Pres1. PROPERTIES OF FOOD AND PROCESSING THEORY

CHAPTER ONE
PROPERTIES OF FOODS
&
PROCESSING THEORY

1. Properties of liquids, solids and gases
Introduction
• Liquids, gases and some solids (e.g. powders and particulate
materials) are termed ‘fluids’ and can flow without
disintegration when a pressure is applied to them. In
contrast, solids deform when pressure is applied to them.
• The transition from solid to fluid and back is known as a
phase transition and this is important in many types of food
processing (e.g. water to water vapour in evaporation and
distillation, and dehydration, water to ice in freezing and
freeze drying or freeze concentration , or crystallisation of
fats.
• Phase transition takes place isothermally at the phase
transition temperature by release or absorption of latent
heat,

A. Density and specific gravity
• A 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.
• The density of a material is equal to its mass divided by its
volume and has units of kg/m3
. The density of materials is
not constant and changes with temperature (higher
temperatures reduce the density of materials) and pressure.
• for particulate solids and powders there are two forms of
density: the density of the individual pieces and the density
of the bulk of material, which also includes the air spaces
between the pieces. This latter measure is termed the bulk
density and is ‘the mass of solids divided by the bulk
volume’.

A. Density and specific gravity…Cont’d
• The fraction of the volume that is taken up by air is termed
the porosity and is calculated by: є = Va/Vb, where Va
volume of air (m3
) and Vb volume of bulk sample (m3
).
• The bulk density of a material depends on the solids density
and the geometry, size and surface properties of the
individual particles.
• The density of liquids can be expressed as specific gravity
(SG), a dimensionless number , which is found by dividing
the mass (density) of a liquid by the mass (density) of an
equal volume of pure water at the same temperature:

Density and specific gravity…continued
SG = mass of liquid/mass water or SG = density
of liquid/density water
• If the specific gravity of a liquid is known at a
particular temperature, its density can be found
using:
• ρL = (SG)T * ρw , where ρL = liquid density (kg /m3
)
and ρw density of water, each at temperature T (o
C. )
• Specific gravity is widely used instead of density in
brewing and other alcoholic fermentations.
• The density of gases depends on their pressure and
temperature.

Density and specific gravity…continued
• The density of gases and vapors is also referred to
as specific volume which is ‘the volume occupied
by unit mass of gas or vapor’ and is the inverse of
density.
• This is used for example in the calculation of the
amount of vapor that must be handled by fans
during dehydration or by vacuum pumps in freeze
drying or vacuum evaporation.
• When air is incorporated into liquids (for example
cake batters, ice cream, whipped cream) it creates a
foam and the density is reduced.

B. Viscosity
• Viscosity is an important characteristic of liquid foods
in many areas of food processing. For example the
characteristic mouth feel of food products such as
tomato ketchup, cream, syrup and yoghurt is dependent
on their consistency or viscosity.
• The viscosity of many liquids changes during heating,
cooling, concentration, etc. and this has important
effects on, for example, the power needed to pump
these products.
• Viscosity may be thought of as a liquid’s internal
resistance to flow. 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.

B. Viscosity…
• If shear stress is plotted against shear rate, most simple
liquids and gases show a linear relationship (line A in Fig.
below) 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 (lines B–E), it is non-
newtonian.
• For all liquids, viscosity decreases with an increase in
temperature.

• Many liquid foods are non-
Newtonian, including
emulsions and suspensions,
and concentrated solutions
that contain starches,
pectins, gums and proteins.
• Non-Newtonian fluids can
be classified broadly into
different type:
• for example as shown in
figure
B. Pseudoplastic fluid
C. Dilatant fluid
D. Bingham plastic fluid
E. Casson plastic fluid

C. Surface activity
• A large number of foods comprise two or more immiscible
components, which have a boundary between the phases.
The phases are known as the dispersed phase (the one
containing small droplets or particles) and the continuous
phase (the phase in which the droplets or particles are
distributed).
Emulsions
• Chemicals that reduce the surface tension in the surface of
a liquid are termed surface active and are known as
‘surfactants’, ‘emulsifying agents’ or ‘detergents’.
• By reducing the surface tension, they permit new surfaces
to be produced more easily when energy is put into the
system (for example by homogenisers) and thus enable
larger numbers of droplets to be formed.

C. Surface activity… Emulsions…
• There are naturally occurring surfactants in foods,
including alcohols, phospholipids and proteins and these are
sometimes used to create food emulsions (for example using
egg in cake batters).
• However, synthetic chemicals have more powerful surface
activity and are used in very small amounts to create
emulsions. Others are used in detergents for cleaning
operations.
• Surface active agents contain molecules which are polar (or
‘hydrophilic’) at one end and non-polar (or ‘lipophilic’) at
the other end.
• In emulsions, the molecules of emulsifying agents become
oriented at the surfaces of droplets, with the polar end in the
aqueous phase and the non-polar end in the oil phase.

Foams
• Foams are two-phase systems which have gas bubbles dispersed
in a liquid or a solid, separated from each other by a thin film. In
addition to food foams, foams are widely used for cleaning
equipment. The main factors needed to produce a stable foam
are:
a low surface tension to allow the bubbles to contain more air
and prevent them contracting
gelation or insolubilisation of the bubble film to minimise
loss of the trapped gas and to increase the rigidity of the foam
and
a low vapour pressure in the bubbles to reduce evaporation
and rupturing of the film.
• In food foams, the structure of the foam may be stabilised by
freezing (ice cream), by gelation (setting gelatin in
marshmallow), by heating (cakes, meringues) or by the addition
of stabilizers such as proteins or gums.

D. Rheology and texture
• The texture of foods has a substantial influence on
consumers’ perception of ‘quality’ and during chewing (or
‘mastication’), information on the changes in texture of a
food is transmitted to the brain from sensors in the mouth,
from the sense of hearing and from memory, to build up an
image of the textural properties of the food.
• Rheology is the science of deformation of objects under the
influence of applied forces.
• When a material is stressed it deforms, and the rate and type
of deformation characterize its rheological properties.

2. Material transfer
• The transfer of matter is an important aspect of a large number of food
processing operations: it is a key factor in solvent extraction,
distillation and membrane processing and it is an important factor
in loss of nutrients during blanching.
• Mass transfer of gases and vapours is a primary factor in
evaporation, dehydration, baking and roasting, frying, freeze
drying, the cause of freezer burn during freezing and a cause of loss
in food quality in chilled, MAP and packaged foods.
• the two factors that influence the rate of mass transfer are a driving
force to move materials and a resistance to their flow.
• When considering dissolved solids in liquids, the driving force is
difference in the solids concentration, whereas for gases and
vapours, it is a difference in partial pressure or vapour pressure.
• The resistance arises from the medium through which the liquid, gas
or vapour moves and any interactions between the medium and the
material.

2. Material transfer…Continued
• An example of materials transfer is diffusion of water vapour
through a boundary layer of air in operations such as
dehydration, baking, etc.
Mass balances
• The law of conversion of mass states that ‘the mass of material
entering a process equals the mass of material leaving’. This
has applications in, for example, mixing, fermentation, and
evaporation.
• In general a mass balance for a process takes the following form:
mass of raw materials in = mass of products and wastes out +
mass of stored materials + losses

2. Material transfer…Continued
• Many mass balances are analysed under steady-state conditions where
the mass of stored materials and losses are equal to zero.
• Mass balances are used to calculate
• The quantities of materials in different process streams
• to design processes,
• to calculate recipe formulations
• the composition after blending, process yields and separation
efficiencies.
• A simple method to calculate the relative masses of two materials that
are required to form a mixture of known composition is the Pearson
Square.
• A method to calculate the relative masses of two materials is to use a
total balance and a component balance method.

3. Fluid flow
• Many types of liquid food are transported through pipes
during processing, and powders and small-particulate foods
are more easily handled as fluids.
• The study of fluids is therefore of great importance in food
processing. It is divided into fluid statics (stationary fluids)
and fluid dynamics (moving fluids).
• When a fluid flows through pipes or processing equipment,
there is a loss of energy and a drop in pressure which are
due to frictional resistance to flow.
• The loss of pressure in pipes is determined by a number of
factors including the density and viscosity of the fluid, the
length and diameter of the pipe and the number of bends,
valves, etc., in the pipeline.

3. Fluid flow…
• In any system in which fluids flow, there exists a boundary
film (surface film) of fluid next to the surface over which
the fluid flows (Fig. 1.7(a)).
• The thickness of the boundary film is influenced by a
number of factors, including the velocity, viscosity, density
and temperature of the fluid. This produces movement of
the fluid, which is termed streamline (or laminar) flow.
• Above a certain flow rate, which is determined by the nature
of fluid and the pipe, the layers of liquid mix together and
turbulent flow is established (Fig. 1.7 (c)) in the bulk of the
fluid, although the flow remains streamline/laminar in the
boundary film. Higher flow rates produce more turbulent
flow and hence thinner boundary films.

• Fluid flow is characterized
by a dimensionless group
named the Reynolds
number (Re). This is
calculated using,
• Re number < 2100 describes
streamline flow and a Re
number of >4000 describes
turbulent flow. For Re numbers
between 2100 and 4000,
transitional flow is present,
which can be either laminar or
turbulent at different times.

• In turbulent flow, particles of fluid move in all directions
and solids are retained in suspension more readily. This
reduces the formation of deposits on heat exchangers and
prevents solids from settling out in pipework.
• Streamline flow produces a larger range of residence times
for individual particles flowing in a tube. This is especially
important when calculating the residence time required for
heat treatment of liquid foods, as it is necessary to ensure
that all parts of the food receive the required amount of
heat.
• Turbulent flow causes higher friction losses than
streamline flow does and therefore requires higher energy
inputs from pumps.

4. Heat transfer
• There are three ways in which heat may be transferred:
by radiation, by conduction and by convection.
• Radiation: is the transfer of heat by electromagnetic
waves, for example in an electric grill.
• Conduction is the movement of heat by direct transfer
of molecular energy within solids (for example through
metal containers or solid foods).
• Convection is the transfer of heat by groups of
molecules that move as a result of differences in density
(for example in heated air) or as a result of agitation (for
example in stirred liquids).

4. Heat transfer…
• In the majority of applications all three types of heat
transfer occur simultaneously but one type may be more
important than others in particular applications.
• Energy balances
– An energy balance states that ‘the amount of heat or
mechanical energy entering a process = the total
energy leaving with the products and wastes + stored
energy + energy lost to the surroundings’.

Mechanisms of heat transfer
• Steady-state heat transfer takes place when there is a
constant temperature difference between two materials.
The amount of heat entering a material equals the
amount of heat leaving, and there is no change in
temperature of the material.
• However, in the majority of food-processing applications,
the temperature of the food and/or the heating or cooling
medium are constantly changing, and unsteady-state heat
transfer is more commonly found.

Sources of heat and methods of application to foods
• The following sources of energy are used in food
processing:
•electricity
•gas (natural and liquid petroleum gas)
∀liquid fuel oil.
Direct heating methods
• In direct methods, the heat and products of combustion
from the burning fuel come directly into contact with
the food.
• There is an obvious risk of contamination of the food by
odours or incompletely burned fuel and, for this reason,
only gas and, to a lesser extent, liquid fuels are used.

Indirect methods
• Indirect heating methods employ a heat exchanger to
separate the food from the products of combustion. At
its simplest, an indirect system consists of burning fuel
beneath a metal plate and heating by energy radiated
from the plate.
• The most common type of indirect-heating system used
in food processing is steam or hot water generated by a
heat exchanger (a boiler) located away from the
processing area.
• A second heat exchanger transfers the heat from the
steam to the food under controlled conditions or the
steam is injected into the food.

Effect of heat on micro-organisms
• The preservative effect of heat processing is due to the
denaturation of proteins, which destroys enzyme
activity and enzyme-controlled metabolism in micro-
organisms.
• The rate of destruction is a first-order reaction; that is
when food is heated to a temperature that is high
enough to destroy contaminating micro-organisms, the
same percentage die in a given time interval regardless
of the numbers present initially.
• This is known as the logarithmic order of death and is
described by a death rate curve.

Effect of heat on micro-organisms…
• The time needed to destroy 90% of the micro-
organisms (to reduce their numbers by a factor of
10) is referred to as the decimal reduction time or D
value (5 min in Fig. 1.13).
• D-values differ for different microbial species, and
a higher D value indicates greater heat resistance.
• There are two important implications arising from
the decimal reduction time:
first, the higher the number of micro-organisms present
in a raw material, the longer it takes to reduce the
numbers to a specified level.

• Second, because microbial destruction takes place
logarithmically, it is theoretically possible to destroy all
cells only after heating for an infinite time.
• Processing therefore aims to reduce the number of
surviving micro-organisms by a pre-determined amount.
This gives rise to the concept of commercial sterility.
• The destruction of micro-organisms is temperature
dependent; cells die more rapidly at higher
temperatures. By collating D values at different
temperatures, a thermal death time (TDT) curve is
constructed (Fig. 1.14).

Pres1. PROPERTIES OF FOOD AND PROCESSING THEORY

• The slope of the TDT curve is termed the z value and is
defined as “the number of degrees Celsius required to
bring about a ten-fold change in decimal reduction time”
(10.5ºC in Fig. 1.14).
• The D value and z value are used to characterise the heat
resistance of a micro-organism and its temperature
dependence respectively.
• There are a large number of factors which determine the
heat resistance of microorganisms;
Type of micro-organism
Incubation conditions(T0
, age of culture, culture medium)
Conditions during heat treatment (pH, Wa, composition of food,
the growth media and incubation conditions)

5. Water activity or RVP
• Water content is very important factor controlling the rate
of deterioration of food.
• The moisture content of foods can be expressed either on a
wet-weight basis:
m = mass of water *100 or m = mass of water *100
mass of sample mass of water + solids
or on a dry-weight basis
m = mass of water
mass of solids
• A knowledge of the moisture content alone is not sufficient to predict
the stability of foods. It is the availability of water for microbial,
enzymic or chemical activity that determines the shelf life of a food,
and this is measured by the water activity (aw) of a food, also known
as the Relative Vapour Pressure (RVP).

5. Water activity…Cont’d
• Water in food exerts a vapour pressure. The size of the
vapour pressure depends on:
• the amount of water present
• the temperature
• the concentration of dissolved solutes (particularly salts and
sugars) in the water.
• Water activity is defined as ‘the ratio of the vapour
pressure of water in a food to the saturated vapour pressure
of water at the same temperature’.
aw = P/P0,
• where P (Pa) = vapour pressure of water in the food, P0 (Pa)
vapour pressure of pure water at the same temperature.

• A proportion of the total water in a food is strongly bound to
specific sites (for example hydroxyl groups of polysaccharides,
carbonyl and amino groups of proteins, and hydrogen bonding).
• When all sites are (statistically) occupied by adsorbed water, the
moisture content is termed the BET (Brunauer–Emmett–Teller)
monolayer value.
• Typical examples include gelatin (11%), starch (11%),
amorphous lactose (6%) and whole spray dried milk (3%).
• The BET monolayer value therefore represents the moisture
content at which the food is most stable.
• At moisture contents below this level, there is a higher rate of
lipid oxidation and, at higher moisture contents, Maillard
browning and then enzymic and microbiological activities are
promoted.

• The movement of water vapour from a food to the
surrounding air depends upon both the food (moisture
content and composition) and the condition of the air
(temperature and humidity).
• At a constant temperature, the moisture content of food
changes until it comes into equilibrium with water vapour in
the surrounding air.
• The food then neither gains nor loses weight on storage
under those conditions. This is called the equilibrium
moisture content of the food and the relative humidity of
the storage atmosphere is known as the equilibrium relative
humidity.

6. Effects of processing on the sensory
characteristics of foods
A. Texture
• The texture of foods is mostly determined by the moisture
and fat contents, and the types and amounts of structural
carbohydrates (cellulose, starches and pectic materials)
and proteins that are present.
• Changes in texture are caused by loss of moisture or fat,
formation or breakdown of emulsions and gels, hydrolysis
of polymeric carbohydrates, and coagulation or hydrolysis
of proteins.
B. Taste, flavour and aroma
• Taste attributes consist of saltiness, sweetness, bitterness
and acidity and some of these attributes can be detected in
very low thresholds in foods

6. Effects of processing on the sensory
characteristics of foods…Cont’d
• The taste of foods is largely determined by the formulation used
for a particular food and is mostly unaffected by processing.
• Exceptions to this include increased sweetness due to respiratory
changes in fresh foods and changes in acidity or sweetness
during food fermentations.
• Fresh foods contain complex mixtures of volatile compounds,
which give characteristic flavours and aromas. These
compounds may be lost during processing, which reduces the
intensity of flavour or reveals other flavour/aroma compounds.
• Volatile aroma compounds are also produced by the action of
heat, ionizing radiation, oxidation or enzyme activity on
proteins, fats and carbohydrates.

C. Color
• Many naturally occurring pigments are destroyed by heat
processing, chemically altered by changes in pH or oxidized
during storage. As a result, the processed food may lose its
characteristic color and hence its value.
• Synthetic pigments are more stable to heat, light and
changes in pH, and they are therefore added to retain the
color of some processed foods.
• Maillard browning is an important cause of both desirable
changes in food colour (e.g in baking or frying), and in the
development of off-colours (e,g. during canning and
drying).

7. Effects of processing on nutritional properties
• Many unit operations, especially those that do not involve
heat, have little or no effect on the nutritional quality of
foods.
• Unit operations that intentionally separate the components
of foods alter the nutritional quality of each fraction
compared with the raw material.
• Unintentional separation of water-soluble nutrients
(minerals, water-soluble vitamins and sugars) also occurs in
some unit operations (for example blanching, and in drip
losses from roast or frozen foods).

7. Effects of processing on nutritional properties…Cont’d
• Heat processing is a major cause of changes to nutritional
properties of foods. For example gelatinisation of starches
and coagulation of proteins improve their digestibility, and
anti-nutritional compounds (for example a trypsin inhibitor
in legumes) are destroyed.
• However, heat also destroys some types of heat-labile
vitamin, reduces the biological value of proteins (owing to
destruction of amino acids or Maillard browning reactions)
and promotes lipid oxidation.

7. Effects of processing on nutritional properties…Cont’d
• Oxidation is a second important cause of nutritional
changes to foods. This occurs when food is exposed to air
(for example in size reduction or hot-air drying or as a result
of the action of heat or oxidative enzymes (for example
peroxidase or lipoxygenase).
• The main nutritional effects of oxidation are:
the degeneration of lipids to hydroperoxides and
subsequent reactions to form a wide variety of carbonyl
compounds, hydroxy compounds and short chain fatty
acids, and in frying oils to toxic compounds
the destruction of oxygen-sensitive vitamins

Individual Assignment
1. Explain density in general and specifically for
solid, liquid and gas?
2. What is viscosity and identify between Newtonian
and non-Newtonian fluid using graph?
3. What is surfactant or emulsifying agents and how
they used as emulsifying agent?
4. Define rheology?
5. What are the two factors that affect the rate of mass
transfer (explain using example)? What is the
application of mass transfer in food processing?

Individual Assignment
6. Compare laminar and turbulent flow (please use
figure in addition to written explanation).
7. What are sources of heat and methods of
application to foods?
8. What is D-value and Z-value? (support your answer
using figure)
9. What is water activity and BET monolayer value?
There implication in food processing?
10. Explain in short the effect of processing methods
sensory and nutritional properties of food?

Pres1. PROPERTIES OF FOOD AND PROCESSING THEORY

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Pres1. PROPERTIES OF FOOD AND PROCESSING THEORY