Engineering property teaching material

CHAPTER ONE
Introduction
Out lines :
➢ Definition of food properties
➢ Classification of food properties
➢ Application of food properties
➢ Structural levels in foods

Introduction
Definition of Food Properties
➢ A property of a material is any observable attribute or characteristic of that
material.
➢ The state of a system or material can be defined by listing its properties.
➢ A food property is a particular measure of the food’s behavior as a matter, its
behavior with respect to energy, its interaction with the human senses, or its
efficacy in promoting human health and well-being.

Introduction
➢ Food properties, in turn, define the food functionality
➢ Food functionality refers to the control of food properties that provides a desired
set of organoleptic properties, wholesomeness (including health-related
functions), as well as properties related to processing and engineering, in
particular, ease of processing, storage stability, and minimum environmental
impact

Introduction
Classification of Food Properties
It is proposed that food properties can be classified into four classes:
✓ Physical & physicochemical properties
✓ Kinetic properties
✓ Sensory properties
✓ Health properties

Introduction
1. Physical & physicochemical properties
A. Mechanical properties
B. Thermal properties
C. Thermodynamic properties
D. Mass transfer properties
E. Electromagnetic properties

Introduction
Mechanical properties:
❖ Mechanical properties are related to food’s structure and its behavior when
physical force is applied. These are:
✓ Acoustic properties
✓ Mass–volume–area-related properties
✓ Morphometric properties
✓ Rheological properties
✓ Structural characteristics
✓ Surface properties

Introduction
Thermal properties:
Thermal properties are related to heat transfer in food. These are:
✓ Specific heat
✓ Thermal conductivity
✓ Thermal capacity
✓ Thermal diffusivity
✓ Surface conductance
✓ Emissivity, Transmissivity &Absorptivity

Introduction
Thermodynamic properties:
Thermodynamic properties are related to the characteristics indicating phase or state
changes in food. These are:
➢ Water activity
➢ Sorption Isotherm properties
➢ Glass transition

Introduction
Mass transfer properties:
Mass transfer properties are related to the transport or flow of components in food.
It includes:
➢ Drying
➢ Extraction
➢ Distillation
➢ Absorption etc.

Introduction
Electromagnetic properties:
Electromagnetic properties are related to the food’s behavior with the interaction of
electromagnetic energy.
➢ dielectric constant
➢ dielectric loss
➢ electrical resistance

Introduction
2. Kinetic properties
▪ Kinetic properties are kinetic constants characterizing the rates of changes in
foods.
▪ These can be divided into two groups.
1. Kinetic constants characterizing the rates of biological, biochemical, chemical,
physicochemical, and physical changes in food.
2. Kinetic constants characterizing the rates of growth, decline, and death of
microorganisms in food.

Introduction
3. Sensory Properties:
▪ A sensory property can be defined as the human physiological–psychological
perception of a number of physical and other properties of food and their
interactions.
▪ Sensory properties can be subdivided into:
o tactile properties
o textural properties
o color and appearance
o taste, odor, and sound.

Introduction
4. Health Properties
• Health properties relate to the efficacy of foods in promoting human health and
well-being. Foods have positive or negative impacts on health.
• Positive effects can be subdivided into:
✓ Nutritional properties: defined by nutritional composition tables
✓ Medical properties: these are those that prevent and treat diseases
✓ Functional properties: these are those that impact on an individual’s general
health, physical well-being, and mental health, and slow the aging process

Introduction
• Negative health properties are grouped as:
✓ toxic at any concentration
✓ toxic above a critical concentration level
✓ Toxic at excessive or unbalanced intake.

Introduction
Applications of Food Properties in Food engineering
Knowledge of food properties is necessary for:
o defining and quantifying a description of the food material,
o providing basic data for food engineering and unit operations, and
o predicting behavior of new food materials
The application of food properties are:
✓ Process design and simulation
✓ Quality and safety, and Packaging design

Introduction
Food structural levels
Food structure can viewed at three levels:
1. Molecular level
2. Microscopic level
3. Macroscopic level
These structural levels can be applied in:
✓ The study and evaluation of food texture and quality
✓ The analysis and correlation of the transport properties of foods, such as rheology
(viscosity), thermal properties (conductivity/diffusivity), and mass transfer

Introduction
Molecular level:
At the molecular level, food biopolymers (proteins, carbohydrates, and lipids) of
importance to transport properties are:
➢ structural proteins (collagen, keratin, and elastin),
➢ storage proteins (albumins, globulins, prolamins, and glutenins)
➢ structural polysaccharides (cellulose, hemicelluloses, pectins, seaweed, and plant
gums)
➢ storage polysaccharides (starch–amylose and amylopectin)
➢ Lignin (plant cell walls).

Introduction
Microscopic level:
• The microscopic level refers to the cells of food materials that contain several
components, essential in living organisms, such as water, starch, sugars, proteins,
lipids, and salts.
• As food materials undergo different processes, their microstructure may be
preserved or destroyed for the production of useful processed products (e.g.,
refining of starch, oil seeds, sugars, grain, and milk), while other processes such
as freezing, crystallization, milling, and emulsification cause changes in the food
material structure

Introduction
Macroscopic level:
Macroscopic level structural properties can be defined as quantitative parameters of
physical meaning, for the characterization of structural changes of foods during
processing and storage. These are:
✓ Particle size
✓ Shape
✓ Density
✓ Porosity and Shrinkage
These are strongly related to the transport properties of solid and semisolid food.

Chapter two
Water activity and moisture sorption

Introduction
Definition:
o Water has a chemical formula of H2O which represents two hydrogen atoms
covalently bound to one oxygen atom. Water is an odorless, tasteless and
transparent liquid at room temperature.
o Water is the most abundant molecule in food and is an essential ingredient to
support life and since all foods come from living organisms, water is an essential
component of foods.

Cont.…
It may occur as:
➢ an intracellular or extracellular component in vegetable and animal products,
➢ as a dispersing medium or solvent in a variety of products,
➢ as the dispersed phase in some emulsified products such as butter and margarine,
and
➢ as a minor constituent in other foods.

Water content of foods
➢ The physicochemical properties of water are important considerations in
understanding and showing how water contributes to food processing.
➢ The exceptionally high values of the thermodynamic parameters (energy to thaw
ice and convert water to steam) of water are of importance for food processes and
operations such as freezing and drying.
➢ The considerable expansion of water during freezing may contribute to structural
damages to foods when they are frozen.

Cont.…
Here is water content of some common foods.
Food type Water availability
Tomatoes, lettuce 95%
Apple juice, milk 87%
Potato 78%
Meats 65-70%
Bread 35%
Honey 20%
Rice, wheat flour 12%
Shortening 0

Cont.…
✓ The presence of water influences the chemical and microbiological deterioration
of foods.
✓ Also, removal (drying) or freezing of water is essential to some methods of food
preservation.
✓ Fundamental changes in the product may take place in both instances.

Forms of water
Commonly three basic forms of water.
1. Free water (capillary water or Type III)
o Water that can be easily removed from a food
o Water that is responsible for the humidity of a food
o Water from which water activity is measured

Cont.…
2. Bound water (adsorbed or Type II)
✓ Water that is tied up by the presence of soluble solids
✓ Salts, vitamins, carbohydrates, proteins, emulsifiers, etc.
3. Water of hydration (Structured or Type I)
✓ Water held in hydrated chemicals
✓ Na2SO4 . 10H2O

Water works
➢ Water must be “available” in foods for the action of both chemical and enzymatic
reactions.
➢ The “available” water represents the degree to which water in a food is free for:
o Chemical reactions
o Enzymatic reactions
o Microbial growth

Cont.…
o Quality characteristics
→Related to a simple loss of moisture
→Related to gel breakdown
→Food texture (gain or loss)
➢ Very important (#1 ingredient in many foods)
➢ Structure
→Polar nature, hydrogen bonding

Cont.…
➢ Can occur in many forms (S,L,V)
➢ Acts as a dispersing medium or solvent
→Solubility
→Hydration
Emulsions
Gels
Colloids

Cont.…
➢ The amount of “free” water, available for these reactions and changes is
represented by Water Activity.
➢ As the percentage of water in a food is “bound” changing from its “free” state,
the water activity decreases
➢ Water Activity is represented by the abbreviation: Aw
Aw = P/ Po
P = Vapor pressure of a food
Po = Vapor pressure of pure water

Cont.…
➢ Vapor pressure can be represented as equilibrium RH
➢ Any food substance added to water will lower water activity….so, all foods have
a water activity less than 1.0

Water Activity
o Water activity and sorption properties of foods is important physical properties
in food formulations and processes.
o Most of the biochemical and microbiological reactions are controlled by the
water activity of the system,
› It is thus useful parameter to predict food stability and shelf life.

Cont.…
➢ Water content alone is not a reliable indicator of stability.
➢ Differences in the intensity with which water is associated with nonaqueous
constituents
→water engaged in strong associations would be less likely to be able to support
degradative activities such as growth of microorganisms and hydrolytic chemical
reactions, than would weakly associated water
→Water Activity” (aw) was developed to reﬂect the intensity with which water
associates with various nonaqueous constituents.

Cont.…
➢ Food stability, safety, and other properties can be predicted far more reliably
from aw than from water content
➢ aw correlates sufficiently well with rates of microbial growth and rates of many
degradative reactions to make it a useful indicator of potential product stability
and microbial safety

Water activity of different foods

Cont.…
➢ Water activity (aw)is, at a given temperature, the ratio of the
fugacity, fw, of water in a system, and its fugacity, fo
w, in pure liquid
water at the same temperature.” i.e.
𝐴𝑤 =
𝑓𝑤
𝑓𝑤
0 𝑇
➢ Fugacity is measure of escaping tendency of a substance. It can be
replaced by vapor pressure, p, provided that the vapor pressure
behaves as an ideal gas.

Cont.…
➢ Substance “activity”: is a ratio of the fugacity of the solvent (f) to the fugacity of
the pure solvent (fo) in a defined standard state.
➢ At low pressures (e.g., ambient) the difference between f /fo and p/po is less than
1%, so
𝐴𝑤 =
𝑃
𝑃0
𝑇
NB! This equality is based on the assumption of thermodynamic equilibrium.

Cont.…
➢ and Relative Vapor Pressure (RVP) are the same and can
interchangeably be used
 RVP thus, does not imply equilibrium

s Moisture sorption isotherm.…

s How to Use the Isotherm
Moisture sorption isotherms
 Shows the relationship between water activity and moisture at a given
temperature (the two are not equivalent)
 Represent moisture content at equilibrium for each water activity
 Allow for predictions in changes of moisture content and its potential effect on
water activity
 If the temperature is altered, then the relationships can not be compared
equivalently

s Influences on Water Activity
 Foods will naturally equilibrate to a point of equilibrium with its environment
 Therefore, foods can adsorb or desorb water from the environment
 Desorption is when a “wet” food is placed in a dry environment
 Analogous to dehydration; but not the same
 Desorption implies that the food is attempting to move into equilibrium (ie. in
a package)
 Dehydration is the permanent loss of water from a food
 In both cases, the Aw decreases

s Cont.…
 Desorption is generally a slow process, with moisture gradually decreasing until it is in
equilibrium with its environment.
 Adsorption is when a “dry” food is placed in a wet environment
 As foods gain moisture, the Aw increases
 The term “hygroscopic” is used to describe foods or chemicals that absorb moisture
 A real problem in the food industry (lumping, clumping, increases rxn rates)

s Water Activity in Practice
 Bacterial growth and rapid deterioration
 High water activity in meat, milk, eggs, fruits/veggies
 1.0-0.9
 Yeast and mold spoilage
 Intermediate water activity foods such as bread and cheese
 0.75-0.9
 Analogous to a pH < 4.6, an Aw < 0.6 has the same preservation effect

s Aw in Low Moisture Foods
 Water activity and its relationship with moisture content help to predict and
control the shelf life of foods.
 Generally speaking, the growth of most bacteria is inhibited at water activities
lower than 0.9 and yeast and mold growth prevented between 0.80 and 0.88.
 Aw also controls physiochemical reactions.
 Water activity plays an important role in the dehydration process. Knowledge of
absorption and desorption behavior is useful for designing drying processes for
foods.

s How to “Control” water
 The ratio of free to bound water has to be altered
 You can either remove water (dehydration or concentration)
 Can change the physical nature of the food
 Alter is color, texture, and/or flavor
 Or you can convert the free water to bound water
 Addition of sugars, salts, or other water-soluble agents

s Cont.…
 You can freeze the food
 This immobilizes the water (and lowers the Aw)
 However, not all foods can be or should be frozen
 Frozen foods will eventually thaw, and the problem persists

s Chemical and functional properties of water
➢ Solvation, dispersion, hydration
➢ Water activity and moisture
➢ Water as a component of emulsions
➢ Water and heat transfer
➢ Water as an ingredient

s Water activity measurements
 Methods based on the following properties can be used for measuring water
activity of foods:
 Colligative properties
 Isopiestic transfer
 Hygroscopicity of salts

s Cont.…
Measurements based on Colligative properties:
 In this method, water activity of foods can be determined by:
A. measuring the vapour pressure of water in food directly
B. using freezing point depression

s Cont.…
A. Vapor Pressure Measurement method
 This method gives a direct measure of vapor pressure exerted by sample.
 Water activity is calculated from the ratio of vapor pressure of sample to that of
pure water at the same temperature.
Vapor Pressure Measurement setup

s Cont.…
Measurement Procedure:
 A sample weighing 10 to 50 g is put into a sample flask and sealed on to the
apparatus. The air space in the apparatus is evacuated.
 After the vacuum source is isolated and equilibration for 30 to 50 min, the
pressure exerted by the sample is recorded as h1.
 The level of oil in the manometer will change by the vapor pressure exerted by
the sample.
 The sample flask is excluded from the system and the desiccant flask is opened.

s Cont.…
 Water vapor is removed by sorption onto CaSO4 and the pressure exerted by
volatiles and gases are indicated by h2 after equilibration.

s Cont.…
Measurement procedure
The pressure exerted by volatiles & gases are indicated by h2 after equilibration.
Water vapor is removed by sorption onto CaSO4
The sample flask is excluded from the system and the desiccant flask is opened.
Level of oil in manometer will change by vapor pressure exerted by the sample.
The pressure exerted by the sample is recorded as h1
after the vacuum source is isolated & equilibration for 30 to 50 min
The air space in the apparatus is evacuated.
A sample (10 - 50 g) is put into a sample flask & sealed on to the apparatus.

s Cont.…
 Then, water activity can be calculated using the following equation:
 Temp. must be constant during measurement. If the temperatures of the sample
(Ts) & vapour space in the manometer (Tm) are different, water activity is
corrected as:

s Cont.…
B. Freezing Point Depression method
 This method is applicable only to liquid foods & gives water activity
values at freezing point rather than at room temperature
 It is very accurate at water activities above 0.85
 It is suitable for materials having large quantities of volatile substances
which may cause error in vapor pressure measurement and in electric
hygrometers.

s Cont.…
 In two-phase systems (ice and solution) at equilibrium the vapor pressure of
water as ice crystals and the interstitial concentrated solution are the same and
water activity depends only on temperature.
 Thus, water activity of solution at a certain temperature below freezing can be
expressed as:
quid water
sure of li
Vapor pres
e
sure of ic
Vapor pres
aw =

s Cont.…
Isopiestic Transfer method:
 Water activity measurement in this method is achieved by equilibration of water
activities of two materials in a closed system.
 Mostly, microcrystalline cellulose is used as the reference adsorption substrate
because:
✓ It is stable in the temperature change of −18 to 80 oC with little changes in sorption
characteristics.
✓ It is stable in its sorption properties after two to three repeated adsorption and desorption
cycles.

s Isopiestic Transfer…
✓ Its sorption isotherm is in sigmoid shape and its sorption model is known.
✓ It is available as a standard biochemical analytical agent.
 This method is not recommended for samples that are susceptible to foaming
such as protein solutions during evacuation of desiccators

s Isopiestic Transfer.…
Desiccator Method

➢ To prevent mold or bacterial growth, use of aseptic techniques are suggested.
➢ Desiccators can be used for preparation of sorption isotherms.
➢ In the desiccator method,
 saturated salt solutions,
 sulfuric acid or glycerol solutions are put into the bottom of desiccators

 Although the desiccator method is very commonly used for water activity
determination and preparation of sorption isotherms, there are some errors arising
from this method
 It was shown that error comes from the disturbance of equilibrium caused by
opening the desiccators, taking the sample, and closing it again.

 These disturbances cause adsorption of water from the surrounding air by
samples with low water activities and desorption of water from samples having
high water activities
 If desorption occurs, the results are not affected significantly since desorption
occurs slowly.
 However, if adsorption occurs, water activity is affected significantly since
adsorption is a fast process.

Measurement Procedure:
The moisture content is calculated & water activity is determined from a
standard cellulose isotherm that was previously prepared with H2SO4-water
mixtures as the medium.
The cellulose is reweighed and the change in weight is recorded.
After 24 hours, the vacuum on the desiccator is slowly released.
The desiccator is closed & evacuated for about 1 min & then held at constant
temperature for 24 hours
Sample & microcrystalline cellulose are placed in vacuum type desiccator.

s Cont.…
Measurements using hygrometers:
➢ In this method, the sample is equilibrated with in a closed vessel and then the
relative humidity of the air is determined by using a hygrometer.
➢ Many hygrometric instruments work on the principle of measuring:
 Wet and dry bulb temperature
 Dew point
 Change in length of material
 Electrical resistance or capacitance of salt

s Cont.…
Measurements using hygrometers
Electrical resistance or capacitance hygrometers:
o These are based on measurement of the conductivity of salt solution that is in
equilibrium with the air. Usually LiCl is used for this purpose.
o These types of hygrometers provide rapid and reliable means of measuring water
activity.

s Hygrometer measurement.…
Electronic Sensors (Pawkit)
➢ uses a capacitance humidity sensor to measure the water activity of a sample.
➢ The sensor is suspended in the headspace of the chamber and uses a special
polymide material sandwiched between two electrodes to sense humidity
changes.
➢ The sensor converts the humidity value into a specific capacitance, which is then
measured electronically by the circuit & translated to aw

s Hygrometer measurement.…
Paw kit

s Moisture Sorption Isotherms (MSI)
Definition:
➢ MSI is a plot of water content in g H2O/g DM of a food vs. (p/po)T
➢ It is also called the equilibrium moisture content curve.
➢ Information derived from MSI are useful for:
✓ studying and controlling concentration and dehydration processes, because the ease or
difficulty of removing water is related to RVP
✓ formulating food mixtures so as to avoid moisture transfer among the ingredients
✓ determining the moisture barrier properties needed in a packaging material required to
protect any particular system

✓ determining what moisture content will curtail growth of microorganisms of interest within
a system
✓ predict the chemical and physical stability of foods as a function of changes in their water
content

MSI (for high moisture foods)

 MSI data of great interest are those in low MC.
 Omission of high MC region is normal practice
 Low MC region expanded
 Gives much more useful MSI
 Several substances have MSIs of markedly different shapes

MSI (for low moisture foods)

Legends:
1- confection
2 - spray dried chicory
3 - roasted coffee
4 - meat powder
5 - native rice starch

❖ The shapes and positions of the isotherms are determined by several factors:
 sample composition
 Molecular weight distribution and
 hydrophilic/hydrophobic characteristics of solutes
 Physical structure of the sample (Crystalline/ amorphous)
 Sample pre-treatment (grinding, drying, …)
 Temperature
 Methodology (adsorption/desorption)

 Brunauer et al., 1940 classified sorption isotherms according to their shape and
processes, establishing five different types;

s Interpreting MSIs
➢ MSI regions are divided into different zones
ZONE I:
 Most strongly sorbed and least mobile water
 This water is associated with accessible polar sites by water–ion or water–dipole
interactions
 It remains unfrozen at −40◦C
 It does not act as a solvent
 it is not present in sufficient amount to have a plasticizing effect on the solid

s Interpreting MSIs
 It behaves simply as a part of the solid
 It is an amount corresponding to just a tiny fraction of the total water content in a
high moisture food material
 This amount of water clearly is less than the potential “sorption sites” represented by
all of the polar or other active groups of the solute molecules

s Interpreting MSIs
BET Monolayer:
 It is moisture content of the food at the end of zone I (boundary of zones I and
II)
 BET Monolayer water corresponds to the amount of water needed to form a
monolayer over only the readily accessible, highly polar groups of the dry
matter

s Interpreting MSIs
ZONE 2:
 It is a second water population
 Additional water added in an amount not exceeding the limit set by the zone II
boundary is considered to populate the first-layer sites (left unoccupied by monolayer
water) that are still available
 It associates with neighbouring water molecules in this first layer and solute
molecules primarily by hydrogen bonding
 It is slightly less mobile than bulk water
 Most of it remains unfrozen at −40◦C

s Interpreting MSIs
Vicinity of the low moisture end of zone II (B):
 Moisture added in the vicinity end of zone II exerts a significant plasticizing
action on solutes
 causes incipient swelling of the solid matrix
 Exchange of all water molecules is enhanced
 leads to acceleration in the rate of most reactions due to increasing
interaction and accessibility

s Interpreting MSIs
Vicinity of the junction of zones II and III :
 the amount of water is sufficient to complete a true monolayer hydration shell
for individual macromolecules

s Interpreting MSIs
ZONE 3:
 It is a third population water
 Further addition of water causes a glass–rubber transition in samples containing
glassy regions
 Leads to very large decrease in viscosity
 Leads to a large increase in molecular mobility
 Leads to large increase in the rates of many reactions
 This water can be frozen
 It is available as a solvent and readily supports the growth of microorganisms

s Interpreting MSIs
Beyond ZONE 3:
 Beyond zone 3, additional water behaves as bulk-phase water
 Its addition to the system does not alter the properties of existing solutes

s Preparation of MSIs
 Sorption data of foods is obtained by storing a weighed sample of food in an
enclosed container maintained at a certain relative humidity, at constant
temperature, and reweighing it after equilibrium is reached.
 Theoretically, at equilibrium water activity of the sample is the same as that of
the surrounding environment.
o However, in practice a true equilibrium is never attained because that would
require an infinitely long period of time.

s Preparation of MSIs
o Therefore, the sample is weighed from time to time during equilibration.
 When the difference between successive weights of the sample becomes less than
the sensitivity of the balance being used, it is accepted that equilibrium is
reached.
 The moisture content of the sample is then determined.
 Desiccators can be used for preparation of sorption isotherms.
 In the desiccator method, saturated salt solutions, H2SO4 or glycerol solutions are
put into the bottom of desiccators

s MSI Models
 MSI Models are semi-empirical equations with two or three fitting parameters to
describe moisture sorption isotherms.
Langmuir Equation:
where C is a constant and M0 is the monolayer sorbate content.

s MSI Models
➢ Brunauer-Emmett-Teller (BET) Equation
➢ where Mw (m) is moisture content, M0 (m1) is the monolayer moisture content
and C is the energy constant related to the net heat of
sorption
➢ Monolayer moisture content represents the moisture content at which water
attached to each polar and ionic groups starts to behave as a liquid-like phase.

s Determining the Monolayer Value
 Data of low moisture end of MSI are needed
 BET equation is used:
m – moisture content
m1 – BET monolayer moisture value
c – constant
aw values of this eqn is (p/po)T value

s Exercise
A. Estimate the BET monolayer value (m1) of a food sample whose data is given
below measured at constant temperature of 20oC.
B. Sketch MSI

s Chapter three
Physical characteristics of foods

s Introduction
⚫ The characteristics of a food material that are independent of the observer,
measurable, can be quantified, and define the state of the material are considered
as its physical properties.
⚫ Physical properties describe the unique, characteristic way a food material
responds to physical treatments involving mechanical, thermal, electrical, optical,
sonic, and electromagnetic processes.

s Introduction
➢ It is important in the design of any particular machine or analysis of the
behavior of the product during process.
➢ Physical properties are important:
• Cleaning unit
• Grading unit
• Separating unit
• Handling unit
• Sorting unit
• Storing and drying system.

s Size and shape
➢ Shape is the rigid form of body while, Size is the measurement of dimensions.
Used in :-
• Shape & Size of Screen openings.
• Selection of Disk
• Adjustment of Cylinder Clearance.
• Angle of inclination.
• Vibration amplitude & Frequency of Screen.

s Parameters for determining the shape & size
Roundness:- measure of the sharpness of the solid materials.
Ap = largest projected area
of object in natural rest position (m2)
Ac = Area of the smallest
circumscribing circle (m2)
c
p
A
A
Roundness =

s Cont.…
We can also express roundness as follow.
where
r = radius of curvature
R = radius of the maximum inscribed circle (m),
N = total number of corners summed in numerator.

s Cont.…
Sphericity:-
It Expresses the characteristic shape of a solid object relative to
that of a sphere of the same volume
Sphericity, = Di/Dc
Di=dia. of largest inscribed circle
Dc=dia. of smallest circumscribed circle

s Sphericity…
✓ Almost all spherical objects have sphericity approaching 1
✓ The value of sphericity ranges from 0 to 1.
✓ An orange has sphericity of the order of 0.95 and
✓ Paddy has 0.130 to 0.1520

s Methods for measuring Size
o Vernier calliper :- least count 0.01 cm
o Micrometer Method :- least count 0.01mm
o Projection Method :- Ac = (A1+A2+A3)
o Travelling Microscope
o Sieve Analysis Method

s Instruments for Measuring Shape & Size
Vernier calliper

s Particle size distribution
⚫ If the particle size ranges are all equal, the data can be plotted directly.
⚫ However, it gives a false impression if the covered range of particle sizes differs
from increment to increment.
⚫ Less material is retained in an increment when the particle size range is narrow
than when it is wide.
⚫ Therefore, average particle size or size range versus Xi
w/Dpi+1 − Dpi should be
plotted, where Xi
w is the mass fraction and (Dpi+1 − Dpi)
⚫ is the particle size range in increment I - D
D
X
pi
pi
w
i
1
+

B. Cumulative Analysis
⚫ Cumulative analysis is obtained by adding, consecutively, the individual
increments, starting with that containing the smallest particles and plotting the
cumulative sums against the maximum particle diameter in the increment in
percentage.
⚫ In a cumulative analysis, the data may be represented by a continuous curve

Typical screen analysis

Cumulative size analysis

⚫ In a sample of uniform particles of diameter Dp, the number of particles in the
sample is:
Where
N = the number of particles
m = mass of the sample (Kg)
p = density of the sample (Kg/m3)
Vp = volume of one particle (m3)
p
pV
m
N

=

⚫ Thus, if the particle density and sphericity are known, the surface area of the
particles in each fraction may be calculated as:
⚫ The surface area results for each fraction are added to give the specific surface
area of mixture.
⚫ The specific surface area is defined as the total surface area of a unit mass of
particles.
p
p
p
D
m
NS
A


=
=
6

⚫ For constant density (ρp) and sphericity (), specific surface area (Aw) of the
mixture is:
where
i = subscript showing individual increments,
Xi
w = mass fraction in a given increment,
n = number of increments,

=

=
n
i pi
w
i
p
w
D
X
A
1
6


= average particle diameter taken as the arithmetic mean of the smallest and
largest particle diameters in the increment and expressed as:
⚫ Various types of particle size distribution can be defined, depending on the
parameter by which the individual diameters are measured:
– If the particles are counted, the result is the ‘arithmetic’ or ‘number’ PSD.
– If the particles are weighed (as they are in sieving analysis), the result is the ‘mass’ PSD.
pi
D
2
)
1
( −
+
=
i
p
pi
pi
D
D
D

⚫ Assuming that the true density of the particles is uniform, this is also the ‘volume’
PSD.
⚫ Similarly, ‘surface’ and ‘surface/volume’ PSDs can also be defined.
⚫ Each type of PSD provides a different type of ‘mean particle diameter’ .

⚫ Average particle diameter of a mixture can be calculated in different
ways
– Volume surface mean diameter (Sauter mean diameter),
– Mass mean diameter,
– Arithmetic mean diameter,
– Volume mean diameter,
s
D
w
D
N
D
V
D

⚫ Volume surface mean diameter
⚫ Mass mean diameter
⚫ Volume mean diameter

=
= n
i pi
w
i
D
X
s
D
1
1

=
=
n
i
w
i
pi
w X
D
D
1
3
/
1
1
3
)
(
1












=

=
n
i pi
i
w
V
D
X
D

s Exercise
1. Wheat flour is made by grinding the dry wheat grains. Particle size is an
important characteristic in many of the wheat products. For example, in making
wafers, if the flour is too fine, light and tender products are formed. On the other
hand, incomplete sheets of unsatisfactory wafers are formed if the flour is too
coarse. Therefore, it is important to test the grinding performance of flour by
sieve analysis in wafer producing factories.

s Exercise cont.…
⚫ Determine:
– Show tabulated differential Analysis results
– Volume surface mean diameter
– Mass mean diameter,
– Volume mean diameter
by differential analysis using the data given in given table

s Exercise cont.…
Sieve Analysis of Wheat Flour

s CHAPTER FOUR
MECHANICAL PROPORTIES

s Introduction
❑The mechanical properties mainly result from the structure, physical state, and
rheology.
❑They can be subdivided into two groups: structural and geometrical properties,
and strength properties.
❑Structural and geometrical properties include mass–volume–area-related
properties (density, shrinkage, and porosity), and morphological properties (surface
area, roundness, and sphericity).

s Introduction
❑Strength properties are related to solid and semi-solid stress and deformation, and
intervene in food texture and rheological characterization.
❑These properties are needed for process design, estimating other properties,
characterizing foods, and quality determination.

s Cont.…
Density:-
is defined as mass per unit volume (the SI unit of density is kg/m3). Indeed, there
are different forms of density such as true, material, particle, apparent, and bulk that
can be used, depending on its application in process calculations or product
characterization

s Cont…
A. True Density
True density (ρT) is the density of a pure substance or a composite material
calculated from its components’ densities (excludes the pores and voids) considering
conservation of mass and volume.
B. Material Density
Material density (ρm) is the density measured when a material has been
thoroughly broken into pieces small enough to guarantee that no closed pores
Remain.

s Cont.…
C. Particle Density
Particle density (ρp) is the density of a particle, which includes the volume of all
closed pores but not the externally connected pores.
D. Apparent Density
Apparent density (ρa) is the density of a substance including all pores remaining in
the material .

s Cont.…
E. Bulk Density
Bulk density (ρB) is the density of a material when packed or stacked in
bulk. The bulk density of packed materials depends on the geometry, size, and
surface properties of individual particles.

s Cont.…
Porosity:-
➢ indicates the volume fraction of void space or air space inside a material. Volume
determination is relative to the amount of internal (or closed) or external (or
open) pores present in the food structure.
➢ Therefore, like density, different forms of porosity are also used in food
processing studies, namely open pore, closed pore, apparent, bulk, and total
porosities.

s Cont.…
➢ mathematically it can be expressed as :
Different forms of porosity are used in food process calculations and food products
characterization.

s Cont.…
A. Open Pore Porosity
✓ Open pore porosity is the volume fraction of pores connected to the
exterior boundary of a material and is given by (εop):
B. Closed Pore Porosity
Closed pore porosity (εcp) is the volume fraction of pores closed inside the material
and not connected to the exterior boundary of the material. It can be defined as:

s Cont.…
C. Apparent Porosity
Apparent porosity is the volume fraction of total air or void space in the material
boundary and is defined as (εa = εop + εcp):

s Cont.…
D. Bulk Porosity
Bulk porosity (εB) is the volume fraction of voids outside the boundary of
individual materials when packed or stacked as bulk:

s Cont.…
E. Bulk-Particle Porosity
Bulk-particle porosity is the volume fraction of the voids outside the individual
particle and open pore to the bulk volume when packed or stacked as bulk.

s Cont.…
F. Total Porosity
Total porosity is the total volume fraction of air or void space (i.e., inside and
outside of the materials) when material is packed or stacked as bulk.

s Cont.…
Surface area:-
Two types of surface area are used in process calculations: outer boundary surface
of a particle or object, and pore surface area for a porous material.

s Cont.…
volume :-
Mainly two types of volumes.
1. Boundary Volume
Boundary volume is the volume of a material considering the geometric boundary. A
material’s volume can be measured by buoyancy force; liquid, gas, or solid
displacement; or gas adsorption; it can also be estimated from the material’s
geometric dimensions.

s Cont.…
2. Pore volume
Pore volume is the volume of the voids or air inside a material.

s CHAPTER FIVE
Rheology of Foods

s Out lines
✓ Introduction: Definition
✓ Classification: Flow and Deformation
✓ Flow of materials
✓ Viscos fluids: Newtonian and Non-Newtonian
✓ Plastic fluids: Bingham and Non-bingham

s Introduction
Definition :
➢ “Rheology” comes from Greek rheos, meaning ‘to flow’
➢ The Greek philosopher Heraclitus described rheology as panta rei - everything
flows (if you wait long enough!)
➢ Rheology aims at measuring those properties of materials that control their
deformation and flow behavior when subjected to external forces

s Introduction
➢ The subject of rheology is concerned with the study of deformation and flow of
matter
➢ When subjected to external forces, solids (or truly elastic materials) will
deform, whereas liquids (or truly viscous materials) will flow.

s Introduction
➢ Rheological properties are defined as mechanical properties that result in the
flow and deformation of material in the presence of a stress.
➢ Rheological properties are used:
✓ in product quality evaluation
✓ engineering calculations
✓ process design
✓ to determine the size of the pump and pipe and the energy requirements

s Application areas of food rheology
A. Cereals
❖ Viscosity , elastic modulus and tensile strength are the out standing factors in
determining the behavior of wheat flour dough.
❖ Compressibility, crispness and breaking strength of baked product play a role in
acceptability by the consumer.
❖ Kernel hardness : Methods to determine wheat hardness include determination
of :

✓ Power to grind a sample.
✓ Time to grind under specified conditions on a burr-type mill.
✓ Resistance to grinding.
✓ Particle size of ground wheat. Recording dough miners : types of recording
dough miners such as:
Barbender farinograph.
Mixo graph.
✓ Barbender farinograph : Measures plasticity and mobility of dough subjected to a
prolonged , relatively gentle mixing action at constant temperature.

B. Starch
➢ For the evaluation of starch products three aspects of starch rheology are
important:
✓The alterations of rheological properties during pasting.
✓ Hot paste viscosity and its variations with time.
✓Change in rheological properties during and after coding of the paste

C. Dairy products
Butter:
The resistance to deformation under its own weight is an essential quality
of butter.
✓ Investigation have been under way to measure instrumentally some of rheological
properties of finished butter.
✓ The different methods of measurement include the penetrometer method, sectility
method, extrusion method, single bean method, spread ability.

Cheese :
✓ The rheological behavior of cheese, milk, and curd have been evolved.
✓ Plastic bowl and new torsiometer for measuring the firmness of the coagulation
before cutting
✓ Ball compressor or hardness tester have been developed to the stage where they
can be usefully employed in cheese making.

D. Meat
o Tenderness is the most important factor affecting consumer evaluation of meat
quality and acceptability,
o Meat tenderness devices are divided into four types depending on upon their
mode of action,
1. Shear.
2. Penetration.
3. Biting.
4. Mincing.

E. Fruits and vegetables
The instruments used for evaluating texture of fruits and vegetables are ,
✓ Compressimeter,
✓ Penetrometer,
✓ Shear –testing devices, consumption during grinding.
Smaller extent instruments measuring energy.

s Flow of material
Newton’s Law of Viscosity:
➢ Let us take a pair of large parallel plates, each one with area A and the plates are
separated by a distance Y. In the space between them is a fluid either gas or
liquid.

s Newtons law cont.…
➢ When the final sate of steady motion has been attained a constant force F is
required to maintain the motion of the lower plat .
➢ This means, the force is proportional to the area and to the velocity difference
between the plate and inversely proportional to the distance between the plates

s Newtons law cont.…
𝑭
𝑨
= 𝝁
𝑽
𝒀
➢ The microscopic form of this equation is known as Newton’s law of viscosity:
𝝉𝒚𝒛 = −𝝁
𝒅𝑽𝒛
𝒅𝒚
= −𝝁𝜸𝒚𝒛

s Exercise
1. Two parallel plates are 0.1 m apart. The bottom plate is stationary while the
upper one is moving with a velocity V. The fluid between the plates is water,
which has a viscosity of 1 cp.
(a) Calculate the momentum flux necessary to maintain the top plate in motion at a
velocity of 0.30 m/s.
(b) If water is replaced with a fluid of viscosity 100 cp, and momentum flux remains
constant, find the new velocity of the top plate.

s Viscosity
➢ Viscosity is defined as the resistance of a fluid to flow.
➢ The unit of dynamic viscosity is (Pa · s) in the SI system and poise (g/cm · s) in
the CGS system.
➢ Viscosity varies with temperature.
➢ Viscosity of most of the liquids decreases with increasing temperature.

s Viscosity cont.…
➢ The temperature effect on viscosity can be described by an Arrhenius type
equation:
𝝁 = 𝝁∞ 𝒆𝒙𝒑
𝑬𝟎
𝑹𝑻
➢ Liquid molecules are closely spaced with strong cohesive forces between them.
➢ Viscosities of liquids show little dependence on density, molecular velocity, or
mean free path.

s Viscosity cont.…
➢ In most liquids, viscosity is constant up to a pressure of 10.134MPa, but at higher
pressures viscosity increases as pressure increases.
➢ In gases, in contrast to liquids, molecules are widely spaced and intermolecular
forces are negligible.
➢ In most gases, viscosity increases with increasing temperature, which can be
expressed by the kinetic theory.

s Viscous Fluids
➢ Viscous fluids tend to deform continuously under the effect of an applied stress.
➢ They can be categorized as Newtonian or non-Newtonian fluids.

s Newtonian Fluids
✓ Fluid that experiences shear stress that is linearly correlated to strain rate
✓ Newton law’s of viscosity
✓ All gases and water, milk and dilute solutions of low molecular weight solutes
composed of small molecules (up to molecular weight of 5000)

s Non Newtonian
✓ Fluid where the viscosity varies based on applied stress
✓ Don’t obey Newton law’s of viscosity
✓ Most structurally complex fluids including suspension, emulsions, paste and
some biological fluids

s Newtonian & non Newtonian cont.…
➢ In the case of flow inside cylindrical pipes, laminar regime prevails for Re 2300
approximately.
➢ A laminar regime is common in food processes where the velocities are relatively
low and the viscosities relatively high.
➢ The viscosity of liquids is strongly temperature-dependent and almost pressure
independent.
➢ The viscosity of gases increases with pressure and decreases slightly with increasing
temperature.

s Viscosity measurement
Most commonly used viscosity measurement devices are:
✓ Capillary flow viscometers
✓ Orifice type viscometers
✓ Falling ball viscometers
✓ Rotational viscometers

A. Capillary Flow Viscometers
⚫ Capillary flow viscometers are generally in the form of a U-tube.
⚫ These types of viscometers are very simple,
inexpensive, and suitable for low-viscosity fluids.
⚫ There are different designs of capillary viscometers.

Measurement procedures:

⚫ Assuming that the flow is laminar, fluid is incompressible, velocity of the fluid at
the wall is zero (no-slip condition) & end effects are negligible
⚫ Macroscopic force balance eqn for a fluid flowing through a horizontal
cylindrical tube of length (L) and inner radius (r), is:
⚫ Where P is the pressure drop causing flow and  is the shear stress resisting
flow.
πrL
τ
ΔPπr 2
2
=

⚫ From force balance eqn, shear stress can be solved:
For a Newtonian fluid
⚫ Both shear stress and shear rate vary linearly from zero at the centre (r = 0) of the
capillary to a maximum at the wall (r = R).
– this results in the parabolic velocity profile
⚫ The shear stress on the fluid at the wall (w) is related to the pressure drop along the
length of the tube:
2L
ΔPr
τ =
L
PR
w
2

=


Hagen Poiseuille equation
➢ The flow in capillary viscometers is described by the Hagen Poiseuille
equation:
➢ Substituting Hagen Poiseuille eqn into
➢ for Newtonian fluid,
where Q is the volumetric flow rate
2
v
8
R
L
P

=

L
PR
w
2

=

R
w
v
4

 =
3
4
v
4
R
Q
R
w

 =
=


⚫ Newton’s law of viscosity can be written in terms of pressure gradient and
volumetric flow rate as:
⚫ Viscosity of the fluid thus can be determined from the pressure drop and
volumetric flow rate or velocity data.






=

3
4
2 R
Q
L
PR



B. Orifice Type Viscometers
✓ In orifice type viscometers, the time for a standard volume of fluid to flow
through an orifice is measured.
✓ They are used for Newtonian or near-Newtonian fluids when extreme accuracy is
not required.
✓ In the food industry, the most commonly used one is a dipping type Zahn
viscometer that consists of a 44-mL capacity stainless steel cup with a handle and
with a calibrated circular hole in the bottom.

✓ The cup is filled by dipping it into the fluid and withdrawing it.
✓ The time from the start of withdrawing to the first break occurring in the issuing
stream is recorded.

C. Falling Ball Viscometers
⚫ These types of viscometers involve a vertical tube where a ball is allowed to fall
under the influence of gravity.
⚫ It operates on the principle of measuring the time for a ball to fall through a
liquid under the influence of gravity.
⚫ When the ball falls through the fluid, it is subjected to gravitational force, drag
force, and buoyancy force

Net force (FNet) =
Gravitational force (FG)− Buoyancy force (FB)− Drag force (FD)
𝜋𝐷𝑝
3
𝜌𝑝
6
𝑑𝑣
𝑑𝑡
=
𝜋𝐷𝑝
3
𝜌𝑝𝑔
6
−
𝜋𝐷𝑝
3
𝜌𝑓𝑔
6
−
𝑐𝐷𝜋𝐷𝑝
2
𝜌𝑓 𝑣2
8

➢ When equilibrium is attained, the upward and downward forces are balanced
and the ball moves at a constant velocity.
➢ That is, the falling ball reaches a terminal velocity (vt) when the acceleration due
to the force of gravity is exactly
compensated by the friction of the fluid on the ball.
𝒅𝒗
𝒅𝒕
= 𝟎

➢ In the Stoke’s region, the drag coefficient is: 𝒄𝑫 =
𝟐𝟒
𝑹𝒆
Substituting
𝝅𝑫𝒑
𝟑𝝆𝒑
𝟔
=
𝝅𝑫𝒑
𝟑𝝆𝒑𝒈
𝟔
+
𝟔𝝅𝑫𝒑𝝁𝒗𝒕
𝟐
➢ If the terminal velocity of the ball is calculated, it is possible to determine the
dynamic viscosity of the fluid.
➢ Falling ball viscometers are more suitable for viscous fluids where the terminal
velocity is low.

D. Rotational Viscometers
➢ In rotational viscometers, the sample is sheared between the two parts of the
measuring device by means of rotation. In agitation, the shear rate is
proportional to the rotational speed.
➢ It is possible to measure the shear stress as the shear rate is changed. In addition,
a sample can be sheared for as long as desired.

➢ Therefore, rotational viscometers are the best for characterization of non-
Newtonian and time-dependent behavior. There are different forms of these
viscometers.

s Introduction
➢ Thermal properties data are required in:
✓ Engineering and process design
✓ An energy balance for a heating or cooling process
✓ Determination of temperature profile within a material

s
Introduction
Terms used to define thermal properties:
✓ Specific heat
✓ Thermal conductivity
✓ Thermal diffusivity
✓ Thermal expansion coefficient
✓ Surface heat transfer coefficient
✓ Sensible and Latent heat
✓ Enthalpy

s Introduction
➢ Processing and Storage of agricultural Products
– Heating
– Cooling
– Combination of heating and cooling
➢ Grain dried for storage, Noodles dried Fruits/Vegetables rapidly cooled
➢ Vegetables are blanched, maybe cooked and canned Powders such as spices
and milk: dehydrated
➢ Cooking, cooling, baking, pasteurization, freezing, dehydration: all involve heat
transfer
➢ Design of such processes require knowledge of thermal properties of material

s Temperature
• The temperature of a system is an indication of the kinetic energy exhibited by the
molecular motion taking place within the constituent substances of the system.
• This kinetic energy increases with increasing temperature (molecules move about
at greater speed).
• The mathematical product of absolute temperature T and Boltzmann’s constant k is
called the thermal energy E of a system.
𝐸 = 𝑘. 𝑇

s Cont.….
1. Eq. for calculating Cp based on moisture Content
For liquid H2O
• Cp= 0.837 + 3.348 M above freezing
For solid H2O
• Cp= 0.837 + 1.256 M below freezing
2. Eq. based on composition
• Cp=4.18Xw+1.711Xp+1.928Xf+1.547 Xc+0.908Xa
X is the mass or weight fraction of each component The subscript denote following components:
w=water, p= protein, f=fat, c= carbohydrate, a=ash

s Exercise
1. Estimate the specific heat of potatoes containing 85% water.
• Data Given
• Cp water = 4186.80 J/kgK
• Cp nonfat solids = 837.36 J/kgK

s Thermal Conductivity (k)
K,water = 0.566 at 0°C
= 0.602 at 20°C
= 0.654 at 60°C
✓ At room temp. value of k for endosperm of cereal grains, flesh of fruits and
vegetable., dairy products, fats and oil and sugar are less than that of water.
✓ Higher the moisture content higher will be thermal conductivity of food product
✓ Another factor is porosity e.g. freeze dried products and porous fruits like apple
have low thermal conductivity

Thermal conductivity Models
• There are models that assume an isotropic physical structure.
• The most well known models in literature are:
• Parallel and series models;
• Krischer model,
• Maxwell-Eucken
• Kopelman models

A. Parallel Model
• In the parallel model, components are assumed to be placed parallel to the
direction of heat flow
• The effective thermal conductivity of a food material made of n components can
be calculated using volume fractions (Xi
v) and thermal conductivities (ki) of each
component (i) from the following equation:

)
nt (kg/m
constitue
of the i
= density
ρ
nt,
constitue
the i
action of
= mass fr
X
nt,
constitue
f the i
fraction o
= volume
X
where
)
/ρ
(X
/ρ
X
X
where
X
k
k
th
i
th
w
i
th
v
i
n
i
i
w
i
i
w
i
v
i
n
i
v
i
i
pa
3
1
1


=
=
=
=

• The parallel distribution results in a maximum thermal conductivity value.
• If the food material is assumed to be composed of three components (water, solid,
and air), effective thermal conductivity can be calculated from:
ities
conductiv
ng thermal
orrespondi
are the c
, k
, k
k
air and
ctions of
volume fra
X
air
solid, and
ctions of
volume fra
X
moisture
ctions of
volume fra
X
where
v
a
X
a
+ k
v
s
X
s
+ k
v
w
X
w
= k
pa
k
a
s
w
v
a
v
s
v
w
=
=
=

B. Series (perpendicular) Model
➢ In the perpendicular model, components are assumed to be placed perpendicular
to the direction of heat flow
➢ The effective thermal conductivity of a food material can be calculated from the
following equation:

=
=
n
i i
v
i
se k
X
K 1
1

➢ The perpendicular distribution results in a minimum thermal conductivity value.
➢ If the food material is assumed to be composed of three components (water, solid,
and air), effective thermal conductivity can be calculated from:
a
v
a
s
v
s
w
v
w
se k
X
k
X
k
X
k
+
+
=
1

C. Krischer Model
• Krischer proposed a generalized model by combining the parallel and series
models using a phase distribution factor.
• The distribution factor, fk, is a weighing factor between these extreme cases.
• Krischer’s Model is described by the following equation:

el
ies
by the ser
ductivity
hermal con
ffective t
are the e
k
el
l
he paralle
ivity by t
al conduct
tive therm
the effec
k
el
e Krischer
vity by th
l conducti
ive therma
the effect
k
where
se
k
k
f
pa
k
k
f
k
se
pa
mod
mod
mod
1
1
=
=
=
+
−
=

D. Maxwell-Eucken Model
The effective thermal conductivity of a food, for a two-component food system
consisting of a continuous and a dispersed phase, by this model is defined as:
phase
dispersed
vities of
l conducti
the therma
k
phase
continuous
vities of
l conducti
the therma
k
where
)
d
k
c
(k
v
d
X
d
k
c
k
)
d
k
c
(k
v
d
X
d
k
c
k
c
k
k
d
c
2
2
2
=
=










−
+
+
−
−
+
=

E. Kopelman Model
Kopelman model describes the thermal conductivity of a composite material as a
combination of continuous and discontinuous phases.
 
 








−
=
−
−
−
=
c
d
v
d
v
d
c
k
k
X
Q
where
X
Q
Q
k
k
1
)
(
)
(
1
1
1
3
/
2
3
/
1

➢ The approach chosen in Kopelman equation is to successively determine the
thermal conductivity of two-component systems, starting with water continuous
and carbohydrate dispersed phases.
➢ Then, the water–carbohydrate phase is taken as continuous and protein as
dispersed phases and the iterative procedure is continued through all phases using
the order: water (phase 1), carbohydrate
(2), protein (3), fat (4), ice (5), ash (6), and air (7).

➢ The following iterative algorithm was obtained for the thermal conductivity of a
system of i + 1 components:
( )

+
+
+
+
+
+
+
+
+
+
=








−
=
=
1
1
v
1
i
d,
1
3
/
2
v
1
i
d,
1
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s Surface heat transfer coefficient (h)

s Sensible and Latent Heat
Sensible heat:
Temperature that can be sensed by touch or measured with a thermometer.
Temperature change due to heat transfer into or out of product
Latent heat:
Transfer of heat energy with no accompanying change in temperature. Happens
during a phase change...solid to liquid...liquid to gas...solid to gas

s Latent Heat…
✓ Heat that is exchanged during a change in phase
✓ Dominated by the moisture content of foods
✓ Requires more energy to freeze foods than to cool foods (90kJ removed to lower 1 kg of
water from room T to 0 °C and 4x that amount to freeze food)
✓ 420 kJ to raise T of water from 0 ° C to 100 ° C, 5x that to evaporate 1 kg of water
✓ Heat of vaporization is about 7x greater than heat of fusion (freezing)
✓ Therefore, evaporation of water is energy intensive (concentrating juices, dehydrating
foods…)

s Enthalpy (h)
Units: (kJ/kg or BTU/lb)
✓ Heat content of a material.
✓ Used frequently to evaluate changes in heat content of steam or moist air
Combines latent heat and sensible heat changes
ΔQ = M(h2-h1) Where,
ΔQ = amount of heat needed to raise temperature from T1 to T2
M = mass of product
h2= enthalpy at temp T1
h1= enthalpy at temp T2

s Enthalpy (h)…
Approach useful when one of the temperature is below freezing
✓ Measurements based on zero values of enthalpy at a specified temperature e.g.
at -40°C, -18°C or 0°C.
✓ Enthalpy changes rapidly near the freezing point
Change in enthalpy of a frozen food can be calculated from eq. below:
Δh = M cp(T2– T1) + MXwL

s Enthalpy (h)…
▪ Xw is the mass fraction of water that undergoes phase change(frozen fraction) L
is the latent heat of fusion of water
▪ M is the mass of product
▪ Δh = Change in enthalpy of frozen food

s 1. Specific Heat
✓ Specific heat is the amount of heat required to increase the temperature of a unit
mass by 1 ⁰C or 1 ⁰F. This is denoted by Cp.
Q = MCp∆T
Factors that influence the specific Heat :
✓ Moisture content
✓ Temperature
✓ Pressure

s 2. Thermal conductivity
➢ thermal conductivity of a product gives in quantitatives terms the rate of heat that
will be conducted through a unit thickness of the materials if a unit temperature
gradient exists across that thickness.
Q = KA dT/dx
Where ;
K= thermal conductivity
A = area through which heat flows X = length
T = Temperature

s Applications of thermal conductivities
✓ Prediction of processing time
✓ Heat calculation
✓ Thermo physical properties prediction
3. Thermal diffusivity
X density

s Thermal diffusivity…
Applications:
✓ estimating the processing time i.e. canning, cooking, freezing etc.
✓ the higher value of TD indicates high heat energy will passes
through the process.

s Introduction
❑There are two main electrical properties in food engineering:
✓ Electrical conductivity and
✓ Electrical permittivity
❑Electrical properties are important when processing foods involving electric
fields, electric current conduction, or heating through electromagnetic waves.
❑These properties are also useful in the detection of processing conditions or the
quality of foods.

s Electrical conductivity
➢ is a measure of how well electric current flows through a food of unit cross-
sectional area A, unit length L, and resistance R. It is the inverse value of
electrical resistivity (measure of resistance to electric flow) and is expressed in SI
units s/m in the following relation: σ = L / (AR)

s Electrical permittivity
➢ is a dielectric property used to explain interactions of foods with electric fields. It
determines the interaction of electromagnetic waves with matter and defines the
charge density under an electric field.

s Cont..
In solids, liquid, and gases the permittivity depends on two values
✓ The dielectric constant ε’, related to the capacitance of a substance and its ability
to store electrical energy; and
✓ The dielectric loss factor ε”, related to energy losses when the food is subjected
to an alternating electrical field (i.e., dielectric relaxation and ionic conduction).
✓ The parameter that measures microwave absorptivity is the loss factor. The
values of dielectric constant and loss factor will play important roles in
determining the interaction of microwaves with food.

s Cont..
➢The rate of heat generation per unit volume (Q) at a location inside the food
during microwave heating can be characterized by
where f is the frequency, ε0 is the dielectric constant of free space (8.854 × 10−12
F/m), ε’’is the dielectric loss factor, and E is the electric field.
Q = 2π f ε0ε’’E2

s Cont..
✓ As microwaves move through the slab at any point, the rate of heat generated per
unit volume decreases. For materials having a high loss factor, the rate of heat
generated decreases rapidly and microwave energy does not penetrate deeply.
✓ A parameter is necessary to indicate the distance that microwaves will penetrate
into the material before it is reduced to a certain fraction of its initial value.
✓ This parameter is called power penetration depth (δp), which is defined as the
depth at which power decreases to 1/e or (36.8%) of its original value. It depends
on both dielectric constant and loss factor of food.

s Cont..
where λ0 is wavelength of the microwave in free space.

s Dielectric constant & loss factor of some foods

s Exercise
Estimate the penetration depth of a chicken meat during processing in home type
microwave oven. Chicken meat has a dielectric constant of 53.2 and dielectric loss
factor of 18.1. Assume that dielectric properties are constant during heating.

s Solution
The frequency of a home type microwave oven is 2450 MHz. Wavelength in free
space is calculated as:

s Cont.…
Electrical properties are important in processing foods with pulsed electric fields,
ohmic heating, induction heating, radio frequency, and microwave heating.
Conductivity plays a fundamental role in ohmic heating, in which electricity is
transformed to thermal energy when an alternating current (a.c.) flows through food.

s Dielectric properties
➢ A dielectric material is a substance that is a poor conductor of
electricity, but an efficient supporter of electrostatic field.
➢ When a dielectric is placed in an electric field, electric charges do
not flow through the material as they do in a conductor, but only
slightly shift from their average equilibrium positions causing diele-
Ctric polarization.

s Cont.…
➢ Dielectric property used to explain interactions of foods with electric fields.
➢ It determines the interaction of electromagnetic waves with matter and defines
the charge density under an electric field.
➢ Major factors that influence these properties of agricultural and food materials
are:

s Cont.…
✓ Frequency of the applied radio frequency or microwave
✓ electric fields
✓ Water content
✓ Temperature
✓ Density of the materials

s Factors influencing dielectric property
o The dielectric properties of most materials vary with several influencing factors.
The dielectric properties also depend on the:
1. Frequency of the applied alternating electric field,
2. Temperature of the material,
3. Density,
4. Composition,
5. Structure of the material.

s
MAGNETIC AND ELECTROMAGNETIC PROPORTIES

s Magnetic properties
✓ When a material is brought into a magnetic field, it becomes magnetically
polarized.
✓ materials being made up of small sub microscopic particles like polar molecules
that act as tiny magnets.
✓ When brought into the presence of a magnetic field, they will orient themselves
in order to become aligned with the polarity of the magnetic field.

s Cont.….
• The material property that quantifies the extent to which a material is capable of
becoming magnetically polarized when placed in a magnetic field is called
magnetic permeability.
• Materials with high magnetic permeability will develop strong levels of
magnetic polarization in response to an external magnetic field, whereas
materials with low magnetic permeability will polarize to a much lesser extent,
if at all.

s
• When we consider the atomic basis for magnetic polarization, we need to distinguish
between paramagnetism and diamagnetism.
• Paramagnetism occurs in materials which have an atomic angular momentum, which
mostly is the case when there are unpaired electrons. These types of atoms are said to
have a magnetic momentum, and are responsible for the magnetic behavior of
paramagnetic materials.
• Diamagnetism occurs in materials made up of atoms with paired electron spins. These
types of atoms are said to have no magnetic momentum, and are responsible for poor
magnetic polarization in diamagnetic materials.

s Electromagnetic properties
➢ Electromagnetic properties are those properties which govern the rate at which
a material will respond to absorption or emission of electromagnetic radiations.
➢ Examples of electromagnetic radiations with which we might be familiar are
radio waves,microwaves,ultraviolet rays, infrared rays and visible light rays,
among others.

s Cont.…
Knowledge of the electromagnetic properties in foods is important because:
✓ food materials possess polar molecules that will respond to microwave radiation.
✓ When foods are placed into a microwave field in which microwaves are
emitted at special frequencies, polar molecules can become electrically polarized,
and can absorb microwave energy.

s Introduction
• Optical properties are those material properties resulting from physical
phenomena occurring when any form of light interacts with the material under
consideration. Only optical properties detected by the human eye are discussed
here.
• In the case of foods, the main optical property considered by consumers in
evaluating quality is color, followed by gloss and translucency, or turbidity, and
other properties of much lower interest.

s
Introduction
➢ Light can be defined as the visible part of the whole electromagnetic spectrum
(wavelengths between 380 and 770 nm).
➢ When light strikes an object (food or other) several optical phenomena take
place: absorption, refraction, transmission, scattering, and reflection. The change
in intensity and/or quality undergone by light when interacting with the material
greatly depends on the composition and the structure of the said material.

s
Cont.…
➢ Foods contain chromophores that are capable of altering the nature of light, thus determining whether the
food has color or not.
Properties of light :
1.) Particles and Waves
➢ Light waves consist of perpendicular, oscillating electric and magnetic fields
➢ Parameters used to describe light
- amplitude (A): height of wave’s electric vector
- Wavelength (l): distance (nm, cm, m) from peak to peak
- Frequency (n): number of complete oscillations that the waves makes each second
▪ Hertz (Hz): unit of frequency, second-1 (s-1)
▪ 1 megahertz (MHz) = 106s-1 = 106Hz

s
Cont..
Parameters used to describe light
Energy (E): the energy of one particle of light (photon) is proportional to its
frequency. 
h
E = where: E = photon energy (Joules)
 = frequency (sec-1)
h = Planck’s constant (6.626x10-34J-s)
As frequency () increases, energy (E) of light increases

s
Cont.…
2.) Types of Light – The Electromagnetic Spectrum
➢ Note again, energy (E) of light increase as frequency () increases

s
Absorption of Light
1.) Colors of Visible Light
➢ Many Types of Chemicals Absorb Various Forms of Light
➢ The Color of Light Absorbed and Observed passing through the Compound are
Complimentary

s
Absorption of Light
2.) Ground and Excited State
➢When a chemical absorbs light, it goes from a low energy state (ground state) to a
higher energy state (excited state)
➢ Only photons with energies exactly equal to the energy difference between the two electron states will be
absorbed
➢ Since different chemicals have different electron shells which are filled, they will each absorb their own
particular type of light
Energy required of photon
to give this transition:
E = E1 − Eo

s
Color
⚫ Color is one of the important quality attributes in foods. Although it does not
necessarily reflect nutritional, flavor, or functional values, it determines the
acceptability of a product by consumers.
⚫ Sometimes, instead of chemical analysis, color measurement may be used if a
correlation is present between the presence of the colored component and the
chemical in the food since color measurement is simpler and quicker than
chemical analysis.

s
Color Measuring Equipment’s
Color measuring instruments are categorized into two types:
➢ spectrophotometers
➢ colorimeters

s
Spectrophotometers
⚫ Spectroscopy deals with the production, measurement, and interpretation of
spectra arising from the interaction of electromagnetic radiation with matter.
⚫ Spectroscopic methods based on the absorption or emission of radiation in the
ultraviolet (UV), visible (Vis), infrared (IR), and radio (nuclear magnetic
resonance,NMR) frequency ranges are most commonly encountered in
traditional food analysis laboratories.

s An instrument used to make absorbance or transmittance measurements is known as a
spectrophotometer .

s
Colorimeters
⚫ Tristimulus colorimeters were developed since spectrophotometer integration was
expensive.
⚫ A tristimulus colorimeter has three main components:
1. Source of illumination
2. Combination of filters used to modify the energy distribution of the incident/reflected light
3. Photoelectric detector that converts the reflected light into an electrical output

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