Mass-Volume-Area Properties of Food

Mass-Volume-Area Related Properties of Food
Md. Sajib Reza
Lecturer
Department of Food technology and Nutritional Science
Mawlana Bhashani Science and Technology university
Tangail-1902, Bangladesh
Email: sajib.ftns2010@gmail.com

MECHANICAL PROPERTIES
• The mechanical properties of food mainly result from its structure, physical state, and
rheology.
• These properties are NEEDED for –
- process design,
- estimating other properties,
- characterizing foods,
- quality determination.
• They can be subdivided into two groups:
- structural and geometrical properties,
- strength properties.
• Structural and geometrical properties include mass–volume–area-related properties
(density, shrinkage, and porosity), and morphological properties (surface area,
roundness, and sphericity).
• Strength properties are related to solid and semi-solid stress and deformation, and
intervene in food texture and rheological characterization.
• Mechanical properties can be sub divided into 5 categories: acoustic properties, mass-
volume-area-related properties, morphometric properties, rheological properties,
surface properties.

PHYSICAL PROPERTIES OF FOODS
• Dr Alina Szczesniak defined the physical properties of foods as
‘those properties that lend themselves to description and
quantification by physical rather than chemical means’ (
Szczesniak, 1983 ).
• The physical properties of foods are of utmost INTEREST TO THE
FOOD ENGINEER, MAINLY FOR TWO REASONS:
- Many of the characteristics that define the quality (e.g. texture,
structure, appearance) and stability (e.g. water activity) of a food
product are linked to its physical properties.
- Quantitative knowledge of many of the physical properties, such as
area, volume, size, shape, density, viscosity, porosity, weight,
surface area is essential for the rational design and operation of
food processes and for the prediction of the response of foods to
processing, distribution and storage conditions.

APPLICATION OF PHYSICAL PROPERTIES
• The geometric characteristics of size, shape, volume, surface area,
density, and porosity are important in Design of specific machine or
analysis of product behavior during handling and processing
operations.
• Separation of seeds and grains from undesirable material. (shape,
size, density).
• Fruits and vegetables are usually graded depending on size, shape,
and density. Impurities in food materials are separated by
density differences between impurities and foods.
• Conveying of solid materials (density, size, shape)
• Calculation of other properties for further use in design of
systems (for terminal velocity, thermal diffusivity etc.)
• Knowledge of the bulk density of food materials is necessary to
estimate floor space during storage and transportation.

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• Sorting, grading and for capacity determination of machines and
during storage.
• When mixing, transportation, storing and packaging particulate
matter, it is important to know the properties of bulk material.
• Surface areas of fruits and vegetables are IMPORTANT in
investigations related to spray coverage, removal of residues,
respiration rate, light reflectance, and color evaluation, as well as in
heat transfer studies in heating and cooling processes.
• In many physical and chemical processes, the rate of reaction is
proportional to the surface area; thus, it is often desirable to
maximize the surface area.
• Density and porosity have a direct effect on the other physical
properties. Volume change and porosity are important parameters in
estimating the diffusion coefficient of shrinking systems.
• These properties are needed for process design, estimating other
properties, characterizing foods, and quality determination.

Continue
• Size, shape, sphericity, volume, surface area, density, and porosity
are important physical characteristics of many food materials in
handling and processing operations.
• Sphericity and shape factors are also needed in heat and mass
transfer calculations
• Fruits and vegetables are usually graded according to size, shape,
and density
• Density and the shape factor of food materials are also necessary for
predicting the freezing and thawing rate

IMPORTANCE OF PHYSICAL PROPERTIES
• The study of food engineering focuses on the analysis of
equipment and systems used to process food on a
commercial production scale.
• Engineering of systems for food materials can be more
thorough if there is an understanding of the changes that
occur in food as it is processed by the system.
• Raw food materials are biological in nature and as such have
certain unique characteristics which distinguish them from
other manufactured products.

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• Because food materials are mainly of biological
origin they have –
(a) irregular shapes commonly found in naturally
occurring raw materials
(b) properties with a non-normal frequency distribution
(c) heterogeneous composition
(d) composition that varies with variety, growing
conditions, maturity and other factors
(e) they are affected by chemical changes, moisture,
respiration, and enzymatic activity.

• Dealing with materials that have these unique characteristics
requires additional consideration, mostly indirectly, in that
there are additional sources or causes of variation.
• People unfamiliar with this natural variability of biological
materials may overlook these factors or be frustrated by lack
of control over the input parameters
• The characteristics of a food material that are independent of
the observer, measurable, can be quantified, and define the
state of the material (but not how it attained that state) are
considered as its physical properties.

WHY KNOWLEDGE OF A FOOD‘S PHYSICAL
PROPERTIES IS NECESSARY?
• “Physical properties describe the unique, characteristic
way a food material responds to physical treatments
involving mechanical, thermal, electrical, optical, sonic,
and electromagnetic processes”.
• A better understanding of the way food materials
respond to physical and chemical treatments allows for
optimum design of food equipment and processes to
insure food quality and safety.

Continue … …
• KNOWLEDGE OF A FOOD‘S PHYSICAL PROPERTIES
IS NECESSARY FOR:
- defining and quantifying a description of the food
material,
- providing basic data for food engineering and unit
operations
- predicting behavior of new food materials.
• It is common for the physical properties of a food to change
during processing operations. These changes not recognizing
to the potential processing failures.
• Physical properties are an important aspect of food quality
and relate to food safety. They are the basis for instruments
and sensors.

LIST OF COMMON PHYSICAL PROPERTIES
• Physical characteristics of raw, unprocessed, as well as processed food
materials include -
Size
Shape
Volume
Density
Porosity
Surface area
Color

Size
• The size of a raw food material can vary widely.
• The variation in shape of a product may require additional parameters
to define its size.
• SIZE IS AN IMPORTANT PHYSICALATTRIBUTE OF FOODS
used in - screening solids to separate foreign materials, - grading of
fruits and vegetables, - evaluating the quality of food materials.
- In fluid flow, and heat and mass transfer calculations, it is necessary to know
the size of the sample.
- Size of the particulate foods is also critical as it affects the viscosity and
dispersibility and stability of the product.
- Sort the various agro produces into size groups for fresh market. This is helps in
assessing market and price differentials of large and small produce, to match
consumer preferences and to allow pattern packing.
- Determine produce surface area
- Mandatory for modern or on-line fruit, vegetables, grain, spices density sorting
for which two size related parameters, volume and weight, are required.

MEASUREMENT OF SIZE
• The size of spherical particles like peas or cantaloupes ( a
kind of melon) is easily defined by a single characteristic
such as its diameter.

Continue ….
• The size of non-spherical objects like wheat kernels, bananas,
pears, or potatoes may be described by multiple length
measurements.
• The longest diameter (major) and shortest diameter (minor) will
adequately describe the size of an ellipsoidal object such as grain
kernel or potato.
•

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• The size of pear-shaped objects such as pears, avocados, carrots,
or beets can be expressed by
- diameter or circumference of the largest part and
- an overall length in the direction of the stem.

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• Size of a carrot may be expressed only in length or in diameter of
its large end.

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• The size of irregular-shaped materials like banana or okra
requires more extensive considerations.
• The size of larger objects may be expressed only in terms of its
largest diameter or circumference

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• The size of larger objects may be expressed only in terms of its
largest diameter or circumference.
• The size of a banana might be given only in overall length.
• Precise methods incorporating optical, light, or lasers in machine
vision systems exist to define shape and size of irregular-shape
objects.
• These systems are costly; their use is warranted in applications of
high value materials more commonly found in highly
processed, final products rather than raw, unprocessed materials.

Continue ….
• Size may be indicated by weight since it is so easily
determined by simply placing on a scale.
• Thus, the physical property size is actually related or
correlated to the property weight.
• In practice, there is often a compromise between ease or cost
of measurement and usefulness or value of that property in the
market channel.

METHOD OF MEASUREMENT OF SIZE
• Projected area method: Photographic enlarger
• Micrometer method: slide calipers and micrometer
• Electronic devices: Image analysis (Precise method)
Precise methods incorporating optical, light, or lasers in
machine vision systems exist to define shape and size of
irregular-shape objects.
These systems are costly, their use is warranted in applications
of high value materials more commonly found in highly
processed, final products rather than raw unprocessed
materials.

Projected area method
• Size can be determined using the projected area method. In
this method, three characteristic dimensions are defined:
1. Major diameter, which is the longest dimension of the
maximum projected area.
2. Intermediate diameter, which is the minimum diameter of
the maximum projected area
or
the maximum diameter of the minimum projected area
1. Minor diameter, which is the shortest dimension of the
minimum projected area.
Projected area: The area which corresponds to the shape
produced by projecting a three-dimensional object on to a
plane surface.

PROPERTIES OF FOOD POWDER ….
Food powder properties contribute to the understanding
of operations like grinding, filtration,
sedimentation, centrifugation, spray drying,
conveying, dosing, hopper storage, mixing etc.
MEASUREMENT FOR PARTICULATE FOOD:
• Size of the particulate foods are expressed as particle
size.
• Particle size is used in sieve separation of foreign
materials or grading (i.e., grouping into size
categories).
• Particle size is particularly important in grinding
operations to determine the condition of the final
product and determines the required power to reduce
the particle‘s size.
• Small irregular-shaped objects can be sized with
sieves by expressing particle size as the smallest
sieve opening through which the particle passes.
The size of the larger objects may be expressed only
in terms of its largest diameter or circumference. SI
unit for particle size is micrometers or millimeters.

Continue …..
• More important than individual size is the size distribution
among the particles.
• Particle size distribution is directly related to material
behavior and physical properties of products.
• Bulk density, compressibility and flowability of a food
powder are highly depend on particle size and its distribution.
• In quality control or system property description,
measurement of the particle size distribution in food powders
becomes paramount.
• Different types of methods such as seiving, microscopic
counting techniques, sedimentation and stream scanning
are available for measuring particle size distribution.
• Fineness from sieve analysis is the common measurement.

Measurement unit of irregular particle
It is easy to specify size for regular particles, but for irregular
particles the term size must be arbitrarily specified.
• Particle sizes are expressed in different units depending on the
size range involved.
- Coarse particles are measured in millimeters,
- Fine particles in terms of screen size, and
- Very fine particles in micrometers or nanometers.
- Ultrafine particles are sometimes described in terms of their
surface area per unit mass, usually in square meters per gram

Shape
• Shape describes the object in terms of a geometrical body.
• SHAPE IS ALSO IMPORTANT in -
- Heat and mass transfer calculations
- Screening solids to separate foreign materials
- Grading of fruits and vegetables
- Evaluating the quality of food materials
❑ The shape of food materials is usually expressed in terms of
its –
o Roundness ○ Sphericity ○ Aspect
ratio ○ Ellipsoid ratio ○ Slenderness
ratio

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• Shape affects the grade given to fresh fruit.
• To make the highest grade a fruit or vegetable must have the
commonly recognized expected shape of that particular
fruit/vegetable.

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• Shape is also related to the maturity of fruits and vegetables.

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• Shape influences the consumer perception.
• Misshapen fruit and vegetables will be down-graded and may sell
at a lower price in high volume markets.

THE SHAPE OF AN IRREGULAR OBJECT
• The shape of an irregular object can be described by terms such as
the following:

SOME COMMON SHAPES WITH EXAMPLE

MEASUREMENT OF SHAPE
• Various methods are used to measure or characterize the shape and size
characteristics of foods and food products.
• In several cases, actual measurements are made to estimate the major
dimensions and cross sections of the product.
• Mohsenin (1970) illustrates the use of standard charts in the
describing and defining the shape of a product.
• Various formulas and methods have been devised to estimate cross
sections and other characteristics of the materials.
• The shape of a food material is usually expressed in terms of its
roundness, sphericity and aspect ratio.

1. Roundness
• Roundness, as defined by Mohsenin (1970),
“is a measure of the sharpness of the corners
of the solid”.
• Curray (1951) and Mohsenin (1970) provided the equations
for estimating roundness under different conditions of
geometry and application.
• The most commonly used ones are given below (Mohsenin,
1970):

Continue … … …
Roundness = Ap / Ac
Where:
Ap = largest projected area of object in natural rest position
(m2),
Ac = Area of the smallest circumscribing circle as defined (m2).

Roundness can also be estimated from –
Roundness = Σr / NR
where:
r = radius of curvature as defined
R = radius of maximum inscribed circle
N = total number of corners summed in numerator

Curvature - বক্রতা
Curvature is the -
• the act of curving
• the state of being curved
• a measure or amount of curving.
• Certain parameters are important for the design of conveyors for
particulate foods, such as radius of curvature, roundness, and angle of
repose.
• Radius of curvature is important to determine how easily
the object will roll.
• The more sharply rounded the surface of contact, the greater will be
the stresses developed.

• Roundness can also be estimated from –
Roundness ratio = r / R
Where:
R = is the mean radius of the object
r = is the radius of curvature of the
sharpest corner.
The use of the radius of curvature of a
single corner determines the
roundness or flatness of an object
N.B. Roundness values will differ for each of the above methods. Thus, the
method for roundness determination should always be noted

2. Sphericity
• Sphericity expresses “the characteristic shape of a
solid object relative to that of a sphere (গ োলক) of the
same volume” (Mohsenin, 1970)
• Simply, sphericity indicates “how the shape of an
object deviates from a sphere”.
• Sphericity is defined from the volume, surface area, or
geometric dimensions of an object.
• Sphericity and shape factors are also needed in fluid
flow, heat and mass transfer calculations.
• According to the most commonly used definition,
“sphericity is the ratio of volume of solid to the volume of
a sphere that has a diameter equal to the major
diameter of the object so that it can circumscribe the
solid sample”.

• Curray (1951) suggested the following equation for estimating
the sphericity of an object:
Sphericity = Di / Dc
Where,
Di = diameter of largest inscribed circle
Dc = diameter of smallest circumscribed circle

Calculate the sphericity of a cylindrical object of diameter 1.0 cm and height 1.7 cm.
STEP 1: The formula for the volume of a cylinder is, V = πr2h
So, The volume of the cylindrical object can be calculated by,
= π(0.5)2(1.7) = 1.335 cm3
STEP 2: The formula for volume of a sphere is, V = 4/3 π r³
Let, The radius of the sphere (rs) having this volume can be calculated as:
4/3 π r³ = 1.335 cm3
⇒ rs = 0.683 cm
STEP 3: The formula for Surface area of a sphere is = 4πr2
So, the surface area of sphere of the same volume as the particle is:
Di = 4πr2 = 4π(0.683)2 = 5.859 cm2
STEP 4: The total surface area of a given cylinder object whose radius is r and height is h,
then
Total Surface area = Curved Surface area + Area of Circular bases
Total Surface area = 2πrh + 2πr2
Total Surface area = 2πr (h + r)
The surface area of the object is Dc : 2π r(h + r) = 2π(0.5)(1.7 + 0.5) = 6.908 cm2
STEP 5: Then, sphericity is calculated as: = Di / Dc
= diameter of largest inscribed circle / diameter of smallest circumscribed circle
= 5.859 cm2 / 6.908 cm2 = 0.848

3. The aspect ratio
• It is calculated using the length (a) and the width (b) of the sample
Aspect ratio (Ra) = width (b) / length (a)

4 & 5. Slenderness ratio and Ellipsoid ratio

Sagittal height
• Sagittal depth or vault is the distance
between the center of the posterior
surface to the of the curvature. plane of
the edges

MEASURING THE RADIUS OF CURVATURE
• A simple device for measuring the radius of curvature is
shown in Figure.
• It consists of a metal base that has a dial indicator and holes
into which pins are placed.
• Two pins are placed within these holes according to the size
of the object.
• When the two pins make contact with the surface,
the tip of the dial indicator is pushed upwards.
• Then, the dial indicator reads the sagittal height (S).
• The radius of curvature is calculated from the measured
distances using this simple device and the
following formula:

DEVICE FOR MEASURING THE RADIUS OF
CURVATURE

The minimum and the maximum radii of
curvature
• The minimum and the maximum radii of curvature for larger objects
such as apples are calculated using the larger and smaller dial indicator
readings, respectively.
• For smaller objects of relatively uniform shape, the radius of curvature
can be calculated using the major diameter and either the minor or
intermediate diameter.
• where:
Rmin = Minimum radius of curvature (m),
Rmax = Maximum radius of curvature (m),
H = intermediate diameter or the average of minor and major diameters
(m),
L = major diameter (m).

Problem Example:
• The major diameter (L) and the average of the minor and
major diameters (H) of barley are measured as 8.76 mm and
2.83 mm, respectively. Calculate the minimum and maximum
radii of curvature for the barley.
Solution: The minimum and maximum radius of curvatures can be
calculated using Equations:

Angle of repose
• This physical property used in particulate foods such as seeds,
grains, and fruits.
• “When granular solids are piled on a flat surface, the sides of the pile
are at a definite reproducible angle with the horizontal. This angle is
called the angle of repose of the material”
• The angle of repose is IMPORTANT for the design of processing,
storage, and conveying systems of particulate material.
• When the grains are smooth and rounded, the angle of repose is
low.
• For very fine and sticky materials the angle of repose is high.

Determination of angle of repose

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• For determination of this property, a box with open sides at the
top and bottom is placed on a surface.
• The angle of repose is determined by filling the box with
sample.
• lifting up the box gradually, allowing the sample to accumulate
and form a conical heap on the surface.
• Then, the angle of repose is calculated from the ratio of the
height to the base radius of the heap formed.

PARTICLE SIZE DISTRIBUTION
• The range of particle size in foods depends on the cell structure
and the degree of processing.
• The hardness of grain is a significant factor in the particle size
distribution of flour.
• The particle size distribution of flour is known to play an important
role in its functional properties and the quality of end products.
• Particles can be separated into fractions by using one of the
following methods:

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• Air elutriation method: In this method, the velocity of an air stream is
adjusted so that particles measuring less than a given diameter are suspended.
After the particles within the size range are collected, the air velocity is
increased and the new fraction of particles is collected.
• Settling, sedimentation, and centrifugation method: In settling and
sedimentation, the particles are separated from the fluid by gravitational forces
acting on the particles. The particles can be solid particles or liquid drops.
Settling and sedimentation are used to remove the particles from the fluid. It is
also possible to separate the particles into fractions of different size or density.
Particles that will not settle by gravitational force can be separated by
centrifugal force.
• Screening: This is a unit operation in which various sizes of solid particles are
separated into two or more fractions by passing over screen(s). A dispersing
agent may be added to improve sieving characteristics. Screen is the surface
containing a number of equally sized openings. The openings are square. Each
screen is identified in meshes per inch. Mesh is defined as open spaces in a
network. The smallest mesh means largest clear opening.

VOLUME
“Volume is the space occupied by an object”
“Volume is defined as the amount of three-dimensional space occupied
by an object”
usually expressed in units such as cubic inches and cubic centimeters, or
in units of liquid measure, such as gallons and liters. In the SI system,
the unit of volume is m3.
The unit of volume in the metric system is liter (L)

VOLUME IS ONE OF THE IMPORTANT ISSUES IN THE
PRODUCTION AND PROCESSING OF FOOD PRODUCT, WHY?
• Volume together with other physical properties plays an
important role to - calculate water loss, - heat transfer, -
quantity of pesticide applications, - respiration rates, etc.
• In food processing, volume is useful for - size sorting, - quality
grading, and - microbial concentration.
• If volume of a food product can be determined then the other
physical properties such as mass and density can be easily
estimated.
• In BREAD, volume is a good indicator for gas retention
properties of dough, quality of flour protein, quality of
gluten development, and balancing recipe and processing
requirement.
• In egg, volume is related to many important things in
bird life such as female mass, lay date, egg composition etc.

EXPRESSION OF VOLUME
• Volume can be expressed in different forms. The most
commonly used definitions are:
1. Apparent volume (Va): the volume of a substance
including all pores within the material (internal pores).
Apparent volume of regular geometries can be calculated
using the characteristic dimensions. Apparent volume of
irregularly shaped samples may be determined by solid or
liquid displacement methods.
2. Solid volume (Vs): the volume of the solid material
(including water) excluding any interior pores that are
filled with air. It can be determined by the gas displacement
method in which the gas is capable of penetrating all open
pores up to the diameter of the gas molecule.

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3. Bulk volume (Vb): the volume of a material when packed or
stacked in bulk. It includes all the pores enclosed within the
material (internal pores) and also the void volume outside the
boundary of individual particles when stacked in bulk (external
pores).
4. Boundary volume: 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.
5. Pore volume: the volume of the voids or air inside a material.

VOLUME DETERMINATION METHOD
Volume of solids can be determined by using the following
methods:
1. Volume can be calculated from the characteristic
dimensions in the case of objects with regular shape.
2. Volumes of irregular solids can be determined experimentally
by liquid, gas, or solid displacement methods.
3. Volume can be measured by the image processing method.
An image processing method has been recently developed to
measure volume of ellipsoidal agricultural products such
as eggs, lemons, limes, and peaches.

1. Measuring solid volume
• The volume of REGULAR OBJECT is calculated by certain
dimensions. i.e.

a) LIQUID DISPLACEMENT METHOD (Continue)
• The volume of IRREGULAR OBJECT can be calculated using
simply liquid displacement method.
• Traditionally the volume of a solid object can be measured using
water displacement method based on Archimedes‘ principle.
• The object is immersed into water that placed in a container
until all surfaces are immersed.
• The volume of displaced water is considered as the volume of
the object.

LIQUID DISPLACEMENT METHOD (continue)
✓ If the SOLID SAMPLE DOES NOT ABSORB LIQUID VERY
FAST, the liquid displacement method can be used to measure its
volume. In this method, volume of food materials can be measured by
pycnometers (specific gravity bottles) or graduated cylinders.
✓ The volume of a sample can be measured by direct measurement of
volume of the liquid displaced by using a graduated cylinder or burette.
✓ The difference between the initial volume of liquid in a graduated
cylinder and the volume of liquid with immersed material gives us the
volume of the material.
✓ That is, the increase in volume after addition of solid sample is equal to
the solid volume.
✓ In the liquid displacement method, liquids used should have a low
surface tension and should be absorbed very slowly by the particles.
✓ Most commonly used fluids are water, alcohol, toluene, and
tetrachloroethylene. For displacement, it is better to use a nonwetting
fluid such as mercury. Coating of a sample with a film or paint may
be required to prevent liquid absorption.

LIQUID DISPLACEMENT METHOD (Continue)
• FOR LARGER OBJECTS, a platform scale can be used (Mohsenin, 1970).
• The sample is completely submerged in liquid such that it does not make contact
with the sides or bottom of the beaker.
• Weight of the liquid displaced by the solid sample is divided by its density.
• The method is based on the Archimedes principle, which states that a body
immersed in a fluid will experience a weight loss in an amount equal to the weight
of the fluid it displaces.
• That is, the upward buoyancy force exerted on a body immersed in a liquid is equal
to the weight of the displaced liquid.
where
G = the buoyancy force (N),
ρl = the density of liquid (kg/m3),
Wair = the weight of sample in air (kg),
Wl = the weight of sample in liquid (kg).

b) SOLID DISPLACEMENT METHOD
• Liquid (water) displacement method is inaccurate, especially
for porous or fragile object.
• For example in bread volume measurement, water
displacement method fails because it can damage the bread.
Moreover the water will be absorbed by the bread.
• In measuring bread volume water displacement method is
modified using solid displacement method.
• Usually rape seed or pearl barley is used.
• This method is less accurate because the bread may be
compressed when it is merged in the seed.
• Currently, volume measurement instrument using laser scanner
for food product had been developed.
• However, this instrument may be expensive and cannot be
applied directly in production line.

SOLID DISPLACEMENT METHOD (Continue)
• The VOLUME OF IRREGULAR SOLIDS can also be
measured by sand, glass bead, or seed displacement
method.
• Rapeseeds are commonly used for determination of
volume of baked products such as bread.
• PROCEDURE: In the rapeseed method, first the bulk
density of rapeseeds is determined by filling a glass
container of known volume uniformly with rapeseeds
through tapping and smoothing the surface with a ruler.
The densities of the seeds are calculated from the measured
weight of the seeds and volume of the container.

SOLID DISPLACEMENT METHOD (Continue)
• Then, the sample and rapeseeds are placed together in the
container. The container is tapped and the surface is smoothed
with a ruler. Tapping and smoothing are continued until a
constant weight is reached between three consecutive
measurements. The volume of the sample is calculated as
follows:

c) GAS DISPLACEMENT METHOD
Volumes of PARTICULATE SOLIDS AND MATERIALS WITH
IRREGULAR SHAPE can be determined by displacement of gas or air in
pycnometer.
The most commonly used gases are helium and nitrogen.
The pycnometer consists of two airtight chambers of nearly equal volumes, V1
and V2,that are connected with small-diameter tubing.
The material to be measured is placed in the second chamber.
The exhaust valve (valve 3) and the valve between the two chambers (valve 2) are
closed.
The inlet valve (valve 1) is opened and the gas is supplied to the first chamber
until the gauge pressure is increased up to a suitable value (e.g., 700–1000 Pa).
Then, the inlet valve (valve 1) is closed and the equilibrium pressure is recorded
(p1).

GAS DISPLACEMENT METHOD
After the equilibrium pressure is recorded, valve 3 is closed and the valve
between the two chambers is opened (valve 2) and the gas within the first
chamber is allowed to fill the empty spaces (pores) in the second chamber.
The new pressure (P2) is recorded. When valve 2 is opened, total mass of gas (m)
is divided into two, one of which fills the first tank (m1) and the other fills the
pore space of the second tank (m2).
Under this condition with valves 1 and 3 closed, the volume of sample in tank 2 is
measured .
• Then the volume of the sample in tank 2 is estimated based on ideal gas law as

2. Measuring liquid volume
• To find the volume of liquids and other objects, graduated
cylinders can be used.

Continue … …
What is the volume
of water in each
cylinder?
Notice scales.

Density
This is defined as mass per unit volume (the SI unit of density is
kg/m3).
It is a measure of how tightly packed and how heavy the
molecules are in an object.
Density is the amount of matter within a certain volume.

Continue … …
In most engineering designs, solids and liquids are assumed to be
incompressible—in other words, density changes moderately
with changes in temperature and pressure.
In food engineering, the density of solid and liquid foods changes
with temperature and pressure and is dependent on
temperature and composition.
In the literature most of the density data is correlated empirically
as a function of temperature, water, solids, and fat content.
There are different forms of density such as true density,
material density, particle density, apparent density, and
bulk density, depending on its application in process
calculations or product characterization.

Importance of density … …
• Density of food materials is useful in mathematical conversion of
mass to volume.
• The grain industry determines the amount of agricultural grains by
converting the weight to volume.
• The density of processed products dictate the characteristics of its
container or package.
• Product density influences the amount and strength of packaging
material.
• Breakfast cereal boxes contain a required weight of cereal.
• More weight of material can be placed into a box if the cereal density
is greater.
• Also, food density influences its texture or mouth feel.
• Processing can affect product density by introducing more air, such
as is done in the manufacture of butter or ice cream.
• In products like whipped cream, which are primarily air, control of
density is essential.

TYPES OF DENSITY
1. True density (ρt): is the density of a pure substance or a composite material
calculated from its components’ densities considering conservation of mass and
volume.
2. 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.
3. Particle density (ρp): is the density of a particle, which includes the volume of
all closed pores but not the externally connected pores.
4. Apparent density (ρa): is the density of a substance including all pores
remaining in the material.
5. 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.

DENSITY DETERMINATION
• Regular Shapes – mass, then determine the volume by
formula.
• Example: cubes, cylinders, spheres, cones, etc.
• Irregular shapes – mass, then measure displacement of a
liquid (usually water) by that irregularly shaped object.
• To find the density -
1- Find the mass of the object
2- Find the volume of the object
3- Divide mass to volume

Continue … …
Density Measurement methods :
1. Apparent Density –
a. Geometric Dimension Method
b. Buoyant Force Method
c. Volume Displacement Method –
✓ Liquid Displacement Method
✓ Gas Pycnometer Method
✓ Solid Displacement Method
2. Material Density –
- Pycnometer Method
- Mercury Porosimetry
- Particle Density
- Bulk Density

Determination of LOOSE AND TAPPED BULK DENSITY
1. A graduated measuring cylinder (small size, 20 to 100 ml) was
weighed.
2. Filled to 10 ml (sometimes 20 ml) mark with flour.
3. The volume of the sample was determined before tapping the base of
the measuring cylinder on a laboratory bench.
4. The bottom of the cylinderwas gently tapped on a laboratory bench
several times (20-30 times).
5. Tapping continue until there was no further diminution/ decrease of
the sample upper level /layer, after filling to the 10 ml mark.
6. Take reading (volume) from cylinder wall where the upper surface of
the flour is fixed.
7. The bulk density of flour samples was thereafter calculated by the
following relation:
Bulk density (g/mL)= weight of sample (g)/volume of sample (mL)
Math

Porosity
• Porosity is the percentage of air between the particles
compared to a unit volume of particles.
• In other words, porosity (Ɛ) is defined as the volume fraction
of air or the void fraction in the sample and expressed as:
• Porosity (Ɛ) = Air or Void volume/ Total volume

Importance of porosity
• Porosity is an important pyhsical property characterizing the
water holding capacity, texture and the quality of dry and
intermediate moisture foods.
• Porosity data is required in modeling and design of various
heat and mass transfer processes such as drying, frying,
baking, heating, cooling, extrusion.
• It is an important parameter in predicting diffusional
properties of cellular foods.
• Some examples: bakery products, extruded materials….

Continue … …
• Porosity allows gases, such as air, and liquids to flow
through a mass of particles referred to as a packed bed in
drying and distillation operations.
• Beds with low porosity (low percentage air space) are more
resistant to fluid flow and thus are more difficult to dry,
heat, or cool.
• With high porosity, air flows easily through the bed, drying
is fast, and the power required by fans and pumps is low.

Continue … …
• Porosity of the materials are influenced from food processing.
• The formation of pores in foods during drying may show
different trends.
• Porosity may show maximum or minimum as a function of
moisture content.
• It may show also decrease or increase exponentially during
drying without showing an optimum point.
• The porosity of the apple rings increased linearly when
moisture content decreased during the drying and then
reached a constant value.

Continue … …
• A linear increase in porosity was observed
during drying of starch samples.
• The presence of pores and degree of porosity
affect mechanical properties of extruded
materials such as their texture, crumb
strength etc.

Continue … …
• Porosity is also important in frying, since it affects oil
uptake of the product.
• Porosity increased during frying of reconstructed potato
product and after a short initial period, it was found to be
linearly correlated with oil uptake.

TYPES OF POROSITY
1. 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):
2. 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.

Continue … …
3. 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)
4. Bulk Porosity: Bulk porosity (εB) is the volume fraction of
voids outside the boundary of individual materials when
packed or stacked as bulk.

Continue … …
5. 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.
6. 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 in bulk.

Measurement Techniques of Porosity
1. Direct Method
In this method, the bulk volume of a piece of porous material is measured
first, and then the volume is measured after compacting the material to
destroy all its voids. Porosity can be determined from the difference of the
two measured volumes.
This method can be applied if the material is very soft and no repulsive or
attractive force is present between the surfaces of solid particles.
2. Optical Microscopic Method
In this method the porosity can be determined from the microscopic view of a
random section of the porous medium.
This method is reliable if the sectional (two-dimensional) porosity is same as the
three-dimensional porosity.
Image analysis is necessary to estimate the surface area of pores.

Continue … …
3. Density Method
Porosity can also be estimated from the densities of the
materials.
Alternatively, pore volume can be measured directly by liquid or
gas displacement methods, described earlier in the discussion
of density measurements.
• Porosity due to the enclosed air space within the particles is
named apparent porosity (Ɛ app) and defined as the ratio of
total enclosed air space or voids volume to the total volume. It
can be also named internal porosity.
• Apparent porosity is calculated from the measured solid (ρs )
and apparent density (ρapp) data as:

Gas pycnometer method
• Porosity can be measured directly by measuring the volume
fraction of air using the air compression pycnometer.

EMULSION
• Emulsion - a mixture of two or more immiscible liquids
• one liquid (the dispersed phase) is dispersed in the other
(the continuous phase).
• A thermodynamically unstable system consists of two liquid
one is dispersed into another liquid by emulsifying agent.
• E.g., Milk, butter, mayonnaise etc.

Components of emulsion:
• Dispersed phase: The phase in an emulsion that consists of
finely divided particles, droplets or bubbles of one
substance distributed through another substance.
E.g. Antioxidants, wax, silicon etc.
• Continuous phase: Dispersion medium of an emulsion is
called continuous phase.
E.g. water, preservative, thickener etc.

Types of emulsion
• An emulsion may be
1. oil-in-water (o/w) in which case small oil droplets are
dispersed through water, e.g. milk (o/w), salad dressing
(o/w),, cream (o/w), mayonnaise (o/w) etc.
None greasy and easy to remove from skin
2. water-in-oil (w/o) in which case small water droplets are
dispersed through oil, e.g. butter.
Emulsion examples: butter (w/o), margarine (w/o), cod liver oil
(w/o)
Greasy
Difficult to remove from skin

Continue … …
3. Multi emulsion: like simple emulsions the multiple emulsions
are also considered to be of two types –
• Oil-in-Water-in-Oil (O/W/O): is a system in which water
droplets may be surrounded in oil phase, which encloses one
or more oil droplets.
• Water-in-Oil-in-Water (W/O/W): is a system in which oil
droplets may be surrounded by an aqueous phase, which
encloses one or several water droplets.

Continue … …
• According to droplet size of dispersed phase –
a) Micro emulsion: Dispersed phase droplets size 0.01-0.2
mm
- Thermodynamically stable
- Clear solution
- Simply mixing of components and do not require shear
b) Macro emulsion: Dispersed phase droplet size 0.2-50 mm
- Kinetically stable
- Hazy solution
- Mixing is difficult thus require shear

Continue … …
When water and oil are shaken together, they
form an emulsion.
This emulsion is unstable.
If left to stand, the oil will form a separate layer
on top of the water, e.g. traditional French
dressing.

Emulsion stability
• Emulsion stability: is defined as the ability to resist changes
in physicochemical properties with time.
• Stability of emulsions is influenced from:
1. Dispersed particle size
2. Viscosity of continuous phase
3. Dispersed phase concentration
4. Density difference between 2 phases

Emulsifying agent
• A stable emulsion is formed when two immiscible liquids
are held stable by a third substance, called an emulsifying
agent.
• Emulsifying agent / emulsifier: are the substances added
to an emulsion to prevent the coalescences of the globules
of the dispersed phase. These are also known as
emulsifier.
• Mayonnaise is an example of a stable emulsion of oil and
vinegar, when egg yolk (lecithin) may be used as an
emulsifying agent.

Some desirable characteristics of food
emulsifiers
• Emulsifiers must posses some desirable characteristics.
It should be –
- stable
- compatible with other ingredients
- non toxic
- posses little odor, taste or color.
Ability to reduce interfacial tension below 10 dynes/cm
Ability to be rapidly absorbed at the interface
Ability to function effectively at low concentrations
Resistance to chemical change
Lack of odor, color, and toxicity
Economical

Continue … …
• Importance:
✓ These are used to stabilize oil in water, water in oil, gas in liquid (soft
drinks) and gas in solid emulsion.
✓ Emulsifiers are used to facilitate the formation of emulsion by lowering
o/w interfacial tension & impart short-term stability by forming a
protective film around the droplets.
✓ Emulsifiers/stabilizers impart long-term stability to emulsions by
restricting interfacial interactions.
✓ Emulsifiers play an important role in the manufacture of food products
such as margarine, mayonnaise, creamy sauces, candy, many packaged
processed foods, confections and a range of bakery products.
✓ Commonly used emulsifiers are mono and diglycerides of fatty acids,
lecithin, sugar esters, proteins (i.e. egg proteins), phospholipids etc…..
✓ Stabilisers are often added to emulsions to increase the viscosity of the
product. These help improve the stability of the emulsion, as over time
the emulsion may separate.
✓ Stabilisers also increase shelf life, methylcellulose, used in low fat
spreads.

How emulsifiers work
An emulsifying agent is made up of two
parts.-
❖ One is hydrophilic (water loving) and
the other is hydrophobic (water hating).
❖ The emulsifier holds the disperse phase
within the continuous phase. This results
a stable emulsion.
❖ Its hydrophilic head binds with water
and hydrophobic tail binds with
inorganic solvents.

Types of emulsifier
1. Natural – emulsifier from hydrocolloids
Vegetable source: acacia, agar, pectin, lecithin
Animal source: gelatin, lanolin, cholesterol
2. Synthetic –
Cationic: quarternary ammonium salt, benzothonium chloride
Anionic: Na, K, NH4, Salt of fatty acid
Nonionic: Sorbitan esters, glyceryl esters

Use of emulsifier in foods:
• Acts as lubricants
• Build structure
• Improve eating quality
• Extend shelf life
• Prevent sticking
• Retention moisture
• Modify rheology of chocolate
• Crumb softening
• Provide stability

Emulsion breakdown mechanism (instability)
1. Creaming---the process in which droplets move upwards
(droplets density<density of continuous phase).
2. Sedimentation---the process in which droplets move
downwards (droplets density>density of continuous phase).
3. Flocculation---the process in which 2 or more droplets
“stick” together to form an aggregate (but the droplets still
retain their individual integrity).
4. Coalescence---the process in which 2 or more droplets merge
together to form a single larger droplet.
5. Phase Inversion---the process in which o/w emulsion changes
to w/o emulsion, or vice versa.

Foam
• Foam- Gas is dispersed in liquid or semi-
liquid.
Dispersed-phase: gas
Continuous-phase: liquid

Continue … …
• A surface active foaming agent is essential for the
formation of a stable foam.
• The foaming agent lowers the surface tension of the
liquid phase and allows expansion of its surface area.
• The surfactant forms a closely packed film around the
dispersed gas bubbles. This surface layer must have a
certain amount of strength or rigidity for foam
stability

Formation of a foam.
• Three process steps are important for formation of a
foam.
1. First, air has to be injected into the liquid (e.g.
using a mixer).
2. Second, large air bubbles have to be broken up
into smaller bubbles.
3. Finally, the smaller bubbles have to be prevented
from fusing during the formation of a foam.

Factors influencing foam stability are:
1. Surface tension
2. Concentration of separate phase
3. Presence of foaming agent to lower surface tension
4. Viscosity of liquid - the higher the viscosity, the more stable
the foam.
5. Presence and thickness of adsorption layer

Breakdown mechanisms for foam stability:
❑Once a foam has been formed, long-term stability is of
major importance.
❑There are three breakdown mechanisms for foam
stability:
1. drainage: the draining of liquid from foam
2. disproportionation: the change in foam bubble size
distribution caused by gas diffusion from small to large
bubbles
3. coalescence: the fusion of foam bubbles

Stability of foam
• Because food foams are thermodynamically unstable, the
bubbles must be stabilized.
• Stability is largely related with rheology of the system, but the
stabilization mechanism can vary. Rheology of system can be
improved by adding hydrocolloids.
• Bubbles are also stabilized by surface active agents (proteins,
emulsifiers).
• Gelatin is widely used to set the continuous matrix of aerated
food foams such as whipped cheese cakes and fruit jellies.
• Proteins and emulsifiers are both surface active, i.e.,
reducing the surface tension at an interface.
• However, they have very different chemical structures,
and the mechanism by which they stabilize bubbles in
food differs.
• Typical emulsifiers are small fatty acids chains with a
polar head; at a bubble interface, the water-loving polar
head can sit in aqueous phase, while the hydrophobic lipid
chain sticks out into the air.

Continue … …
• Proteins, by contrast, are long chain molecules made up
of amino acid units.
• Some parts of the protein molecule are hydrophobic,
other parts hydrophilic.
• The molecule can therefore unfold at the bubble interface,
such that the hydrophilic portions are in water and the
hydrophobic portions in the air.

CONCENTRATION
• Concentration: Concentration is a measure of the amount of
substance contained in a unit volume.
Unit:
• It may be expressed as weight per unit weight, or weight per
unit volume.
• Normally, concentration is given in percentage when weight
per unit weight measurement is used.
Example: what it mean when a food containing 20% fat?
Which means it will contain 20g of fat in every 100g of food.
• mass per unit volume
Example: mass of a solute dissolved in a unit volume of the
solution.

What is molarity and molality
• Both molality and molarity are measures of a chemical solution’s
concentration. The primary difference between the two comes down to
mass (kg)versus volume (L).
• The MOLALITY of a solution is equal to the moles of solute divided
by the mass of solvent in kilograms.
For example, a 1 molal solution contains 1 mole of solute for every 1 kg of
solvent.
• The MOLARITY of a solution is equal to the moles of solute divided
by the volume of solution in liters.
For example, a 1 molar solution contains 1 mole of solute for every 1 L of
solution.
NOTE: One mole of a substance is equal to 6.022 × 10²³ units of that
substance (such as atoms, molecules, or ions). The number 6.022 × 10²³
is known as Avogadro's number or Avogadro's constant. The concept of
the mole can be used to convert between mass and number of particles.

Continue …. … molarity
• Another term used to express concentration is molarity, or molar
concentration.
• It is defined as “no. of moles of solute present in 1 liter of solution”.
• Molarity (M) is the amount of a substance in a certain volume of
solution.
• SI Unit – M or mol/L.
• Equation: M = moles solute / liters solution
• To express these units in a dimensionless form, mole fraction may be
used; Independent of temperature.
• This is the ratio of the number of moles of a substance divided by the
total number of moles in the solution.
• Thus, for a solution containing two components, A and B, with number
of moles nA and nB, respectively, the mole fraction of A, XA, is

Continue …. … molarity
Problem: How much water should be added to 1 liter of 1 M
KOH solution to make it 0.2 M KOH solution?

Continue … …. molality
• Concentration is sometimes expressed by molality. Molality is
also known as molal concentration.
• Molality is defined as the “total moles of a solute contained in a
kilogram of a solvent.”
• The solution is composed of two components; solute & solvent.
• It is a measure of solute concentration in a solution.
• SI unit: mole per kilogram (mol/kg) or m.
• Molality Formula:
m = moles solute / kilograms solvent
• A relationship between molality, MA, and mole fraction, XA, for a
solution of two components, in which the molecular weight of
solvent B is MB, is

Molality Examples
Problem: Calculate the molality of a solution where 5 grams of toluene
(C7H8) is dissolved in 225 grams of Benzene (C6H6). Calculate the moles
of given solute.
Step 1 Step 2
Step 3

Difference between molarity and molality
Meaning

Problem
Calculate concentration units for a sugar solution. The sugar solution is prepared by
dissolving 10 kg of sucrose in 90 kg of water. The density of the solution is 1040
kg/m3. Determine a. concentration, weight per unit weight b. concentration, weight
per unit volume c. Brix d. molarity e. mole fraction f. molality.
Sl no. Given Equation / calculation Results and unit
1 Amount of sucrose 10 kg
2 Amount of water 90 kg
3 Density of solution 1040 Kg/m3
4
Volume of solution
(Amount of sucrose + Amount of water) / Density
of solution
0.0962 m3
5 Concentration (w/w) Amount of sucrose / (Amount of sucrose +
Amount of water)
0.1 kg solute/ kg
solution
6 Concentration (w/v) Amount of sucrose / Volume of solution 104 kg solute/m^3 solution
7 Brix [Amount of sucrose / (Amount of sucrose +
Amount of water)] × 100
10 (kg solute/kg
solution)*100
8 Molarity Concentration (w/v) / 342 0.30 mole solute/liter
of solution
9
Mole fraction
(Amount of sucrose / 342) / {(Amount of water /
18) + (Amount of sucrose / 342)}
0.0058
10 Molality (Amount of sucrose × 1000) / (Amount of water × 0.325 mole solute/liter
of solution

WATER ACTIVITY
• Water is present in all food. It usually takes two forms:
– free or available water or unattached water
– water that is bound to different molecules such as proteins and
carbohydrates
• Water activity is often described as a measure of the “free” or “non-
chemically bound” water.
• Water activity is a measure of water in a material that is available to
react with or attach itself to another material.
• Water activity is a measurement of the availability of water for
biological reactions.
• Available water can support the growth of bacteria, yeast and
mould, which can affect the safety and quality of food.
• It determines the ability of micro-organisms to grow.
• If water activity decreases, micro-organisms with the ability to grow
will also decrease.

Water Content / moisture content
• Moisture content is, simply, how much water is in a product.
• Water Content or moisture content is the total amount of water in a
product both bound water and free water.
• It is usually expressed as a percentage of the total weight:
• Mw (wet basis) =
• Mw = moisture content on a wet per cent basis
w = wet weight
d = dry weight
• It influences the physical properties of a substance, including
weight, density, viscosity, conductivity, and others. It is generally
determined by weight loss upon drying.
• It is easy to assume that foods with higher water content will have
a higher water activity than dry foods. This is not always correct.

Definition of water activity
• Water activity, aW, is a measure of how much of that water is free, i.e.,
unbound in a sample.
• Water activity, aw, is defined as the ratio of the water vapor pressure of the
food (P) to the vapor pressure of pure water (P0) at the same temperature.
• where:
p = partial pressure of water vapor of the food at temperature T
p0 = equilibrium vapor pressure of pure water at temperature T
• Free water or unbound water available to microorganisms to use for growth. It
is therefore important with regard to food safety.
• Water that is not bound to the ingredients themselves can be used by unwanted
microorganisms which could lead to one of the contributing factors for food
spoilage.
• Microorganisms will not grow below a certain water activity level - aW 0.90 for
most pathogenic bacteria, 0.70 for spoilage molds, and 0.60 for all
microorganisms.

Equilibrium Relative humidity (ERH)
• The same type of ratio also defines the relative humidity of
air, RH (usually expressed as a percentage , %)
• The relative humidity (RH) of air in equilibrium with a
sample is also called the Equilibrium Relative Humidity
(ERH) and is usually given as a percentage.
• It is equal to water activity according to

Water Activity scale
Water Activity is based on a scale from 0.0 to 1.0

Classification of foods according to their water
content
• Water is the most abundant
constituent in most foods.
• Classification of foods into three
groups according to their water
content or moisture content.
1. high water content: Fruits,
vegetables, juices, raw meat, fish
and milk etc.
2. intermediate water content :
Bread, hard cheeses and sausages
3. low moisture content :
dehydrated vegetables, grains,
milk powder and dry soup
mixtures.

Importance of water activity
• Water activity is a crucial factor in determining quality and safety
of foods.
• It affects shelf life, safety, texture, flavor, and smell.
• Water activity helps to predict which MO will be possible source
of spoilage.
• Water activity can affect the activity of enzymes and vitamins in
foods.
• It also has implication on color, taste and aroma.
• Water activity describes the degree to which the water is free in
the food and hence available to participate in chemical /
biochemical reactions and growth of MO.
• Important property that can predict the stability and safety of food
withy respect to MO growth, rates of deteriorative reactions and
chemical/physical properties.
• Water activity is often responsible for the microbial, enzymatic
and chemical deterioration of food.

Uses of water activity
1) Food product design: Food designers use water activity to
formulate shelf stable food. If a product is kept below a certain
water activity then mold growth is inhibited. This results in a
longer shelf life. Water activity values can also help limit
moisture migration within a food product made with different
ingredients.
2) Food safety: water activity is used in many cases as a critical
control points (HACCP) programs. Samples of the food product
are periodically taken from the production area and tested to
ensure water activity values are within a specified range for
food quality and safety.

Water Activity of Common Food Products
Food Product Typical water activity
Fresh fruits & vegetables 0.99
Fresh meat 0.99
Milk 0.99
Bacon 0.97
Bread 0.95
Flour tortilla 0.92
Jams 0.8
Soy sauce 0.8
Peanut butter 0.7
Rice 0.5-0.6
Honey 0.5-0.6
Roasted coffee beans 0.1-0.3

Water activity: effect on food quality and
stability
• Bacterial growth does not occur at water activity levels below
0.9.
• With the exception of osmophilic species, the water activity
limit for the growth of molds and yeasts is between 0.8 and
0.9.
• Most enzymatic reactions require water activity levels of 0.85
or higher.

Problem … …
• Estimate the water activity of honey. Consider honey as an 80%
w/w aqueous solution of sugars (90% hexoses, 10%
disaccharides).

Water activity and microorganism
• Water activity indicates the amount of water in the total water content
which is available to microorganisms. Each MO has its own
minimum water activity value below which growth is no longer
possible (MO may present).
• By measuring the water activity of food stuffs it is possible to
determine which MO will be able to develop.

Water activities required for growth of
organisms.

MOISTURE CONTENT
• Moisture content expresses the amount of water present in a moist
sample.
• Moisture content is, simply, how much water is in a product.
• Water Content or moisture content is the total amount of water in a
product both bound water and free water.
• Two bases are widely used to express moisture content;
1. Wet basis moisture content (MCwb) : the amount of water per unit
mass of moist (or wet) sample.
2. Dry basis moisture content (MCdb) : the amount of water per unit
mass of dry solids (bone dry) present in the sample.

Water Contents of Various Foods

Relationship between dry and wet basis MC
• A relationship between MCwb and MCdb may be developed as
follows:
• Divide both numerator and denominator of Equation (2) with
mass of dry solids.
• This relationship is useful to calculate MCwb when MCdb is known.
Similarly, if MCwb is known, then MCdb may be calculated from the
following equation:

Problem
Convert a moisture content of 85% wet basis to moisture content
dry basis
Solution
a. MCwb = 85%
b. In fractional notation, MCwb = 0.85
c. From equation,

Problem
• Problem: 500 kg of paddy at 22% moisture content (wb) is
dried to 14% moisture content (Wb) for milling calculate the
amount of moisture removed in drying.

Importance of Moisture Content in Foods
• Proper moisture content is essential for maintaining fresh, healthy
foods. If a food is too moist or too dry, it may not be suitable to eat
and will not taste as good as it would if it had the correct moisture
content.
1. Storability
2. Agglomeration in the case of powders
3. Microbiological stability
4. Flow properties, viscosity
5. Dry substance content
6. Concentration or purity
7. Commercial grade (compliance with quality
agreements)
8. Nutritional value of the product
9. Legal conformity (statutory regulations governing
food)

PHASE TRANSITION PHENOMENA IN FOOD
The glassy state in foods:
• With few exceptions, foods should be regarded as metastable systems
capable of undergoing change.
• Stability is a consequence of the rate of change. In turn, the rate of change
depends on molecular mobility.
• Molecular mobility is important in solid and semi-solid foods with low to
intermediate water content.
• In the majority of foods belonging to this category, the interaction between
polymeric constituents, water and solutes is the key issue in connection
with molecular mobility, diffusion and reaction rates.

Continue …
• Physically, a glass is an amorphous solid. It is also sometimes
described as a supercooled liquid of extremely high viscosity.
Conventionally, the viscosity assigned to a glass is in the order of
10^11 to 10^13 Pa.s.
• The molecules of a glass do not have an orderly arrangement as in
a solid crystal, but they are sufficiently close and sufficiently
immobile to possess the characteristic rigidity of solids.
• Because of the negligible molecular mobility, the rate of chemical
and biological reactions in glassy material is extremely low.
• The rigidity of the glassy regions affects the texture of the food.
Staling of bread is due to the transition of the starch–water system
from rubbery to glassy state.
• The crunchiness of many snack products is due to their glassy
sructure.

The glass transition phenomena of honey
• Consider a liquid food product, such as honey, consisting of a
concentrated aqueous solution of sugars.
• The physical properties and stability of such a solution depend on
two variables: CONCENTRATION and TEMPERATURE.
• If the concentration is increased by slowly removing some of the
water and the temperature is lowered gradually, solid crystals of
sugar will be formed.
• If the process of concentration and cooling is carried out under
different conditions, the viscosity of the solution will increase until a
rigid, transparent, glass-like material will be obtained. (crystallization
will not take place)

Continue …
• The familiar transparent hard candy is an example of
glassy (vitreous) food.
• The glassy state is not limited to sugar–water systems.
Intermediate and low moisture foods often contain
glassy regions consisting of polymer materials (e.g.
gelatinized starch) and water.
• The phenomenon of passage from the highly viscous,
rubbery semi-liquid to the rigid glass is called ‘ glass
transition ’ and the temperature at which that occurs is
the ‘ glass transition temperature, Tg.

Glass transition temperature, Tg
• The phenomenon of passage from the highly viscous, rubbery
semi-liquid to the rigid glass is called ‘ glass transition ’ and “the
temperature at which that occurs is the ‘ glass transition
temperature, Tg”.
• The glass transition temperature of a given rubbery material is not
a fixed point. It varies somewhat with the rate and direction of the
change (e.g. rate of heating or cooling), therefore the procedure
for its determination has to be specified exactly.
• Glass transition temperature is strongly dependent on
concentration. Dilute solutions have lower Tg.

Application of glass transition temperature
• Just as water activity, glass transition temperature has become
a key concept in food technology, with applications in quality
assessment and product development.
• Since the glassy state is considered as a state where
molecular mobility is at a minimum, it has become a custom
to study food properties and stability, not as a function
of the temperature

Types of food according to pH
pH is defined as the negative log of the hydrogen ion concentration. pH is a measure
of the relative amount of free hydrogen and hydroxyl ions concentration, a measure of the
acidity or alkalinity of a solution.

Continue
On the basis of acidity, food can be classified as follows,
1. Low acid food: pH 5. 3 and higher. This class include peas,
beans, corn, asparagus, meat, fish, poultry, dairy product and most
of vegetables. These are subjected to spoilage by all group of
thermophilic and mesophilic microorganism including botulinum.
Food belonging to this category are also regarded as non-acid food.
2. Medium acid food: pH 4.3 – 5.3. Spaghetti, soup and sauces,
asparagus, beets, pumpkin, spinach, some vegetables and other
partly acid product come under this category. These are also
subjected to spoilage by thermophilic and mesoplilic anaerobes
(not producing H2S) which may cause flat sour spoilage of food.

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3. Acid food: pH: 3.7 – 4.7. This group includes some
vegetables like tomatoes, peas, pineapple and other
fruits. These foods are subjected to spoilage by non-
spore forming microorganism.
4. Highly acid food: pH 3.7 and below. This group
includes Berries, sauerkraut, pickles, jam jellies etc.
Alkaline Foods : Most vegetables, Most fruits, Most beans and
lentils, Soy, Fats like olive oil and avocados.
Magic pH: The lower limit of growth of important poisoning
microorganism, Clostridium botulism is 4.5. Hence pH below 4.5,
it is accepted that no bacterial growth takes place and even if
present, do not harm. Thus the pH 4.5 is also called as ‘magic pH’.

Mass-Volume-Area Properties of Food

Mass-Volume-Area Properties of Food

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