillation is used for thermolabile material as higher B.P. component (organic liquid,
vol. oils) boil below 100° C
Disadvantages:
Product is mixture of water and non-aqueous liquid cannot separate completely.
Application (Uses):
1. For the preparation of volatile oils.
— Clove oil, Anise oil, Eucalyptus oil
2. For the preparation of distilled aromatic waters
— Rose water
— Distilled peppermint water
— Distilled cinnamon water
—Distilled Dill-water
3. For purifying organic compounds that do not react and are immiscible with water. e.g. -
Essential oils (Turpentine Oil)
4. Purification of glycerin and fatty acids.
 VACUUM DISTILLATION (DISTILLATION UNDER REDUCED PRESSURE)
PRINCIPLE: A liquid boils when its vapour-pressure is equal to the Hydrostatic pressure. If
the external pressure on water is reduced to 70 m.m.. Water boils at 40° C. The boiling-point of a
liquid may then be lowered to a desired temperature by reducing the pressure on its surface; Boiling
under reduced pressure will also increase the rate of distillation.

LABORATORY APPARATUS:
It consists of claisen flask having two necks. Through one a thermometer is inserted and to a
side tube a condenser is attached. Through other fine capillary is introduced dipping in the boiling
liquid. In vacuum distillation bumping and foaming occurs, which can be prevented by introducing a
stream of air into the liquid through the capillary. Condenser is attached with receiver flask. Receiver
flasks have side-tube which is connected to a vacuum pump through manometer. Heating of the flask
should not be started until the required vacuum has been attained. Heating the flask should be done on
water-bath or oil-path.
LARGE SCALE APPARATUS: VACUUM STILL:
These are used for distilling substances that have a high boiling point at atmospheric pressure
or for substances that are damaged by a high temperature or for removing last traces of a volatile
solvent. Vacuum stills consist of steam-Jacketed still with a sight glass.
Advantages:
Liquid boils at much lower temperature, so thermo-labile material can be distilled - off.
Applications:
1. Purification of vitamins.
2. To prevent OR Minimize chemical change.
(a) To prevent destruction of enzymes
e.g. Extract of malt
Pancreatin
Pepsin
(b) To prevent hydrolysis of glycosides and Alkaloids
SEDIMENTATION
Sedimentation, or clarification, is the processes of letting suspended material settle by gravity.
Suspended material may be particles, such as clay or silts, originally present in the source water.
Suspended material or floc is typically created from materials in the water and chemicals used in
coagulation or, in other treatment processes, such as lime softening.
Sedimentation is accomplished by decreasing the velocity of the water, when the velocity not supports
the particles, gravitational force act on the particles and remove them from the water flow
Some basic definitions will aid in understanding the basic concept and aim of sedimentation.

 Sedimentation, also known as settling, may be defined as the removal of solid particles from a
suspension by settling under gravity.
 Clarification is a similar term, which usually refers specifically to the function of a
sedimentation tank in removing suspended matter from the water to give a clarified effluent.
Classification implies the sorting of particulate material into size ranges. Particles of different sizes and
densities suspended in a fluid differentially affected by imposed forces such as gravity and centrifugal
fields. Due to this particles settle in different layer and by arrangement of channels they can be
collected separately.
 Thickening in sedimentation tanks is the process whereby the settled impurities are concentrated and
compacted on the floor of the tank and in the sludge-collecting hoppers.
 Concentrated impurities withdrawn from the bottom of sedimentation tanks are called sludge.
 The material that floats to the top of the tank is called scum.
Application of sedimentation processes
1. Water treatment, sedimentation is commonly used to remove impurities that have been rendered settle-
able by coagulation and flocculation, as when removing turbidity and color. Precipitates formed in
processes such as water softening by chemical precipitation are also removed by sedimentation.
2. Municipal wastewater treatment, sedimentation is the main process in primary treatment, where it is
responsible for removing 50 to 70% of the suspended solids (containing 25-40 per cent of the BOD)
from the wastewater.
Classification of settling behavior
Several cases of settling behavior may be distinguished on the basis of the nature of the particles to be
removed and their concentration. Common classifications of settling behaviour are:
 Type 1 - Dilutes, non-flocculent, free-settling. (Every particle settles independently.)
 Type 2 - Dilute, flocculent. (Particles can flocculate as they settle.)
 Type 3 - Concentrated Suspensions, Hindered settling and zone Settling (Sludge Thickening).
 Type 4 - Compression settling (compaction)
 Type 1 sedimentation is the type in which particles settle as individual particles and do not
flocculate or stick to other during settling at constant settling velocity. Example: sand and grit
material.
 Type 2 sedimentation is the type in which particles that flocculate during sedimentation and
because of this their size is constantly changing and therefore their settling velocity is
changing. Example: alum or iron coagulation
 Type 3 sedimentation is also known as zone sedimentation. As the concentration of particles
in a suspension is increased, a point is reached where particles are so close together that they
no longer settle independently of one another. This results in a reduced particle-settling
velocity and the effect is known as hindered settling. The most commonly encountered form
of hindered settling occurs in the extreme case where particle concentration is so high (greater
than 1000 mg/L) that the whole suspension tends to settle as a ‘blanket’. This is termed zone
settling.
 Type 4 sedimentation Very high particle concentrations, where particles settle to the floor of
the sedimentation tanks and particles are actually in contact. Now further settling can occur
only by adjustments within the matrix, created by particles, so it takes place at a reducing rate.
This is known as compression settling zone

s
FACTORS AFFECTING SEDIMENTATION
Several factors affect the separation of settleable solids from water. Some of the more common types of factors
to consider are:
PARTICLE SIZE AND SHAPE
The size and type of particles to be removed have a significant effect on the operation of the
sedimentation tank. Because of their density, sand or silt can be removed very easily. Colloidal material, small
particles that stay in suspension and make the water seem cloudy, will not settle until the material is coagulated
and flocculated by the addition of a chemical, such as an iron salt or aluminum sulfate. The shape of the particle
also affects its settling characteristics. A round particle, for example, will settle much more readily than a
particle that has ragged or irregular edges.
WATER TEMPERATURE
Another factor to consider in the operation of a sedimentation basin is the temperature of the water
being treated. When the temperature decreases, the rate of settling becomes slower. In most cases temperature
does not have a significant effect on treatment. A water treatment plant has the highest flow demand in the
summer when the temperatures as compare to the water is colder
CURRENTS
Several types of water currents may occur in the sedimentation basin:
• Density currents caused by the weight of the solids in the tank, the concentration of solids and temperature of
the water in the tank.
• Eddy currents produced by the flow of the water coming into the tank and leaving the tank.
Some of the water current problems can be reduced by the proper design of the tank. Installation of baffles
helps prevent currents from short circuiting the tank.
OTHER PROBLEMS
 Gases in the water may cause floating scum, which can carry over into the filters.
 Another sedimentation basin problem is algal growth. If sedimentation basins have sufficient
sunlight, algae will grow on the walls of the basin. These algae can break loose and clog the
filter. Algae are best treated with shock chlorination, a method of feeding 5-10 ppm of
chlorine into the raw water.
SEDIMENTATION BASIN ZONES

Sedimentation basins have 4 zones
1. The Inlet zone,
2. The Settling zone,
3. The Sludge zone, and
4. The Outlet zone.
Each zone should provide a smooth transition between the zone before and the zone after.
Zones in Rectangular Sedimentation Basin
Each and every zone has its own unique purpose. All zones are in a rectangular sedimentation basin.
Zones in a Circular Sedimentation Basin
In a square or circular basin (clarifier), water typically enters the basin from the center rather than
from one end and flows out to outlets located around the edges of the basin. But the four zones can
still be found within the clarifier the above figure.
Inlet Zone
The two primary purposes of the inlet zone of a sedimentation basin are to distribute the water and to
control the water’s velocity as it enters the basin. In addition, inlet devices act to prevent turbulence of
the water. The incoming flow in a sedimentation basin must be evenly distributed across the width of
the basin
Inlet arrangement for a rectangular basin

The inlet of rectangular basin water leaves the inlet and enters the settling zone of the sedimentation
basin by flowing through the holes evenly spaced across the stilling wall.
The second type of inlet allows water to enter the basin by first flowing through the holes evenly
spaced across the bottom of the channel and then by flowing under the baffle in front of the channel.
Settling Zone
After passing through the inlet zone, water enters the settling zone where water velocity is greatly
reduced. This is where the bulk of settling occurs and this zone will make up the largest volume of the
sedimentation basin. For optimal performance, the settling zone requires a slow, even flow of water.
The settling zone may be simply a large area of open water.
Outlet Zone
The outlet zone controls the amount of water flowing out of the sedimentation basin. Like the inlet
zone, the outlet zone is designed to prevent short-circuiting of water in the basin. In addition, a good
outlet will ensure that only well-settled water leaves the basin and enters the filter. The
best quality water is usually found at the very top of the sedimentation basin, so outlets are usually
designed to skim this water off the sedimentation basin.
Outlet arrangement in rectangular basin
Sludge Zone
The sludge zone is found across the bottom of the sedimentation basin where the sludge is collected
temporarily. Velocity in this zone should be very slow to prevent re-suspension of sludge.
A drain at the bottom of the basin allows the sludge to be easily removed from the tank. The tank
bottom should slope toward the drains to further facilitate sludge removal. In some plants, sludge
removal is achieved continuously using automated equipment. In other plants, sludge must be
removed manually.
SELECTION OF BASIN
There are many sedimentation basin shapes. They can be rectangular, circular, and square.

 Rectangular Basins
Rectangular basins are commonly found in large-scale water treatment plants. Rectangular tanks are
popular as they tend to have:
• High tolerance to shock overload
• Predictable performance
• Cost effectiveness due to lower construction cost
• Lower maintenance
 Circular and Square Basins
Circular basins are frequently referred to as clarifiers. These basins share some of the
performance advantages of the rectangular basins, but are generally more prone to short circuiting and
particle removal problems. For square tanks the design engineer must be certain that some type of
sludge removal equipment for the corners is installed.
 High Rate Settlers
High rate tube settlers are designed to improve the characteristics of the rectangular
basin and to increase flow through the tank. The tube settlers consist of a series of tubes that are
installed at a 60 degree angle to the surface of the tank. The flow is directed up through the settlers.
Particles have a tendency to flow at an angle different than the water and to contact the tube at some
point before reaching the top of the tube. After particles have been removed from the flow and
collected on the tubes, they tend to slide down the tube and back into the sludge zone.
 Solids-Contact Clarifier

A solids contact unit combines the coagulation, flocculation, and sedimentation basin in one unit.
These units are also called up-flow clarifiers or sludge-blanket clarifiers.
1. In the solids contact clarifier, the liquid stream enters into a central settling zone.
2. There is a re-circulator paddle with in this zone creates pressure differential and pumps
previously settled material from a central settling cone, where chemicals can be added.
3. Due to addition of chemical initial coagulation and flocculation take place.
4. Now material send to secondary mixing zone is used to produce a large number of
particle Due to this settling of solid is favorable.
5. Water passes out of the inverted cone into the settling zone, where solids settle to the
bottom and clarified water flows over the weir.
6. Solids are drawn back into the primary mixing zone, causing recirculation of the large
floc.
7. The concentration of solids in the mixing zones is controlled by occasional or
continuous blow down of sludge.
The solids contact unit is used primarily in the lime-soda ash process to settle out the floc
formed during water softening.

CRYSTALLIZATION
Crystallization is a separation process, widely applied in the chemical and pharmaceutical
industry. The principle of crystallization is based on the limited solubility of a compound in a solvent
at a certain temperature, pressure, etc. A change of these conditions to a state where the solubility is
lower will lead to the formation of a crystalline solid. Although crystallization has been applied for
thousands of years in the production of salt and sugar, many phenomena occurring during
crystallization are still poorly understood.
During crystallization, atoms and molecules bind together with well-defined angles to form a
characteristic crystal shape with smooth surfaces and facets. Although crystallization can occur in
nature, crystallization also has a broad industrial application as a separation and purification step in the
pharmaceutical and chemical industries.
Key Crystallization Definitions
 Crystallization- Crystallization is a process whereby solid crystals are formed from another
phase, typically a liquid solution or melt. Or Crystallization is the process of atoms or
molecules arranging into a well-defined, rigid crystal lattice in order to minimize their
energetic state.
 Crystal- Crystal is a solid particle in which the constituent molecules, atoms, or ions are
arranged in some fixed and rigid, repeating three-dimensional pattern or lattice. Crystals are
solids in which the atoms are arranged in a periodic repeating pattern that extends in three
dimensions. While all crystals are solids, not all solids are crystals.
 Precipitation-Precipitation is another word for crystallization but is most often used when
crystallization occurs very quickly through a chemical reaction.
 Solubility-Solubility is a measure of the amount of solute that can be dissolved in a given
solvent at a given temperature
 Saturated-Solution- At a given temperature, there is a maximum amount of solute that can be
dissolved in the solvent. At this point the solution is saturated. The quantity of solute dissolved
at this point is the solubility. On adding a solid substance in a liquid and stirring it, the solid
dissolves in the fluid. But when added more and more solid to the liquid, a point comes after
which no more solid dissolves in the liquid. This point is called a saturation point and the fluid
is called a saturation solution.
 Crystal Lattice is defined as a three dimensional network of imaginary lines connecting the
atoms or molecules. The distance between the center of two atoms (or molecules) is
called length of unit cell and the angle between the edges of a unit cell is called as lattice
angle.
 Habit –the outward appearance of a crystal. Can be indicative of
crystallographic symmetry.
 Mother liquor –growth medium for a crystal. The crystal is harvested from
the mother liquor and stabilized in a harvest buffer.
 The smallest entity of crystal lattice is called a unit cell, which can accept
atoms or molecules to grow a macroscopic crystal.
Crystallization Steps
1. Choose an appropriate solvent. Common considerations included how
much solute can be dissolved (solubility) and how practical the solvent is to
handle (safety)
2. Dissolve the product in the solvent by increasing the temperature until the
last product molecule disappears. At this insoluble impurities may be
filtered from the hot solution
3. Reduce solubility via cooling, anti-solvent addition, evaporation or
reaction. The solution will become supersaturated.
4. Crystallize the product. As solubility is reduced a point is reached where
crystals will nucleate and then grow. Highly pure product crystals should
form and impurities should remain in solution.

5. Allow the system to reach equilibrium after cooling (or another crystallization method stops).
6. Filter and dry the purified product.
Crystallization cannot occur without supersaturation. There are 5 basic methods of generating
supersaturation
 EVAPORATION – by evaporating a portion of the solvent. Crystallizers that obtain precipitation by evaporating
a solution; applicable for the substance whose yield by cooling is negligible; Crystallizers involve are Salting
Evaporator, Oslo Crystallizer
 COOLING – by cooling a solution through indirect heat exchange. Crystallizers that obtain precipitation by
cooling a concentrated hot solution; Crystallizers involve are Pan Crystallizers, Agitated batch Crystallizers,
Swenson Walker Crystallizer
 Reaction, where feed streams enter and mix resulting in a chemical reaction generating the product, usually at
high levels of supersaturation.
 Drowning out, where a miscible solvent is added resulting in a mixture in which the product is less soluble. This
has similar characteristics to reaction crystallization.
 Salting out, where a salt with a common ion is added to precipitate the product from solution. Again, this has
similar characteristics to reaction crystallization.
There are many examples of natural process that involve crystallization.
Geological time scale process examples include:
 Natural (mineral) crystal formation (gemstone);
 Stalactite/stalagmite, rings formation.
Usual time scale process examples include:
 Snow flakes formation;
 Honey crystallization (nearly all types of honey crystallize).
There are seven types of crystal forms, depending on the arrangement of the faces expressed as crystal
axes and angles between the axes.
1. Cubic - The three crystallographic axes are all equal in length and intersect at right angles (90
degrees) to each other. [a = b = c]
2. Tetragonal - Three axes, all at right angles, two of which are equal in length (a and b) and one (c)
which is different in length (shorter or longer). Note: If c was equal in length to a or b, then we would
be in the cubic system.
3. Orthorhombic - Three axes, all at right angles, and all three of different lengths. Note: If any axis
was of equal length to any other, then we would be in the tetragonal system
4. Hexagonal - Four axes, three of the axes fall in the same plane and at 600
to each other.
5. Monoclinic - Three axes, all unequal in length, two of which (a and c) intersect at an oblique angle
(not 90 degrees), the third axis (b) is perpendicular to the other two axes. Note: If a and c crossed at 90
degrees, then we would be in the orthorhombic system.
6. Triclinic - The three axes are all unequal in length and intersect at three different angles (any angle
but 90 degrees). Note: If any two axes crossed at 90 degrees, then we would be describing a
monoclinic crystal.
Important Factors in a Crystallization Process
 Yield
 Purity of the Crystals
 Size of the Crystals – should be uniform to minimize caking in the package, for ease in pouring,
ease in washing and filtering and for uniform behavior when used
 Shape of the Crystals
Mechanism of Crystallization Process
There are two basic steps in the over-all process of crystallization from supersaturated solution:
(i) Nucleation: i.e. the birth of a new solute particle and

(ii) Crystal growth: i.e. the growth of the nucleus to macroscopic size.
(1) NUCLEATION- Nucleation is the step where the solute molecules dispersed in the solvent start to gather
into clusters, which become stable under the current operating conditions.
OR
Nucleation refers to the birth of very small bodies of new phase within a supersaturated
homogeneous existing phase.
Nucleation may take the following steps:
(i) Primary nucleation
(ii) Secondary nucleation
(i) Primary nucleation- Primary nucleation may be of two types:
(a) Homogeneous nucleation (b) Heterogeneous nucleation
a. Homogenous or Primary Nucleation – occurs due to rapid local fluctuations on a molecular scale in a
homogenous phase; it occurs in the bulk of a fluid phase without the involvement of a solid-fluid inter
face
b. Heterogeneous Nucleation– occurs in the presence of surfaces other than those of the crystals such as
the surfaces of walls of the pipe or container, impellers in mixing or foreign particles; this is dependent
on the intensity of agitation
ii. Secondary Nucleation – occurs due to the presence of crystals of the crystallizing species
(2) CRYSTAL GROWTH – a layer-by-layer process
Once nuclei are formed, either spontaneously or by seeding, the crystals will continue to grow so long
as supersaturation persists. The three main factors controlling the rates of both nucleation and of
crystal growth are
 the temperature
 the degree of supersaturation
 and the interfacial tension between the solute and the solvent.
In practice, slow cooling maintaining a low level of supersaturation produces large crystals and
fast cooling produces small crystals. Nucleation rate is also increased by agitation.
By controlling these condition nucleation growth, crystals sizes and shapes are obtained
(control of crystal size and shape constitutes one of the main challenges in industrial manufacturing,
such as for pharmaceuticals).
Crystallization in food processing
In food industry crystallization process is used for two specific purposes. Firstly, it is used to separate
out a solid phase of different composition from liquid phase and one or both the fractions may be
valuable. Alternatively, crystallization is used without effecting separation of fractions in order to
control or bring about desirable changes to the texture of the solid product.
Controlling crystallization in food processing requires control of the relative rates of nucleation and
growth. To make the appropriate number and size of ice cream requires that the proper conditions are
met during ice cream manufacture.
To make smooth texture of ice cream, many small crystals must be formed during processing.
Crystallization may serve for the recovery of crystalline products – (sugar, glucose, lactose, citric acid,
salt), for the removal of certain undesirable components or for modification of certain food products in
order to obtain a desirable structure. In the process of crystallization of triglycerides, it is a complex
phenomenon characterized by fairly slow growth rates and polymorphic transitions of their
crystallized phases.
In crystallization of sucrose, it is the final step in the recovery of sugar from sugar cane or sugar beet.
Also called ‘sugar boiling’, sugar crystallization of sugar is a complex process requiring precise
control, skill and experience.
In principle, crystals are always pure. Impurities, which are sometimes found can be removed by

washing.
In fondant processing, the temperature at which the syrup is nucleated is a critical parameter.
If nucleation is induced at temperature other than the optimal temperature, fewer crystals will be form
than the maximum number, and the texture of the fondant will be unsatisfactory.
Classification of Crystallizer
 May be classified according to whether they are batch or continuous in operation
 May be classified according on the methods used to bring about supersaturation
 Can also be classified according on the method of suspending the growing product crystals
TANK CRYSTALLIZER
The simplest type of cooling crystallizer. A hot, nearly saturated solution is run into an open
rectangular tank in which the solution is allowed to cool and deposit crystals. No attempt is usually
made to seed these tanks, to provide for agitation or to accelerate or control the rate of crystallization
in any way. Sometimes rods or strings are hung in the tank to give the crystals additional surface on
which to grow. Under these conditions crystal growth is slow, and the crystals formed tend to be large
and considerably interlocked. These interlocking results in the occlusion of mother liquor, thus
introducing impurities. When the solution has sufficiently cooled which normally takes a number of
days, the remaining mother liquor is drained off and the crystals removed manually. The
disadvantages of this method are as follows: it needs much labor, the crystals are contaminated with
impurities that settle to the bottom of the tank, it needs more floor space and material is tied up in the
process for a long time. But the method produces larger crystals and is a simpler and cheaper process.
This method is now almost obsolete.
Labour costs are generally high, but the method may be economical for small batches because capital,
operating, and maintenance costs are low. However, the productivity of this type of equipment is low
and space requirements are high.
Disadvantages
1. Crystal growth is very slow.
2. Crystals formed are large and interlocked, so mother
liquor along with impurity gets entrapped within the
crystals.
3. The floor space required and the amount of material
tied up in this process are both large.

AGITATED BATCH CRYSTALLIZER
Procedure
It is a tank with a central shaft running through it.
Water is circulated through the cooling coils, and the
solution is agitated by the propellers on the central shaft.
Agitation increases the rate of cooling and keeps the
solution at a more uniform temperature. It also keeps the
fine crystals in suspension which can then grow uniformly
without forming too large crystals or aggregates.
Product is collected at the bottom of the crystallizer.
It is a batch process. The final product tends to have a
higher purity because less mother liquor is retained by the
crystals after filteration and more efficient washing is
possible.
Vertical baffles may be fitted inside the vessel to
induce better mixing, but they should terminate below the
liquor level to avoid excessive encrustation. For the same
reason, water jackets are usually preferred to coils for
cooling purposes.
An agitated cooler is more expensive to operate than a
simple tank crystallizer, but it has a much higher
productivity.
The use of external circulation allows good mixing inside the crystallizer and high rates of heat
transfer between the liquor and coolant.
Advantages
1. The agitation increases the rate of heat transfer and keeps the temperature of the solution
uniform throughout the crystallizer.
2. Agitation keeps the smaller crystals in suspension and allows them to grow uniformly– thus
finer crystals can be obtained.
Disadvantages
1. It is a batch process or a discontinuous one.
2. Since the solubility is least at the cooling surface hence the crystals growth is more rapid on
the cooling coils
SWENSON-WALKER CRYSTALLIZER
Swenson Walker Crystallizer is a continuous type crystallizer designed to make the large, uniform
crystals .This operation involves both heat and mass transfer. It works on principle of super saturation
by cooling. Swenson Walker Crystallizer is a continuous type crystallizer
Principle: It works on principle of Super saturation by cooling. A very common type of continuous
crystallizer using cooling alone to bring about supper saturation is the Swenson-walker crystallizer.
Construction:
 It consists of U- shaped open trough with a semi cylindrical bottom.
 A water jacket welded to the outside of trough.
 A slow speed, long pitch, spiral agitator running at about 7 RPM and set as close to bottom of
the trough as possible.
 Water jacket is divided into section so that differential cooling may be used in the various
zones.
 The crystallizer is built in units 10ft long and number of units may join together to increase the
capacity.
 For still higher capacity, this larger unit may be arranged one above the other such that the
solution cascade from one unit to other.

Operation:
 The hot concentrated solution to be crystallized is fed at one end of trough & cooling water
usually flows through the jacket in counter current to the solution.
 Sometimes extra amount of water is introduced into certain sections of trough to control crystal
size.
 Nucleation may be started by short cold zone followed by gradual cooling.
Functions of the spiral stirrer are:
 To prevent an accumulation of crystals on the cooling surface.
 To lift the crystal formed & shower them down through the solution so crystal grow uniformly
and free from aggregate and inclusion of mother liquor.
 At the end of the crystallizer, there is over flow gate where crystals and mother liquor overow
to drain box.
 From the mixture, mother liquor is returned to the process & wet crystals are fed to centrifuge.
Advantages: 1. Less floor space is required 2. Saving labor 3. Continuous process 4. Crystal of
uniform size, free from inclusions/aggregates
Disadvantages : 1. Scrapper may break crystal to the little extent due to agitation.
VACUUM CRYSTALLIZER
Principle: Under vacuum the boiling point of a liquid
reduces. So under vacuum a liquid boils under its
normal boiling point. If a warm saturated solution is
introduced into a vessel in which a vacuum is
maintained and the feed temperature is above the
(reduced) boiling point of the solution then the
solution so introduced must flash (sudden evaporation)
and be cooled due to adiabatic evaporation (taking the
latent heat from the solution). Cooling will cause
supersaturation and thus crystallization. Evaporation
will increase the yield. Vacuum crystallizers are often
operated continuously, but they can also be operated
batch-wise.
Vacuum crystallizers (WITHOUT EXTERNAL
CLASSIFYING SEED BED)- A vacuum crystallizer is
very simple without any moving parts. The capacity
can be as large as desired. The crystallizer proper is
the cone-bottomed vessel. The feed which is a hot
saturated solution enters at a convenient point into the
crystallizer. By use of highly efficient steam jet
ejectors it is possible to produce quite a high vacuum
resulting in the flashing of the feed solution and consequent adiabatic cooling producing low

temperatures enabling large yields to be obtained with minimum amount of mother liquor returning to
the system. The flashing of the solution produces considerable ebullition which keeps the crystals in
suspension until they become large enough to fall into the discharge pipe from which they are
removed as a slurry by the pump. The discharge from the pump goes to centrifuges or to continuous
vacuum filters or to an intermediate settling tank to thicken the slurry. The propellers keep the liquid
thoroughly stiffed and prevent the feed solution from reaching the discharge pipe with-out flashing.
The barometric condenser condenses the vapour coming from the ejector and the condenser is
followed by a 2 or 3 stage ejector to remove air. Vacuum Crystallizers with external classifying seed
bed are not of much importance in pharmacy.
KRYSTAL CRYSTALLIZER
Working principle
In Krystal crystallizer, concentration of
liquid and crystallization are obtained in different
chambers , namely vapor head and crystalline
chamber .
The concentration of liquid (supersaturation) is
induced by evaporation of hot solvent with the
help of a vacuum pump. In the crystallization
chamber, the supersaturated solution and crystals
are maintained in a fluidized state for uniform
crystal growth. As the crystals of desired size
settle down by gravity, the fine crystals and
supersaturated solution is recalculated for further
crystallization. Crystals of desired size are
collected from the crystal growth chamber.
Construction of Krystal crystallizer
Krystal crystallizer is consists of a vapour head and crystallizing chamber. Vapor head consists of a
long tube, which extends almost to the bottom of crystallizing chamber. Other end of vapour head is
connected to condenser and vacuum pump. A pump is provided which allows the feed to enter vapour
head. On its way to vapour head, a heater is provided.
Working of Krystal crystallizer
Solution is pumped, which passes through the heater. The hot solution enters the vapour head. Because
of reduced pressure, the hot solution undergoes flashing, which results in the formation of solvent
vapour and supersaturated solution.
Vapour is removed by Suction pump. Supersaturated solution passes through long tube below . The
operation is controlled in such a way that the crystals must form in the crystallization chamber rather
than in the vapor head.
The crystallizing chamber consists of a bed of crystals suspended in an upward flowing stream of
liquid. Supersaturated - liquid flows through the bed of crystals, which are maintained in a fluidized
state. A uniform temperature is thereby attained.
There is a continuous gradation of crystals in the chamber. Coarse crystals settle at the bottom, while
fine crystals remain above coarser ones. Very fine crystals overflow through the liquid and enter into
the recirculating system, which then combine with fresh feed. From time to time, coarse crystals are
taken out through the opening at the bottom of the chamber.
Advantages of Krystal crystallizer
( 1 ) Krystal crystallizer is preferred when large quantities of crystals of controlled sizes are required .
( 2 ) This crystallizer is available in very large sizes with a body up to 4.5 metres diameter and 6.0
metres height

Application of Crystallization
1. Crystallization is used to separate salt from seawater,
2. Manufacture of sucrose, from sugar cane or sugar beet, is an important example of
crystallization.
3. Crystallization is also used in the manufacture of other sugars, such as glucose and lactose.
4. Manufacture of food additives, such as salt, and in the processing of foodstuffs, such as ice
cream.
5. Crystallization from solution is important industrially because of the variety of materials that
are marketed in the crystalline form.
6. Crystallization is also used for obtaining pure chemical substances in a satisfactory condition
for packaging and storing.
What are the uses of crystallization?
Crystallization is primarily employed as a separation technique in order to obtain pure crystals of a
substance from an impure mixture. Another important application of crystallization is its use to obtain
pure salt from seawater. Crystallization can also be used to obtain pure alum crystals from impure
alum. In such scenarios, crystallization is known to be more effective than evaporation since it also
removes the soluble impurities.
What are the two primary types of crystallization?
Crystallization processes can be broadly categorized into the following two types:
 Evaporative crystallization
 Cooling crystallization
List some examples of crystallization.
Some common examples of crystallization are listed below.
 The crystallization of water to form ice cubes and snow.
 The crystallization of honey when it is placed in a jar and exposed to suitable conditions.
 The formation of stalagmites and stalactites (especially in caves).
 The deposition of gemstone crystals.
What are the advantages of crystallization?
The key advantages of crystallization are listed below.
 A product of high purity can be obtained from one single step via the process of crystallization.
 The dry products formed from crystallization can be directly packaged and stored.
 The energy requirements and the operating temperatures of this process are relatively low.

UOFP- Unit operation in Food Processing

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