fnd605 prof 2024 lecture.pptx. Food analy

1
What is Actually Happening During Cooking Process?
Do You Ever Wonder?

2
INTRODUCTION
The heat transferred during cooking is usually undermined and taken for granted by
a lot of people but this process of heating food also known as heat transfer is an
interesting and an intricate process.
Cooking is the transfer of energy from a heat source to food. This energy alters
food’s molecular structure. Changing the taste, aroma and appearance. Foods can
be cooked using various methods, the method selected gives the finished product
a specific texture, aroma and flavor.
In cooking, heat transfer refers to heating your food items through a cooking
appliance, such as a stove, fryer, microwave, or oven.

3
HEAT TRANSFER
Heat transfer is an exchange of thermal energy between two objects. The rate of
heat transfer depends upon the temperatures of each entity and the medium
through which the thermal energy is being transferred.
Heat transfer is a very important aspect of the cooking process. Heating food
destroys potentially harmful bacteria and other microorganisms, which makes
food safe to eat and easier to digest. When food or liquids become hot, their
molecules absorb energy, begin vibrating rapidly, and start to bounce off of
each other. As they collide, heat energy is produced and transferred, which
warms and cooks food.

Temperature and Its Measurement
• When the physical properties are no longer
changing, the objects are said to be in
thermal equilibrium.
• Two or more objects in thermal
equilibrium have the same temperature.
– If two objects are in contact with one another
long enough, the two objects have the same
temperature.
• This is the zeroth law of thermodynamics.

• The first widely used temperature
scale was devised by Gabriel
Fahrenheit.
• Another widely used scale was
devised by Anders Celsius.
• The Celsius degree is larger than
the Fahrenheit degree
• They are both equal at -40.
TC 
5
9
TF  32
 
TF 
9
5
TC  32

• The zero point on the Fahrenheit
scale was based on the
temperature of a mixture of salt
and ice in a saturated salt solution.
• The zero point on the Celsius scale
is the freezing point of water.
• Both scales go below zero.
– 0 F = -17.8 C
• Is there such a thing as absolute
zero?

• We can then plot the pressure of a gas as a function of the
temperature. PV= nkT
• The curves for different gases or amounts are all straight lines.
• When these lines are extended backward to zero pressure, they all
intersect at the same temperature, -273.2C.
• Since negative pressure has no meaning, this suggests that the
temperature can never get lower than -273.2C, or 0 K (kelvin).

TK TC  273.2

3E-01 Liquid Nitrogen Demos
• liquid nitrogen boils at 77 K (−196 °C; −321 °F)
• freezes at 63 K (−210 °C; −346 °F)

Heat and Specific Heat Capacity
 What happens when objects or fluids
at different temperatures come in
contact with one another?
The colder object gets hotter, and the
hotter object gets colder, until they both
reach the same temperature.
What is it that flows between the objects
to account for this?
• We use the term heat for this
quantity.
– Unit: Joule (SI unit), calorie
– 1 cal = 4.1868 J

• When two objects at different temperatures are placed
in contact, heat will flow from the object with the higher
temperature to the object with the lower temperature.
• Heat added increases temperature, and heat removed
decreases temperature.
• Heat and temperature
are not the same.
• Temperature is a
quantity that tells us
which direction the
heat will flow.

Heat and Specific Heat Capacity
• One-hundred grams of room-temperature water is more
effective than 100 grams of room-temperature steel shot
in cooling a hot cup of water.
Steel has a lower specific heat
capacity than water.

• The specific heat capacity of a material is the quantity
of heat needed to change a unit mass of the material by
a unit amount in temperature.
– For example, to change 1 gram by 1 Celsius degree.
– It is a property of the material, determined by experiment.
– The specific heat capacity of water is 1 cal/gC: it takes 1
calorie of heat to raise the temperature of 1 gram of water by
1C.
• We can then calculate how much heat must be
absorbed by a material to change its temperature by a
given amount:
Q = mcT where Q = quantity of heat
m = mass
c = specific heat capacity
T = change in temperature

• When an object goes through a change of phase or
state, heat is added or removed without changing the
temperature. Instead, the state of matter changes:
solid to liquid, for example.
• The amount of heat needed per unit mass to produce a
phase change is called the latent heat.
– The latent heat of fusion of water corresponds to the amount
of heat needed to melt one gram of ice.
– The latent heat of vaporization of water corresponds to the
amount of heat needed to turn one gram of water into steam.
Phase Changes and Latent Heat

If the specific heat capacity of ice is
0.5 cal/gC°, how much heat would
have to be added to 200 g of ice,
initially at a temperature of -10°C, to
raise the ice to the melting point?
a) 1,000 cal
b) 2,000 cal
c) 4,000 cal
d) 0 cal
m = 200 g
c = 0.5 cal/gC°
T = -10°C
Q = mcT
= (200 g)(0.5 cal/gC°)(10°C)
= 1,000 cal
(heat required to raise the temperature)

Quiz: If the specific heat capacity of
ice is 0.5 cal/gC°, how much heat would
have to be added to 200 g of ice, initially
at a temperature of -10°C, to completely
melt the ice? (Latent heat is 80 cal/g)
a) 1,000 cal
b) 14,000 cal
c) 16,000 cal
d) 17,000 cal
Lf = 80 cal/g Q = mLf
= (200 g)(80 cal/g)
= 16,000 cal
(heat required to melt the ice)
Total heat required to raise the ice to 0 °C and then to melt
the ice is:
1,000 cal + 16,000 cal = 17,000 cal = 17 kcal

Joule’s Experiment and the First
Law of Thermodynamics
• Joule’s experiments led to Kelvin’s statement of the first law of
thermodynamics.
– Both work and heat represent transfers of energy into or out of a system.
– If energy is added to a system either as work or heat, the internal energy of the
system increases accordingly.
• The increase in the internal
energy of a system is equal
to the amount of heat added
to a system minus the
amount of work done by the
system. U = Q - W

Joule’s Experiment and the First
Law of Thermodynamics
• The internal energy of the system is the sum of the kinetic and
potential energies of the atoms and molecules making up the
system.
• An increase in internal energy may show up as an increase in
temperature, or as a change in phase, or any other increase in
the kinetic and/or potential energy of the atoms or molecules
making up the system.
• Internal energy is a property of the system uniquely determined
by the state of the system.

A hot plate is used to transfer 400
cal of heat to a beaker containing ice
and water; 500 J of work are also
done on the contents of the beaker
by stirring. What is the increase in
internal energy of the ice-water
mixture? (note: 1 cal = 4.19J)
a) 900 J
b) 1180 J
c) 1680 J
d) 2180 J
W = -500 J
Q = 400 cal
= (400 cal)(4.19 J/cal)
= 1680 J
U = Q - W
= 1680 J - (-500 J)
= 2180 J

A hot plate is used to transfer 400 cal
of heat to a beaker containing ice and
water; 500 J of work are also done on
the contents of the beaker by stirring.
How much ice melts in this process?
(latent heat: 80 cal/g. 1 cal = 4.19J).
a) 0.037 g
b) 0.154 g
c) 6.5 g
d) 27.25 g
Lf = 80 cal/g
= (80 cal/g)(4.19 J/cal)
= 335 J/g
U = mLf
m = U / Lf
= (2180 J) / (335 J/g)
= 6.5 g

CALORIMETRY I: NOTION OF QUANTITY OF HEAT
Consider two equal quantities of water, at the same temperature t1. Heat one of the two with an
immersion heater: its temperature increases and we consume electrical energy. According to the
principle of conservation of energy, this energy must end up somewhere, it can only be in the water
(if we neglect the losses to the outside). This energy stored by the water was in the form of thermal
or calorific energy. Let us now mix these two masses of water, one at temperature t1 and the other
at temperature t2. The mixture obtained will be at the temperature t' equal to: or: t 2 - t' = t' - t1 If we
did not have the same masses of water, for example the masses m1 and m2, we note that the
temperature t' depends on the ratio of their masses: (m1 + m2)t' = (m1t1 + m2t2) m 2(t2 - t') = m1(t' -
t1) If we had two different liquids, t' would depend on the nature of the two liquids, in particular to
obtain the temperature t2, it would not be necessary to heat in the same way as with water. It is
necessary to bring in two coefficients c1 and c2 which reflect the capacity of bodies to store thermal
energy: m 2c2(t2 - t') = m1c1(t' - t1) m 1c1(t' - t1) + m2c2(t ' - t2) = 0 The quantity mc(tf - ti) is called
the heat Q exchanged with the exterior by a body of mass m, of specific heat c when its
temperature passes from the value ti to the value tf. This quantity of heat is equal to the variation of
thermal energy of the body: we can therefore assimilate the product m.c.t to the quantity of stored
thermal energy. If tf > ti , the body has heated up, it has received energy and Q is positive. If t f < ti ,
the body has cooled, it has given energy and Q is negative. The legal unit of thermal energy and
heat is the joule (J). Other units: the calorie (cal), 1 cal = 4.1868 J; the therm, 1 therm = 106 cal.
Exercise: What volume of water at 60°C must be added to 100 l of water at 20°C to obtain a bath at
35°C? II: SPECIFIC HEAT OR SPECIFIC THERMAL CAPACITY The specific heat C of a body is
the quantity of heat that must be supplied (or taken) from the unit of mass of this body so that its
temperature rises (or drops). lowers) by 1 K (or 1 °C).

The specific heat unit is J.kg-1.K-1 or J.kg-1.°C-1. Body c (J.kg-1.K-1) Body c (J.kg-
1.K-1) water 4.1855.103 Aluminum 0.92.103 ice 2.1.103 Iron 0.75.103 water steam
1.9.103 Air 1.103 Exercise : How much heat must be supplied to a metal vase
weighing 190 g to raise its temperature from 21°C to 41°C? In the interval considered,
the specific heat of the metal is 380 J.kg-1.K-1. III: THERMAL CAPACITY. WATER
VALUE. The product mc is called the heat capacity C of a body: C = mc unit of C: J.K-
1. The water equivalent (or water value) of a system is the mass of water µ
exchanging the same amount of heat with the outside when it undergoes the same
temperature variation: m.c.T = µ.ce.T IV: LATENT HEAT If we have our system that
exchanges heat with the outside, its temperature can remain constant: the heat is
used for something else, for example to make it change state. The heat involved is
then called latent heat. Latent heat is the heat exchanged with the outside during a

change of state of the system. It is noted L. Q = m.L L is expressed in J.kg-
1. V: CALORIMETRY Calorimetry is the science concerned with measuring
the quantities of heat. It is based on the principle of equality of heat
exchange: when two bodies only exchange heat, the quantity of heat
gained by one is equal to that lost by the other (in absolute value).
Exercise: A 1000 g block of aluminum at 80°C is immersed in 1 liter of water
at 20°C. The final temperature is 30.4°C. What is the specific heat of
aluminum? For these measurements, a device is used: the calorimeter. It is
an enclosure that can be considered as thermally insulating. In Berthelot's
calorimeter, the experiment is carried out inside a container called a
calorimetric vessel which contains the calorimetric liquid. This vase is
placed in an insulating enclosure. A second type of calorimeter is the Dewar
calorimeter: the container has a double wall of glass, between which a
vacuum is made. Thermos bottles are the home application of the vase
Dewar. Method of mixtures: Into a Berthelot calorimeter, of water value
µ, a mass m of water is poured, the whole being at the temperature Ti.
We then put the body whose specific heat c' we want to determine, its
temperature being Ti' and its mass m'. We wait for equilibrium to occur,
that is to say for the temperatures of the two bodies to be equal: we
will denote it by Tf. We will therefore have: - m'.c'(Tf - Ti') = (m + µ)ce(Tf -
Ti)

25
CONTENT
• Introduction
• Conduction
• Convection
• Radiation
• Differences
• Conclusion
• References

26
What is Actually Happening During Cooking Process?
Do You Ever Wonder?

27
INTRODUCTION
The heat transferred during cooking is usually undermined and taken for granted by
a lot of people but this process of heating food also known as heat transfer is an
interesting and an intricate process.
Cooking is the transfer of energy from a heat source to food. This energy alters
food’s molecular structure. Changing the taste, aroma and appearance. Foods can
be cooked using various methods, the method selected gives the finished product
a specific texture, aroma and flavor.
In cooking, heat transfer refers to heating your food items through a cooking
appliance, such as a stove, fryer, microwave, or oven.

28
HEAT TRANSFER
Heat transfer is an exchange of thermal energy between two objects. The rate of
heat transfer depends upon the temperatures of each entity and the medium
through which the thermal energy is being transferred.
Heat transfer is a very important aspect of the cooking process. Heating food
destroys potentially harmful bacteria and other microorganisms, which makes
food safe to eat and easier to digest. When food or liquids become hot, their
molecules absorb energy, begin vibrating rapidly, and start to bounce off of
each other. As they collide, heat energy is produced and transferred, which
warms and cooks food.

HEAT TRANSFER MECHANISMS
• Heat is the form of energy that can be transferred from one system to another as a result of
temperature difference. A thermodynamic analysis is concerned with the amount of heat transfer
as a system undergoes a process from one equilibrium state to another. The science that
deals with the determination of the rates of such energy transfers is the heat
transfer.
• The transfer of energy as heat is always from the higher-temperature
medium to the lower-temperature one, and heat transfer stops when the two mediums reach the
same temperature.
• Heat can be transferred in three different modes: conduction, convection,
and radiation. All modes of heat transfer require the existence of temperature difference, and all
modes are from the high-temperature medium to a lower-temperature one.

30
Methods of Heat Transfer during cooking
• Conduction
• Convection
• Radiation
Each of these methods of heat transfer features its own
unique characteristics, but there is some crossovers
between the different types.

31
HEAT TRANSFER BY CONDUCTION DURING COOKING
• The process by which heat or electricity is transferred from the cooking vessel to the
ingredients
• Conduction, in other words, is the process of thermal transfer between a hot object and a
neutral object when they come in direct contact with each other.
• When a substance is heated, particles will gain more energy, and vibrate more.
• These molecules then bump into nearby particles and this allows for the energy to be
transferred.
• In order for food to be in uniform contact with heat, fat or oil is used during cooking.
• Examples of cooking method in which means of heat transfer is mainly conduction:
grilling, boiling, frying
• It’s ideal for cooking methods like searing, sautéing, and pan-frying, which help you
achieve an aromatic and flavorful browning on your foods thanks to the Maillard reaction.

Conduction
• Based on the principle that adding heat to molecules increases their kinetic
energy, thus increases their ability to transfer heat to neighboring molecules
• There is transfer of heat through direct contact from one object or substance to
another
• Transfer can occur in any of the three states: solid, liquid, or vapor
• Heat is transferred from a heat source (gas stove/electrical appliance), through a
cooking utensil to food.
• In preparing foods on a cooker, heat is transferred by conduction
• Heat from the electric coil or gas flame is conducted to the pan or fryer and then
to the food or liquid
• In some cases, the cooking utensil is the conductor; while others, the fat (shallow-
frying) or water (boiling) are the conductor
33

FACTORS THAT AFFECT CONDUCTION
The material of the pan greatly affect the speed and efficiency of heat transfer
– Copper is an excellent heat conductor and is often used to line the bottom of
stainless steel pans
– Iron and aluminum are also effective conductor of heat and thus good for making
cooking utensils
– Stainless steel is not as effective as a heat conductor. It is a metal alloy, in which
chromium is added. Chromium oxide forms on the surface of stainless steel to
prevent it from corrosion, rusting or staining with water
– Copper and aluminum are excellent conductors of heat. They make for cookware
that heats up evenly and responds quickly to sudden changes in the heat dial.
34

35
FACTORS THAT AFFECT CONDUCTION
• Temperature difference. The greater the difference in temperature
between the two ends of the bar, the greater the rate of thermal
energy transfer, so more heat is transferred. The heat, Q, is
proportional to the difference in temperature:
• Cross-sectional area of the cooking material: In general, the amount
of heat conducted, Q, is proportional to the cross-sectional area, A,
• Length (distance heat must travel):
• Time
• Nature of the food
The general equation of the factors that affect conduction is given as

36
CONVECTION COOKING
Convection combines heat transfer and circulation to force molecules in the air to move from
warmer areas to cooler ones. As the molecules closest to the heat source become warm, they
rise and are replaced by unheated molecules. There are two types of convection;
 Natural convection: It occurs when molecules at the bottom of a cooking vessel rise and
warm while cool and heavier molecules sink. This creates a circulating current that evenly
distributes heat throughout the substance being prepared. For example; when a pot of
water is placed on a stove to boil, conduction transfers heat from the pot and into the
water molecules in contact with the interior of the pot. As these molecules heat,
convection causes them to move away from the interior of the pot as they are replaced by
cooler molecules. This continuous current creates convection heat transfer within the
water.

37
 Mechanical convection: It occurs when outside forces circulate heat , which shortens cooking times and
cooks food more evenly. Examples of this include stirring liquid in a pot or when a convection oven uses a
fan and exhaust system to blow hot air over and around the food before venting it back out.
Cooking techniques involving convection
 Boiling : -When water is heated from below, thermal expansion occurs due to which the lower layers of the
water become less dense due to overheating.
-Buoyancy causes the less dense and hotter part of the water to rise and the cooler and denser water takes its
place
-This process is repeated until the water becomes uniformly heated. In this way heat is transferred by
convection.
Convection Steam Cooking: - It brings together two powerful cooking techniques: Convection Cooking
and Steam Cooking. Let’s take a look at each and how they work together:
-Convection Ovens: Circulate the heat with a fan to produce a more even heat in the oven cavity. It keeps
the temperature inside the oven steady, thus eliminating hot spots. The circulation of the air also transfers heat
faster to the food(when cooking or baking in the oven).

38
Steam Ovens: Cook with the vapor from heated water. These ovens have a reservoir that must be filled with water at all
times. The water is pulled from the reservoir and heated by a built-in heating element . Just enough water is pulled from the
reservoir , turned into steam, and then vented into the oven cavity. Cooking food with steam is the fastest cooking method
other than pressure cookers( which is when the container is pressurized to get the steam and contents very hot , thus cooking
very fast). Steam is also the most economical way to cook because it uses minimal water and less energy with less cooking
time . One of the main benefits of preparing a meal with steam is retaining the nutrients in the food.
 Baking: Here the convection oven uses fans to circulate hot air around the product placed on racks in the baking
chamber. This is the convective component of heat transfer. Convection ovens are perfect for baking small-sized goods
such as pastry which is baked free-standing on sheet pans or perforated racks.

39
Radiation
• Heat radiation is another way to dissipate energy.
• Any system or body with temperature above absolute zero emits
electromagnetic radiation.
• Heat radiation does not need a material in which to propagate and
can travel through a vacuum.
• Also heat transfer by radiation does not require a temperature
gradient to proceed and there occurs constantly through out nature.
• In food engineering heat transfer by radiation is limited to the use of
infrared radiation, dielectric heating and microwave heating.

40
Infrared Heating
• Use electromagnetic radiation with wave lengths between visible
lights and radio waves (0.76 – 1000µm).
• The infrared source has a high temperature (500 – 3000oc).
• Heat transfer by convection is also taking and cannot be ignored
unless the process takes place under vacuum as in freeze drying.
• Penetration of this radiation is poor therefore the heating effect is on
the surface and through the body is by conduction and convection.
• Although infrared radiation heat transfer is always present in heat
transfer processes, generally its effects are not as important as the
effect of convection.
Example
- Grilling

41
Dielectric Heating and Microwave
• Dielectric heating such as microwave or radio frequency heating is generally
used when large objects need to be quickly and uniformly heated.
• The principle here is the ability of some wavelengths to induce vibration in
some dipolar molecules such as water, producing intermolecular friction and
there by increasing the temperature.
• Using these methods food products are heated volumetrically, that is, at all
points at the same time.
• The frequency for radio frequency heating are in the range of 10 to 100
MHz.
• While the frequency used for microwave heating are 915 to 2450 MHz.
• These methods involve ‘instantaneous’ generation of heat within the food
product.
• Conduction and convection take command after the heat is generated

Some examples of radiation
• BROILING- is cooking by exposing food directly to radiant heat.
Broiling differs from roasting and baking in that the food is turned
during the process to cook one side at a time.
• GRILLING
• Microwaving food to eat.

43
Differences between Conduction, Convection and
Radiation heat transfer
Basis of
Comparison
Conduction Convection Radiation
Represent How heat travels
between objects in
contact
How heat passes
through fluids
How heat flows
through empty
spaces
Cause Due to temperature
difference
Due to density
difference
Occurs from all
objects at (T>0K)
Occurrence In solids, through
molecular collision
In liquids or gases,
by actual flow of
matter
At a distance and
does not heat the
intervening
substance

44
Differences between Conduction,
Convection and Radiation heat transfer
Basis of
Comparison
Conduction Convection Radiation
Transfer of heat Uses heated solid
substance
Uses intermediate
substance
Uses
electromagnetic
waves
Speed Slow Slow Fast
Examples Stir frying,
simmering
Boiling, steaming,
baking, roasting,
stewing, braising,
deep frying
Broiling, grilling,
microwave cooking

45
CONCLUSION
• Regardless of the method used in cooking (a gas plate, a
convection oven, or a heavy-duty microwave)conduction,
convection, and radiation are all around us. Knowing and
understanding what heat transfer is, how it works, and
which type of heat transfer is happening while cooking can
help better understand the science of cooking and improve
cooking skills.

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