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INTRODUCTION TO FOOD CHEMISTRY (WATER)
Definition of food: Food is any substance consumed to provide nutritional support for an
organism. It is derived mainly from two sources i.e., plants and animals. Composition of food
also provides energy to carry out various life processes.
Composition of food: The main components of food should include nutrients like
carbohydrates, proteins, fats, vitamins, minerals and roughage. These nutrients present in our
meals make for a balanced diet which is essential for us and helps us be healthy.
Definition of water in food: It is a chemical compound composed of two hydrogen atoms
and one oxygen atom. Liquid state – water, solid state – ice and gaseous state – steam. The
physical and chemical stability as well as growth of microbes in a food can be controlled by
optimizing the water within foods by various methods. The water content is measured by
drying, infrared or nuclear magnetic resonance techniques. The measurement of water
activity is more relevant in understanding the quality and safety aspects of water.
Structure of water
Water is relatively small inorganic molecule, but organic life is highly dependent on this tiny
molecule. It is the only substance on the earth that occurs abundantly in all three physical
states (gas, liquid and solid). Water is essential for life: as:
i. regulator of body temperature
ii. solvent
iii. carrier of nutrients and waste products
iv. reactant and reaction medium
v. lubricant and plasticizer
vi. stabilizer of biopolymer conformation
vii. facilitator of the dynamic behaviour of macromolecules (e.g. catalytic activity)
Most of the fresh foods contain large amounts of water. It is one of the major component in
composition of many foods. Each food has its own characteristic amount of this component.
Effect of water on structure, appearance and taste of foods as well as their susceptibility to
spoilage depends on its amount, location, and orientation. Therefore, it is essential to know its
physical properties. Water has unusually high melting point, boiling point, surface tension,
permittivity, heat capacity, and heat of phase transition values. Other unusual attribute of
water include expansion upon solidification, large thermal conductivity compared to those of

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other liquids, moderately large thermal conductivity of ice compared to those of other
nonmetallic solids.
Water Molecule: Some of the unusual properties of water are due strong intermolecular
attractive forces among molecules of water. The unusual properties of water can be explained
from nature of water molecules. In formation of water molecule, two hydrogen atoms form
covalent bonds with oxygen. The highly electronegative oxygen of the water molecule pulls
the single electron from each of the two covalently bonded hydrogen atoms towards its self,
as a result each hydrogen atom becomes partially positively charged and oxygen becomes
partially negatively charged. Consequently, resultant covalent bond formed between oxygen
and hydrogen atoms acquires partial ionic character. The bond angle of individual water
molecule in vapour state is 104.5°.
Fig. 1: Structure of the water molecule

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Fig. 2: Hydrogen bonding of water molecules in a tetrahedral configuration
Association of Water Molecules: The shape of water molecule and the partial polar nature
of the O-H bond in the water molecule create intermolecular attraction force. Such inter
molecular attraction, results in to formation of hydrogen bonds between the water molecules.
Therefore, water molecules associate with considerable tenacity.
Fig. 3: Structure and hydrogen-bond possibilities for a hydronium ion
Each water molecule involves in four hydrogen bonds with neighbouring water molecules.
Multiple hydrogen bonding between water molecules form a structure of three-dimensional
network.

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Fig. 4: Hydrogen bonding (dotted lines) of water to two kinds of functional groups
occurring in proteins
Existence of three-dimensional hydrogen bonded structure of water is responsible for many
of its unusual properties. The extra energy needed to break intermolecular hydrogen bonds.
This leads to large values for heat capacity, melting point, boiling point, surface tension, and
enthalpies of various phase transitions of water. The dielectric constant (permittivity) of
water is influenced by hydrogen bonding. Hydrogen-bonded multi-molecular dipoles increase
the permittivity of water. The hydrogen bonded arrangement of water molecules is highly
dynamic, allowing individual molecules to alter their hydrogen-bonding relationships with
neighboring molecules. This phenomenon facilitates mobility and fluidity of water. The open,
hydrogen-bonded, tetrahedral structure of water molecules in ice is responsible for low
density of water in ice form. The extent of intermolecular hydrogen bonding among water
molecules depends on temperature.
Fig. 5: Hydrogen bonding in Ice

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With input of heat melting of ice occurs; that is, some hydrogen bonds are broken distance
between nearest neighbour increases. The latter factor predominates at temperatures between
0 and 4°C, which causes net increase in density. Further warming increasing distance
between nearest neighbours (thermal expansion) predominates above 4°C, which causes net
decrease in density.
Water in Food Systems
Water is abundant in all living things and consequently is in almost all foods, unless steps
have been taken to remove it. It is essential for life, even though it contributes no calories to
the diet. Water also greatly affects the texture of foods, as can be seen when comparing
grapes and raisins (dried grapes), or fresh and wilted lettuce. It gives crisp texture or turgor to
fruits and vegetables and also affects perception of the tenderness of meat. For some food
products, such as potato chips, salt, or sugar, lack of water is an important aspect of their
quality and keeping water out of such foods is important to maintain quality. The water
contained in raw foods is referred to as the moisture content of those foods. The moisture
content of certain foods depends on the temperature and partial vapor pressure of water in the
surrounding. These are called hygroscopic foods. On the other hand, the foods in which the
moisture content does not get affected by these factors are called non-hygroscopic. The

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hygroscopic foods can absorb water in a number of ways. This is referred to as sorption and
can occur by formation of a hydrate, binding to surface, diffusion in the food, capillary
condensation, formation of a solution etc. Let us first learn about the forms/ types in which
water is present in food products.
TYPES OF WATER IN FOOD:
The water content or the moisture content of a food influences its appearance, texture and
flavour. It varies a great deal in different food items. The green leafy vegetables contain more
than 90 per cent of water while it may be negligible in oils and fats like ghee, butter, oil etc.
Water in foods can be either in free or bound form, depending on its interaction with the
surrounding molecules. The ‘bound water’ refers to water that is physically or chemically
bound to other food components Many compounds like starch, proteins and some salts have
water bound to them in the form of hydrates.
The free water is the bulk water free from any other constituents. It is held in narrow
channels between certain food components due to capillary forces and is held trapped within
the spaces in food. It is surrounded by physical barrier e.g., biological cell that prevents it
from escaping. As you will read in the next subsection, the free water is actually responsible
for the microbial growth and deterioration of food. This form of water is also called the
available water.
Another form of water in food is called imbibed water. This water is found in hydrophilic
gums like gelatin which is a type of protein with ability to absorb a large amount of water.
Gelatin forms a jelly like mass on absorbing or imbibing water. Imbibed water is more or less
like the hydrate formation and involves hydrogen bonding. A yet another form of water in
food is adsorbed water. Some solid foods have the ability or tendency to adsorb water on
surface. The powdered forms of the solids adsorb more water because of a larger surface
area.
Moisture Content: The moisture content of a food item is defined as the amount of water
lost per gram of the food product at about 100o C. Mathematically it can be represented as
follows.
% Moisture = (mw/msample) × 100
where, mw = mass of water
msample = mass of sample
Mass of water can be related to the number of water molecules in the following
manner:

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mw = nw Mw/NA
nw = no. of water molecules
Mw = molecular weight of H2O (18g mol-1)
NA=Avogadro number = 6.022 × 1023 mol-1 (Avogadro number is defined as the
number of atoms, molecules or ions in one mole of a substance). It equals 6.022 ×
1023 mol-1.
It is important to find out the moisture content of a food as it helps to know:
 the probability of microbial growth;
 the food quality as the water content determines the texture, taste, appearance
and stability of foods; and
 the behaviour of foods during and post processing e.g. mixing, drying,
packaging, storage etc.
A number of analytical techniques have been used to measure the moisture content of foods
and their advantages and disadvantages. The loss on drying, infrared, NMR or Karl Fisher
titration are commonly employed tools. Such moisture determination is essential in terms of
product quality, composition, shelf life, package determination etc.
Though important, water content or per cent moisture is not a reliable predictor of microbial
responses and chemical reactions in food products. The water content of a safe product varies
from product to product and from formulation to formulation. One safe, stable product might
have a water content of 15 per cent while another with a water content of just 8 per cent is
susceptible to microbial growth. This is so because the microbial stability or physicochemical
properties of food are often determined by amount of free water present rather than the total
amount of water. It is the free water, not the bound one that supports the growth of bacteria,
yeasts and molds (fungi). This unbound or available water is expressed in terms of water
activity. This is a crucial parameter and is often ill understood.
WATER SORPTION ISOTHERMS
The relationship between water content and water activity of food stuff can be expressed in
terms of a plot, between the two at a given temperature, called sorption isotherm. Higher
moisture content normally means a larger water activity. The relationship, however, is not
linear. In fact, the variation in aw with an increase in water content generally shows a sigmoid
(S-shaped) curve. Such a curve at a given temperature is called the moisture sorption
isotherm.

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These isotherms are obtained by placing completely dry food samples in the atmospheres of
increasing relative humidity at a given temperature and measuring the increase in the weight
of the sample due to absorption of water. The data so obtained is then plotted to get the
sorption isotherm. Thus a sorption isotherm is a plot of the amount of water absorbed
(moisture content) as a function of relative humidity (or water activity). A schematic
representation of a sorption isotherm is given in the figure below:
Fig. 6: Schematic representation of adsorption and desorption isotherms
A typical sorption isotherm may be divided into three different regions depending on the state
of water present in the food. The initial increase in the moisture content with an increase in
the relative humidity corresponds to the formation of a monolayer film of water due to
adsorption on food constituents. This region I is followed by region II that is due to the
adsorption of additional layers of water over the monolayer. The region III is a result of the
condensation of water, in the pores of the food material that dissolves the soluble components
of the food. A simple equation called the Brunauer Emmet Teller (BET) equation can be used
to calculate the monolayer moisture value from aw versus moisture content data of an
isotherm. This value is an important determinant of the shelf life of the food. Similar to the
sorption curves, the desorption isotherms can be obtained by placing the wet food samples in
the atmospheres of same relative humidity as in case of sorption isotherms at a given
temperature and measuring the decrease in the weight of the sample due to desorption of
water. The data so obtained is then plotted to get the desorption isotherm. A look at the
sorption and desorption curves of a given food item reveals that the amount of water
(moisture content) at any aw may be different in the two types of isotherms. This difference
between desorption and adsorption is called hysteresis and is generally observed in most of
the hygroscopic products.

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Significance of Sorption-Desorption Isotherms
The sorption isotherms drawn for different food products are quite significant in a number of
ways. Some of these are listed below:
 These provide useful information for processing and packaging the dehydrated food
items.
 A study of these graphs will be helpful in deciding about the food products that may
be packed together.
 These help in determining the water content at certain critical aw values related to
stability and safety of the food items.
 These can be used in the selection of suitable ingredient to make a safe formulation at
high moisture content.
EFFECT OF WATER ACTIVITY ON PACKAGING AND STORAGE
The conditions during storage and transport should be maintained in such a way so that any
deterioration is kept to a minimum. These problems are influenced by the temperature and
relative humidity during storage and by the type of package in which foods are stored.
Therefore, we should consider the water activity requirement in the packaging, storage and
transportation of food products.
i) Transportation of processed products
During transportation of processed products the moisture migration may cause condensation.
This has proved to be a problem in successful transportation of some commodities due to
differences in climatic zones. For example, if cooling is followed by an increase in
atmospheric temperature then humidity causes condensation on the inner surface of any
container. This causes an increase in the relative humidity of the air above 90 per cent, but
aeration can reduce it to a safe level.
Similarly, condensation of moisture on the surface of containers such as canned foods may
spoil the label, corrode the can and mould attack. This type of problem can be prevented by
drying the container before loading and lacquering the can from outside.
In the atmosphere of equilibrium relative humidity (ERH) a commodity at low water activity,
even small decrease in air temperature may cause moisture condensation. For example, the
air at 70 per cent ERH and 30o C becomes saturated and hence prone to moisture
condensation if its temperature falls by only 6o C, such conditions may also permit the mould
growth.

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ii. Packaged products
Packaging generally reduces the atmosphere of a commodity to a minimum, hence protects it
from outside sources, particularly the migration of moisture and variation in temperature. The
quality of a food depends upon its properties, packaging material and the storage
environment. We are concerned with the changes influenced by water activity, particularly
appearance, taste, odour and texture, resulting from microbiological and biochemical
changes. Packages are expected to provide adequate degree of protection to the product. With
advances in packaging technology, packages are often expected to provide nutritional and
constitutional information to sell the product. The packaging material has to provide desired
water activity, which is the main concern. But, there are special requirements that include
protection from light and oxygen, retention of preservatives, fragility of the product and the
ease with which the package can be filled, handled and stored. Properly sealed cans and glass
bottles give complete protection to the packed food against the intake of gases, moisture and
microorganisms. The more attention should be given to the flexible films or packaging
materials, which exhibit a very wide range of moisture (vapour) and gas permeability.
iii) Packaging materials
The basic flexible materials of importance are aluminium, plastic, regenerated cellulose and
paper. The overall permeability of aluminium foil and laminate is near zero. The permeability
of plastic film is governed by its thickness. Thicker films generally have lower permeability,
greater strength and higher cost. In food packaging application, the materials are combined
by lamination, coating or co-extrusion, and they may have properties different from the basic
or individual film. Type of food product, its hygroscopicity, environment inside and outside
of package, handling during storage, marketing, transportation, etc., usually govern this type
of combination of packaging materials. Sometimes, microbiological problems may arise.
When moist product is packed in impermeable or low permeable materials, such as glass,
metal or films coated with polyvinylidene chloride or polyvinyl chloride, the food will
equilibrate with the internal atmosphere of the package, and cooling may cause condensation
of moisture, resulting in high water activity in which microbial growth may occur. Hence, we
should see that product of relatively low water activity be packed in moisture impermeable
packages and some measure of microbial control should also be used. Although, when high
water activity products are packed in permeable material, then care must be taken to prevent
excessive moisture loss from the product. This can be achieved by careful selection of
wrapping material or by control of humidity in the surrounding atmosphere of the package.

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iv)Unrefrigerated packaged products
Dehydrated vegetables, which have water activity of 0.30 or below, require protection against
moisture and oxygen. Laminates of polyethylene, aluminium foil and paper have been
successfully employed for packaging dehydrated vegetables. Dehydrated soups are also
packaged similar to dried vegetables, which provide full protection to the product. The dried
fruits have much higher water activity levels, may be stored safely in relatively permeable
film, where ambient humidity is very low. The potato chips and roasted nuts which depend
on their appeal and crispness are not susceptible to unsuitable storage conditions. These
products have substantial amount of lipids, hence need additional protection, and therefore
hence packaged with nitrogen gas. But for long storage costly items like nuts vacuum
packaging in cans or glass jars require. However, for retail marketing, these products are
successfully packaged in a wide variety of materials with low water vapour transmission rate,
and oxygen permeability. For example, cellulose-plastic combination, poly vinylidene
chloride coated-cellulose or polypropylene is used.
v) Refrigerated packaged products
Generally, fresh fruits and vegetables are kept in cool store at suitable temperatures and
relative humidity (RH). For example, cured onions and garlic can be stored at 0°C with RH
65-70 per cent, whereas vegetables like cabbage, carrots, cauliflower, leafy greens, green
peas, turnip require 95 per cent of RH. Apples are stored at 0-3°C with 85-90 per cent RH
depending upon non-chilling and chilling sensitive varieties. A compromise is required
between packaging material of fresh fruits and vegetables that retain water and maintain
crispness, but cause condensation and fogging of the films and those which permit loss in
weight and crispness, but do not fog. Sometimes, adequate perforations are made in the
package for minimum respiration without physiological disorders in the living tissues. Frozen
vegetables are commonly packed in polyethylene or paperboard, waxed or plastic coated
moisture proof film, to minimize oxygen uptake and loss of moisture. The frozen fruit juice
concentrates are packaged in hermetically sealed tin cans or aluminium laminated composite
containers.
FOOD SPOILAGE
Most natural foods have a limited shelf life. You must have observed that fresh food items
such as meat, fish, pepper, mangoes or oranges if kept in open for a few days, their
appearance, smell and taste changes. These get covered with whitish or orange substances or
start smelling bad. Some foods can be kept for a considerably longer time. No doubt,

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eventually these also get decomposed. This is known as decay and leads to food spoilage i.e.,
the food become unfit for consumption.
Spoilage can be defined as alterations in foods or the physiological, chemical and biological
changes in a food that make it inedible or hazardous to eat. Certain organisms and chemicals
present in the food and outside it are responsible for the gradual changes causing the spoilage
of foods. The two main ways by which food can spoil are given below:
Natural decay: It is a result of moisture loss through respiration or evaporation of water and
the action of enzymes present in the food by oxidation, browning and ripening processes.
Contamination by microorganisms: The enzymes present in the food facilitate its
contamination by micro-organisms, (like bacteria, moulds and yeasts) and cause food
spoilage. These microorganisms multiply rapidly in favourable conditions of moisture and
temperature.
The effect of contamination is different on different components of food. For example, the
proteins get putrified and produce foul smell while the fats and oils get rancid i.e., they begin
to smell and taste bad. On the other hand cooked cereals containing carbohydrates become
marshy and slimy on contamination and are called stale.
Role of Water Activity: On the basis of moisture content, the food products can be broadly
put into three categories as: Low moisture foods e.g., dried or freeze dried foods having a
moisture content of 5-15 per cent intermediate moisture foods e.g., cakes and dates with a
moisture content of 20-40 per cent and high moisture foods e.g. fresh fruits and vegetables
of greater than 40 per cent of moisture. Though moisture content is important, it is the water
activity (aw) that is critical factor for the shelf life of a food item. It may be used to predict
stability with respect to physical properties like texture and caking, rates of deteriorative
reactions and microbial growth.
Conventionally, the water activity in foods has been controlled by drying, addition of sugar or
salt, and by freezing. The added salt or sugar dissolves in the free water and makes it bound
or unavailable. In other words, it decreases the water activity and makes the food less prone
to spoilage. Lowering the temperature checks the activity of the enzymes in the food and also
makes conditions unfavourable for the growth of microorganisms. Drying of the product, acts
as a means of preserving the food by lowering the water activity.
PROPERTIES OF LIPIDS
The physical and chemical properties of fats are found to depend mainly on their
composition. A number of these properties are made use of in identification and ascertaining

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their purity. Their physical properties help in identifying the utility of a fat for some specific
purpose and in determining the stage of processing.
Physical Characteristics: Contrary to the common belief, pure oils and fats are colourless,
odourless and tasteless. The colours and flavour of the fats are a result of the presence of the
substances that dissolve in them. Fats and oils are lighter (densities are about 0.8 to 0.9 gm3
)
than water and are poor conductors of heat and electricity. Some of the common physical
properties of oils and fats are discussed below:
 Emulsification: the fats are insoluble in water and form two layers on mixing and
shaking. However, they can form emulsions in the presence of emulsifying agents. An
emulsion is a heterogenous system consisting of at least one immiscible liquid dispersed in
another in the form of droplets. The phase present as fine droplets is called disperse or
discontinuous phase while the one in which these droplets are suspended is called the
continuous phase. The emulsion can be oil in water type: e.g. milk, cream, mayonnaise
and salad dressings. It can be water in oil type where water gets dispersed in oil (e.g.
butter). Surface active agents that are added to help in emulsification are called emulsifiers
or emulsifying agents. The emulsification of fats is a necessary step in a number of food
products such as cakes, ice-cream and other frozen desserts. Some of the common natural
food emulsions are milk, cream and egg-yolk.
 Melting point: Fats do not give a sharp melting point like other organic compounds.
Instead they soften over a range of temperatures. The melting point of a fat also depends
on its previous heating-cooling history. This has been attributed to the phenomenon of
polymorphism i.e. existence of fats in different crystalline forms. Another factor
responsible for the absence of sharp melting point of fats is that they consist of mixtures of
glycerides each with its own characteristic melting point.
The consistency of a fat depends on how it has been obtained i.e. whether it is obtained by
gradual or sudden cooling of the oil. However, if kept for sufficient time, it equilibrates
and becomes reasonably consistent. In general, the fats which contain relatively large
amount of unsaturated fatty acids have relatively low melting points and are usually oils at
room temperature. The fats with large amount of saturated fatty acids have higher melting
points. Some related terms are explained as follows:
 Softening point: The temperature at which an equilibrated solid fat, filled in a
capillary tube, starts rising on slow heating indicates its softening point. The
softening point can be used to characterise some fats.

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 Slipping point: It is an empirical characteristic of fats and is related to the
composition of a fat and also air or water embedded into the fat during its
manufacturing process. To measure it, small brass cylinders filled with the solid fat,
are suspended in a bath close to the thermometer. The temperature of the bath is
slowly raised by stirring. The temperature, at which the fat rises in the cylinder or
slips, is called as the slip point.
 Shot melting point: It is the temperature at which a small lead shot will fall through
a fat sample. Thus, for fats the melting point is defined by the specific conditions of
the method by which it is determined.
 Specific Gravity: The specific gravity of fats is found to depend to some extent on the
degree of un-saturation and the chain length of the fatty acid. Since the nature of fats is
such that they are sensitive to temperature, the specific gravity is usually measured at
25°C. However, for high melting fats a higher temperature of 40°C or even of 60°C may
be required.
 Refractive Index: It is defined as the ratio of velocity of light in vacuum to the velocity
of light in the oil or fat. Generally, it is expressed as the ratio of the sine of angle of
incidence to the sine of angle of refraction when a ray of light of known wave length
passes from air into the oil or fat. It is measured with the help of Abbe’s or Butyro
refractometer. When butyro refractometer is used, its reading (called B.R. Reading) is
converted to refractive index with the help of suitable conversion tables. As refractive
index varies with temperature and wavelength, the temperature is maintained during the
measurements. The refractive index of fats can be used for their identification and also for
testing their purity. The refractive index of fats is found to increase with increase in length
of the carbon chains and also with the number of double bonds in it.
 Smoke, Flash and Fire points: Smoke point is the temperature at which a fat or oil gives
off bluish smoke on heating in an open vessel. The flash point is the temperature at which
the mixture of fat vapours with air gets ignited and the fire point is that temperature at
which the fat sustains a continued combustion. For a given sample of oil or fat, the
temperature is progressively higher for the smoke point, flash point and fire point. These
are found to depend on the amount of free fatty acids in the fat. However, the degree of
un-saturation has little effect on them. These are particularly useful in connection with fats
used for any kind of frying.
 Turbidity Point: It is the temperature at which turbidity appears in a solution of fat on
cooling. It is determined by warming a mixture of oil and solvent in which it has limited

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solubility till it dissolves and slowly cooling it till the oil separates out and turbidity
appears. A number of different solvents have been employed to determine the turbidity
point. The turbidity points are found to be sensitive to the presence of free fatty acids. This
test has utility in differentiation of different fats and helps in detection of adulteration.
Chemical Reactions and Fat Constants
The chemical reactions of fats depend on their composition and the nature of their
constituents. A number of tests based on the determination of the chemical composition have
been developed to serve as tools to identify, differentiate and also to check for the purity of
oils and fats. The characteristic parameters that are used for identification of oils and fats are
sometimes called as fat constants. Some of the common chemical reactions of oils and fats
and the tests based on these are discussed below:
 Saponification: The saponification value is an index of mean molecular weight of the
fatty acids of glycerides comprising a fat. Oils and fats when boiled with alcoholic
solution of NaOH or KOH undergo hydrolysis into glycerol and fatty acids. Since the
sodium or potassium salts of fatty acids so obtained act as soaps, the reaction is
known as ‘saponification’. The number of milligrams of potassium hydroxide
required saponifying 1g of the oil or fat is called its saponification number or
saponification value. This number is a measure of the size of the fatty acid chains. A
larger value indicates that the fat contains short chained or low molecular weight fatty
acids whereas the fats containing long chained fatty acids would have a low
saponification value.
Butter with high percentage of butyric acid has the highest saponification value.
 Reichert-Meissl and Polenske Values: Butter fat contains mainly the glycerides of
butyric acid which is volatile and water soluble. As no other fat contains butyric acid
glycerides, the Reichert Meissl value of the butter fat is higher than that for any other
fat.
Butter is different from other fats as it contains the glyceryl esters of relatively low
molecular weight fatty acids, primarily butyric acid and caproic, capric, caprylic,
lauric and myristic acids. These fatty acids are wholly or partially steam volatile and
water soluble. The Reichert-Meissl Value is a measure of water soluble steam
volatile fatty acids mainly butyric and caproic acids present in oil or fat. It is defined
as the number of millilitres of 0.1N aqueous sodium hydroxide solution required to

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neutralise the steam volatile water soluble fatty acids obtained by distillation of 5g of
an oil-fat under the prescribed conditions.
The Polenske Value, on the other hand is a measure of the steam volatile and water
insoluble fatty acids, like caprylic, capric and lauric acids present in oil or fat. It is
defined as the number of mili litres of 0.1N aqueous alkali solution required to
neutralize steam volatile water insoluble fatty acids obtained by the distillation of 5g
of the oil-fat under the prescribed conditions.
As coconut oil and palm kernel oil contain appreciable quantities of steam volatile but
water insoluble caprylic, capric and lauric acid glycerides, these have high Polenske
value.
 Iodine value: The iodine value is a measure of the degree of un-saturation in oil. It is
constant for particular oil or fat. Iodine value is a useful parameter in studying
oxidative rancidity of oils since higher the un-saturation, the greater the possibility of
the oils to go rancid. It is also known as iodine adsorption value or iodine number or
iodine index. Iodine numbers are often used to determine the amount of un-saturation
in fatty acids. This un-saturation is in the form of double bonds, which react with
iodine compounds. The higher the iodine number, the more C=C bonds are present in
the fat. If the iodine number is between 0-70, it will be a fat and if the value exceeds
70 it is oil. Starch is used as the indicator for this reaction so that the liberated iodine
will react with starch to give purple coloured product and thus the endpoint can be
observed.
 Peroxide value: It is the number of milli equivalents of active oxygen that expresses
the amount of peroxide contained in 1000 g of the substance. The peroxide value is
used for identifying the onset of oxidative change in fats and oils, during which the
oxygen (O2) molecule penetrates the fat molecule in the form of a peroxide group
(H2O2). The peroxide value is a quality criterion for explaining the freshness of edible
oils. The lower the figure, the fresher the oil. Crude pressed oils have a PV of 5-20,
refined oils 0-1.
 Saponification value: The saponification value of an oil or fat is defined as the
number of mg of potassium hydroxide (KOH) required for neutralizing the fatty acids
resulting from the complete hydrolysis of 1 g of the sample. It gives an indication of
the nature of the fatty acids constituent of fat and thus, depends on the average
molecular weight of the fatty acids constituent of fat. The greater the molecular
weight (the longer the carbon chain), the smaller the number of fatty acids is liberated

17. [17]
per gram of fat hydrolyzed and therefore, the smaller the saponification number and
vice-versa. The magnitude of saponification value gives an idea about the average
molecular weight of the fat or oil. It also indicates the length of carbon chain of the
acid present in that particular oil or fat. Higher the saponification value, greater is the
percentage of the short chain acids present in the glycerides of the oil or fats.