UNIT 3 LIPIDS

Structure
3.1 Introduction
3.2 Introduction to Lipids
3.3 Classification and Composition
3.3.1 Classification of Lipids
3.3.2 Categories of Fats and Oils
3.4 Functional Properties of Food Lipids
3.5 Deep Fat Frying
3.5.1 Factors Affecting Process of Deep Fat Frying
3.5.2 Maintaining the Quality of Frying Oil
3.6 Deteriorative Changes in Fats and Oils
3.6.1 Autoxidation
3.6.2 Factors Influencing Lipid Oxidation
3.6.3 Lipolysis
3.6.4 Thermal Decomposition
3.7 Antioxidants – Preventing the Deteriorative Changes in Fats and Oils
3.8 Let Us Sum Up
3.9 Glossary
3.10 Answers to Check Your Progress Exercises

3.1 INTRODUCTION

After a detailed study on carbohydrates, we now move on to the next structural

component of all living cells, the lipids. Lipids are the major components of adipose
tissue and together with the proteins and carbohydrates they constitute the principal
structural components of all living cells.

Lipids in food exhibit unique physical and chemical properties. Their composition,
crystalline structure, melting properties and the ability to associate with water and other
non-lipid molecules are especially important to their functional properties in many foods.
We will learn about these properties and the role of lipids in product preparation in this
unit. Further, during the processing, storage and handling of foods, lipids undergo
complex chemical changes and react with other food constituents, producing numerous
compounds, both desirable and deleterious to the food quality. What are these
deteriorative changes in food lipids? Are there any means of controlling such changes?
These are the other issues highlighted in this unit.

You will realize, that like in previous two units, here too we have not dwelt much on the
structural component of lipids. The reason being that the structure, physical/chemical
properties have been discussed in the Nutritional biochemistry course. We do not wish to
duplicate the effort here and make the content bulky. However, we do advise you to look
up the relevant block/unit in the Nutritional Biochemistry course before you start
studying this unit. Best approach would be to have those blocks handy so that you can
refer to them as and when required.

Objectives
After studying this unit, you will be able to:
• enumerate the important sources of food lipids,
• describe the basic composition of food lipids,
• discuss the role of food lipids in product preparation,
• debate on the importance of functionality of food lipids with reference to food
processing and quality of finished products, and
• recognize the deteriorative changes in food lipids and means of controlling such
changes.

3.2 LIPIDS – INTRODUCTION AND SOURCES

In its broadest sense, ‘lipids’ defines substances as oils, fats and waxes which can be only
characterized by a large array of properties. They are in general:
- coming from plant and animal origin;
- insoluble or immiscible with water but soluble in organic solvents such as
chloroform, ether, benzene, acetone; and
- formed of long-chain hydrocarbon groups (carbon and hydrogen), but may also
contain oxygen, phosphorous, nitrogen and sulphur.
Glycerol esters of fatty acids, which make up to 99% of the lipids of plant and animal
origin have been traditionally called fats and oils. This distinction, based solely on
whether the material is solid or liquid at room temperature, is of little practical
importance and the two terms are often used interchangeably.

Food lipids are either consumed in the form of “visible” fats, which have been separated
from the original plant or animal sources, such as vegetable oil and butter, or as
constituents of basic foods, such as milk, cheese and meat. This is referred to as ‘invisible
fat’. You already know that dietary lipids play an important role in nutrition. They supply
calories and essential fatty acids, act as vitamin carriers and increase the palatability of
food. The largest supply of vegetable oil comes from the seeds of soy bean, cottonseed,
peanut and the oil-bearing trees of palm, coconut and olive.

Oil–bearing fruits, nuts and seeds have been grown and used for food for many centuries.
More than 100 varieties of plants are known to have oil–bearing seeds, but only a few
have been commercialized. The largest source of vegetable oil at present is the seeds of
annual plants such as soybean, cottonseed, peanut, sunflower, safflower, mustard and
rapeseed. Many of the oil–bearing seeds are not only a source of oil, but also protein, the
protein portion has the most value. A second source of vegetable oils is the oil–bearing
fruits and nuts of trees such as coconut, palm, palm kernel and olive. The oil from the
palm and olive is extracted from the fruit rather than the seed of the fruit. All the oil–
bearing trees require a relatively warm climate, two of which are tropical: coconut and
palm.
Oil contents for vegetable oil–bearing materials vary between 18% and 68% of the total
weight of the seed, nut, kernel or fruit as indicated in Table 3.1.

Table 3.1: Oil Content of few Vegetable Oil Sources

Oil Bearing Material Oil Content (%)
Coconut 65 – 68
Cottonseed 18 – 20
Olive 25 – 30
Palm 45 – 50
Palm kernel 45 – 50
Peanut 45 – 50
Safflower 30 – 35
Soybean 18 – 20
Sunflower 35 – 45

Meat fats are derived almost entirely from three kinds of domestic animals: hogs, cattle
and sheep. Milk of cow and buffalo is an important source of fat in the form of either
butter or ghee. Bulk of the world’s milk fat production consists of butterfat from cow’s
milk, and in India, butter and ghee have a well-established place in the culinary practices.

Fats and oils are a unique class of agricultural products in that a high degree of
interchangeability among them is possible for many products and uses. Additional
processing and/or blending of one or more source oils may be necessary for a satisfactory
substitution. Knowledge of the physical and chemical properties of each individual raw
material is necessary to successfully duplicate or improve on the functionality of the
original source oil’s functionality. To understand this, we need to first look at the
composition of lipids. The next section is devoted to the classification and composition of
lipids.
3.3 LIPIDS - CLASSIFICATION AND COMPOSITION

The classification and categories of lipids is presented in this section. There may be
different ways of classifying lipids. A general classification is presented herewith.

You may recall reading about the classification of lipids in the Advance Nutrition Course.

3.3.1 Classification of lipids

A general classification of lipids based on their structural components is presented in
table 3.2. Such a classification, however, is possibly too rigid for a group of compounds
as diverse as lipids and should be used only as a guide. The table gives the major, sub-
class and description of the various lipids.

Table 3.2: Classification of lipids

Major Class Subclass Description

Simple lipids Acylglycerols Glycerol + fatty acids

Waxes Long–chain alcohol + long-chain

fatty acids

Compound Phosphoacylglycerols (or Glycerol + fatty acids + phosphate

lipids glycerophospholipids) + another group usually containing
nitrogen

Sphingomyelins Sphingosine + fatty acid +

phosphate + choline

Cerebrosides Sphingosine + fatty acid + simple

sugar

Gangliosides Sphingosine + fatty acid + Complex

carbohydrate moiety that includes
sialic acid

Derived lipids Materials that meet the Examples: fatty acids, carotenoids,
definition of a lipid but steroids, fat-soluble vitamins
are not simple or
compound lipids
It should also be recognized that other classifications may sometimes be more useful. For
example, the sphingomyelins can be classed as phospholipids because of the presence of
phosphate. The cerebrosides and the gangliosides can also be classified as glycolipids
because of the presence of carbohydrate and the sphingomyelins. The glycolipids can be
classed as sphingolipids because of the presence of sphingosine.

The most abundant class of food lipids is the acylglycerols, also known as glycerol esters
of fatty acids, which dominate the composition of depot fats in animals and plants. The
polar lipids are found almost entirely in cellular membranes (phospholipids being the
main component of the bilayer), with only very small amounts in depot fats. In some
plants, glycolipids constitute the major polar lipids in cell membranes. Waxes are found
as protective coatings on skin, leaves and fruits. Major components of lipids are the
acylglycerols. They are the esters of glycerol and fatty acids, having a varying chain
length. Fatty acids are aliphatic monocarboxylic acids that can be liberated by hydrolysis
from naturally-occurring fats. For example, oleic acid, which is a common fatty acid
found in acylglycerols, has 18 carbon atoms in its chain. The carboxyl (COOH) group of
the acids forms the ester by combining with the hydroxyl (OH) group of glycerol. There
are 3 hydroxyl groups in a glycerol molecule. If all the three groups are forming ester
linkage with fatty acids, the resulting compound is called a triacylglycerol or a
triglyceride. Structure of a triacylglycerol is shown:

CH2OOC (CH2)16CH3

CH3 (CH2)16COOHCH

CH2OOC (CH2)16 CH3

The compound shown here is tristearoylglycerol, also known as glycerol tristearate. This
is a triester of glycerol with stearic acid. Many other fatty acids either saturated or
unsaturated and having varying chain length are present in triacylglycerols.
Common fatty acids present in acylglycerols are stearic acid (C-18, saturated), oleic acid
(C-18, monounsaturated), linoleic acid (C-18, diunsaturated) and palmitic acid (C-16,
saturated).

You came across the terms saturated and unsaturated in the above section. Let’s
understand these terms better.

3.3.2 Categories of Fats and Oils

As a student of nutrition, you already know that fatty acids are the lipid-building blocks.
It is customary to divide the fatty acids into different groups, e.g., saturated and
unsaturated ones. Saturated meaning they have as many hydrogens bonded to their
carbons as possible and unsaturated meaning with one or more double bonds connecting
their carbons, hence, fewer hydrogens. This particular division is useful in food
technology because saturated fatty acids have a much higher melting point than the
unsaturated ones, and the ratio of these fatty acids is of major importance for the physical
properties of a fat or oil.

Table 3.3a. Saturated fatty acids

Systematic Short-hand
Name Common Name Formula Description

n-Butanioc Butyric CH3 (CH2)2 COOH 4:0

n-Hexanoic Caproic CH3 (CH2)4 COOH 6:0
n-Octanoic Caprylic CH3 (CH2)6 COOH 8:0
n-Decanoic Capric CH3 (CH2)8 COOH 10:0
n-Dodecanoic Lauric CH3 (CH2)10 COOH 12:0
n-Tetradecanoic Myristic CH3 (CH2)12 COOH 14:0
n-Hexadecanoic Palmitic CH3 (CH2)14 COOH 16:0
n-Octadecanoic Stearic CH3 (CH2)16 COOH 18:0
n-Eicosanoic Arachidic CH3 (CH2)18 COOH 20:0
n-Docosanoic Behenic CH3 (CH2)20 COOH 22:0

Some of the more important saturated fatty acids with their systematic and common
names are listed in table 3.3a, and some of the unsaturated fatty acids in table 3.3b. The
naturally occurring unsaturated fatty acids in fats are almost exclusively in the cis – form,
although trans – acids are abundant in ruminant milk fats and in catalytically
hydrogenated fats. What are cis and trans-acids? You may have learnt about this concept
in the Nutritional Biochemistry Course. We suggest you look up Block 1, Unit 2 of the
Nutritional Biochemistry course for understanding this concept.
Table 3.3 b. Unsaturated Fatty Acids
Systematic Short-hand
Name Common Name Formula
Description

Hexadec-9-enoic Palmitoleic CH3 (CH2)5 CH=CH (CH2)7 COOH 16:1

Octadec-9-enoic Oleic CH3 (CH2)7 CH=CH (CH2)7 COOH 18:1
Octadeca-9:12-dienoic Linoleic CH3 (CH2)4 (CH=CH.CH2)2 (CH2)6 COOH 18:2
Octadeca-9:12:15-trienoic Linolenic CH3 (CH2)3 (CH=CH.CH2)3 (CH2)6 COOH 18:3
Eicosa-5:8:11:14-tetraenoic Arachidonic CH3 (CH2)4 (CH=CH.CH2)4 (CH2)2 COOH 20:4
Docos-13-enoic Erucic CH3 (CH2)7 CH=CH (CH2)11 COOH 22:1

Table 3.4 gives the composition of common vegetable oils.

Table 3.4: Component Fatty acids of some vegetable oils (Wt %)
Fatty Acids
Oil 16:0 18:0 20:0 22:0 24:0 16:1 18:1 18:2 18:3

Cottonseed 22 3 Tr --- --- 1 19 54 1

Peanut 11 2 2 3 1 Tr 48 32 ---
Sunflower 7 5 --- --- --- --- 19 68 ---
Corn 11 2 Tr Tr --- --- 28 58 ---
Sesame 9 4 --- --- --- --- 41 45 ---
Olive 13 3 Tr --- --- 2 71 10 1
Palm 45 4 --- --- --- --- 40 10 ---
Soybean 11 4 Tr Tr --- --- 24 54 7
Safflower 7 2 Tr --- --- --- 13 78 ---
*Mustard 3.5 --- --- --- --- --- 22.4 24.4 13.7
* also contains around 40% Erucic acid (22:1)
Tr- Traces
In continuation of our classification of lipids, it is important to realize that edible fats are
traditionally classified into the following subgroups:

Milk Fats
Fats of this group are derived from the milk of ruminants, particularly cows and
buffaloes. Although the major fatty acids of milk fat are palmitic, oleic and stearic, this
fat is unique among animal fats in that it contains appreciable amounts of the shorter
chain fatty acids (C4 to C12), small amounts of branched and odd numbered acids and
fatty acids with trans-double bonds.

Lauric Fats
Fats of this group are derived from certain species of palm, such as coconut. The fats are
characterized by their high content of lauric acid (40 – 50%), moderate amounts of C6,
C8 and C10 fatty acids, low content of unsaturated acids and low melting points.

Vegetable Butters
Fats of this group are derived from the seeds of various tropical trees and are
distinguished by their narrow melting range, which is due mainly to the arrangement of
fatty acids in the triacylglycerol molecules. In spite of their large ratio of saturated to
unsaturated fatty acids, trisaturated acylglycerol are not present. The vegetable butters are
extensively used in the manufacture of confections, with cocoa butter being the most
important member of the group.
Oleic – Linoleic Fats
Fats in this group are the most abundant. The oils are all of vegetable origin and contain
large amounts of oleic and linoleic acids, and less than 20% saturated fatty acids. The
most important members of this group are cottonseed, corn, peanut, sunflower, safflower,
olive, palm and sesame oils.

Linolenic Acids
Fats in this group contain substantial amounts of linolenic acid (C18 triunsaturated).
Examples are soybean, mustard, rapeseed, flaxseed and wheat germ hempseed and perilla
oils, with soybean being the most important. The abundance of linolenic acid in soybean
oil is responsible for the development of an off-flavour problem known as ‘flavour
reversion’.

Animal Fats
This group consists of depot fats from domestic land animals (e.g., lard and tallow), all
containing large amounts of C16 and C18 fatty acids, medium amounts of unsaturated
acids, mostly oleic and linoleic and small amounts of odd numbered acids. These fats also
contain appreciable amounts of fully saturated triacylglycerols and exhibit relatively high
meting points. Egg lipids are of particular importance because of their emulsifying
properties and their high content of cholesterol.

The lipid content of whole eggs is approximately 12%, almost exclusively present in the
yolk, which contains 32 – 36% lipid. The major fatty acids in egg yolks are 18: 1 (38%),
16: 0 (23%), and 18: 2 (16%). Yolk lipids consist of about 66% triacylglycerols, 28%
phospholipids and 5% cholesterol. The major phospholipids of egg yolk are
phosphatidylcholine (73%) and phosphatidylethanolamine (15%).

Marine oils
These oils typically contain large amounts of long – chain omega-3-polyunsaturated fatty
acids, with up to six double bonds and they are usually rich in vitamins A and D. Because
of their high degree of unsaturation, they are less resistant to oxidation than other animal
or vegetable oils.

With this, we come to the end of first part of this unit i.e., the introduction, classification
and composition of lipids. Look up the points to remember given herewith. They are the
useful hints/tips for remembering the concept on your finger tips. Read them carefully.

Points to Remember
1. Lipids consist of group of compounds that are generally soluble in organic solvents
but only sparingly soluble in water.
2. Glycerol esters of fatty acids (Acylglycerols) which make up to 99% of the lipids of
plant and animal origin have been traditionally called fats and oils.
3. Common fatty acid present in acyl glycerols are stearic acid (C-18, saturated), oleic
acid (C-18, monounsaturated), linoleic (C-18, di unsaturated) and palmitic (C-16,
saturated).
4. Major sources of oils and fats are peanut (groundnut), mustard, soybean, sunflower,
coconut, palm and milk.
5. Fats and oils belonging to oleic-linoleic acid group are the most abundant. They
contain large amounts of oleic acid (C-18 mono unsaturated) and linoleic acid (C-18
diunsaturated) and less than 20% saturated fatty acids.
6. Important members of oleic-linoleic acid group are peanut, sunflower, cotton seed
and sesame oils.
7. Milk fat is unique because it contains appreciable amounts of shorter chain acids (C-4
to C-12).
8. Animal fats contain appreciable amounts of fully saturated triacylgylcerols and
exhibit relatively high melting points.

Check your progress Exercise 1

1. Define lipids and mention main sources of lipids.
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2. What is role of food lipids in human diet?
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3. Mention the major classes of lipids and describe acylglycerols.
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4. Name the categories of fats and oils with examples.
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5. What are the main differences between vegetable oil and animal fats?
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Next, let us look at the functional properties of food lipids.

3.4 FUNCTIONAL PROPERTIES OF FOOD LIPIDS

Chemically, fats and oils, as you may already know by now, are a combination of
glycerol and fatty acids. The glycerol molecule has three separate points, where a fatty
acid molecule can be attached. Physically, fats and oils differ in that fats are solid and oils
are liquid at room temperature. You learnt earlier that the different properties are to a
large extent determined by the fatty acid composition and the extent of saturation or
unsaturation present. These aspects are identified by the carbon chain length and the
number and position of double bonds for the individual fatty acids and their position of
the glycerol. Generally, solid fats indicated by a dominance of saturated fatty acids and
liquid oils, are an evidence of a high level of unsaturated fatty acids.
Carbon chain lengths of fatty acds in edible oils and fats vary between 4 and 24 carbon
atoms with up to three double bonds. The most prevalent saturated fatty acids are lauric
(C-12:0), myristic (C-14:0), palmitic (C-16:0), stearic (C-18:0), arachidic (C-20:0),
behenic (C-22:0) and lignoceric (C-24:0). The most important monounsaturated fatty
acids are oleic (C-18:1) and erucic (C-22:1). The polyunsaturated fatty acids are linoleic
(C-18:2) and linolenic (C-18:3).

Natural fats and oils vary widely in their physical properties even though they are
composed of the same or similar fatty acids. These differences result from differences in
the proportion of the fatty acids and the structure of the individual triglycerides. Factors
that affect the properties of vegetable oil are plant maturity, plant health, microbiological,
seed location within the flower and the genetic variation of the plant. Animal fats and oils
composition varies according to the animal species, diet, health and fat location on the
carcass and maturity.

Physical properties of an oil or fat are of critical importance in determining its functional
characteristics or use in food products. One fundamental physical property of importance
is demonstrated by the terms fats and oils, which indicate whether a lipid is a solid or
liquid at ambient temperatures. But this grouping is not rigid because vegetable oils that
are solid at ambient temperatures in a temperate climate are liquid at the tropical ambient
temperatures. How then can one measure the functional properties? Have you come
across the term performance testing? The next section presents a detailed discussion on
this aspect.

3.4.1 Measurement of Functional Property

Fats and oils have several functional properties that affect the quality of processed foods.
In deep fat frying, the roles played by the frying oil are many: These include:
1. It acts as an effective heat exchange medium leading to cooking of the product
being fried and evaporation of water from the product,
2. It helps in the development of texture of fried food.
 3. Oil absorbed by the products provides characteristic fried taste and flavour. In the
preparation of the baked products, presence of fat contributes to texture and
flavour. Fats and oils form emulsion in batters and doughs leading to the
development of desirable structure and texture on baking or toasting.

Some essential attributes contributed by fats and oils cannot be directly measured with
chemical or physical analytical methods. In these cases, performance testing is the only
means for evaluating the ability of fat or oil to perform the desired functions in a food
product. Actual determinations of the performance qualities of an edible fat and oil
product are made with small scale practical tests that evaluate a finished product.
Performance testing is essential for the development of new products, especially for fats
and oils products designed for a specific food product, a formulation, or a process. After
development, physical or chemical analysis can be related to performance results in most
situations; however, continuation of certain performance evaluations is necessary for
some products to ensure adequate performance or more timely results in some cases.
Initially, most performance testing was designed for bakery products but has now been
expanded to every specialty product situation, i.e., baking, frying, candy, coatings,
formulated foods, nondairy products, and so forth, wherever tailored oils, margarines, oils
and other specialty products are utilized. In many cases the performance tests are
developed to evaluate the fat and oil ingredient as it would be used by a specific food
processor. You would realize, performance evaluation in itself can be a detailed subject
of study. Here, in this unit we shall not dwell on this aspect. Those of you, who are
interested to learn more about performance evaluation, read box 1 for information. It
provides a few examples of performance evaluation.

Box 1: Performance Evaluation, a few examples

Creaming volume – Cake batter aeration can be affected by the plasticity, consistency,
emulsification, bake stock formulation and other fats and oil properties. Creaming
volume evaluations measure the ability of an oil or margarine to incorporate and retain
air in a cake batter. In most cases, batter aeration is an indicator of the baked cake
volume, grain and texture and materially affects the handling qualities of the cake batter.
The creaming volume test formula consists of only three ingredients: (1) Test oil or
margarine, (2) granulated sugar, and (3) whole eggs. This procedure is the first stage of
an old fashion pound cake, where all of the cake batter aeration depended upon the
creaming properties of the oil with whole eggs. Batter specific gravities are determined
after mixing for 15 minutes and again after 20 minutes. Continued aeration, identified by
a decrease in batter specific gravity, indicates that the fat or oil product has a stable
consistency that has not broken down to allow the release of air from the batter. Specific
gravity is expressed as grams per cubic centimeter per 100 grams, calculated by
multiplying the reciprocal of the specific gravity by 100. Specific volume better
illustrates the amount or degree of aeration. The performance test is applicable to
emulsified, as well as non emulsified products, to measure aeration potential in a cake
batter.
Pound cake test – In some cases, oil or margarine creaming volume is most accurately
measured by preparing a regular pound cake, omitting the chemical leavener and
measuring the volume, grain and texture of the baked cake. Creaming volume, as
determined by this method, is affected by batter mixing temperature. Working range or
creaming range can be measured by adjusting the finished batter temperature over the
desired temperature range. The results obtained in this manner provide a good indication
of the creaming range or oil temperature tolerance. The baked pound cake volume is
determined by a seed displacement procedure and the cake appearance rated numerically
with a scale similar to that provided in table 3.5.
Table 3.5: Rating Scale for Cake

Score Rating Description

10 Perfect Fine regular grains; no holes, cracks, or tunnels;

Very thin cell walls and perfect symmetry

9 Very Good Close regular grain; free of holes, cracks or

tunnels, may have occasional hole, good cell wall
thickness

8 Good Grain very slightly open but regular, free of cracks

or tunnels, may have occasional hole, good cell
 wall thickness

7 Satisfactory Grain slightly open, mostly regular, a few small

holes, no tunnels or racks, slightly thick cell walls

6 Poor Open or irregular grain, or frequent holes, some

cracks or tunnels

5 Unsatisfactory Very open or irregular grain, or numerous holes,

cracks or tunnels, or thick heavy cell walls; may
have solid strweaks or gum line

4 and below Bad Increasing degrees of unsatisfactory performance

Cake mix evaluation – Originally, cake mix formulations were very similar to bakery
cakes and utilized standard “Hi-Ratio” cake oils; however, development of improved
cake mixes required rapid aerating oils to minimize mixing times for the house wife,
while at the same time increasing the product’s mixing and baking tolerances. The
competitive nature of the cake mix industry has continued the demands for new and
improved products, of which oil has always been a major contributor. A basic white
mix cake formulation and the make-up procedure can serve to evaluate new or revised
emulsifier systems for aeration, eating qualities and cake shelf-life, as well as the oil
carrier for lubrication and consistency.
Restaurant deep fat frying evaluation – A number of factors are studied when
evaluating frying oils. During deep fat frying, the fat is exposed continuously to
elevated temperatures in the presence of air and moisture. A number of chemical
reactions, including oxidation and hydrolysis, occur during this time, as well as changes
due to thermal decomposition. As these reactions proceed, the functional, sensory and
nutritional quality of the frying fat changes and eventually reaches a point where it is
no longer possible to prepare quality fried products and the fat will have to be
discarded. The rate of frying fat deterioration varies with the food fried, the frying fat
utilized, the fryer design and the operating conditions.
The deep fat frying evaluation consists of controlled heating of the test oils at 360±10°F
(176 to 187°C) continuously until the test is terminated. Fresh French cut potatoes (227
grams) fried three times daily for 7 minutes at 3-hour interval are flavoured once daily.
 Frying observations recorded after each frying includes smoking, odor, clarity, gum
formation and a determination of foam development. Foam development described as
none, trace, slight, definite and persistent should also be measured with a foam test
daily and each time a change in the observed foam is recorded. Samples are taken after
each 24-hour period for analysis of colour, free fatty acid and iodine value for
quantitative measurement of darkening, hydrolysis and polymerization. The frying test
is terminated when persistent foam has been observed and substantiated by foam height
testing.

We have read about the deep fat frying evaluation method for measurement of functional
properties of fat. Deep fat frying is commonly used as a cooking method in most homes.
What are the issues to be considered while using this method of cooking is the focus of
discussion in the next section.

3.5 DEEP FAT FRYING

Deep fat frying, as you may already know, is the method which involves cooking food in
hot fat/oil. The fat immediately surrounds the food and cooks it from all sides, creating an
exterior layer that seals in the food's flavors and juices inside. Deep frying is one of surest
ways of locking in flavor and developing great texture (also known as "crunch") in
cooking. Deep fat frying, in fact, has become one of the more important methods of food
preparation used by the food service, snack and baking industries, as well as the home
kitchen. The deep fat frying process consists most simply of (1) partially or totally
immersing the food prepared for frying into (2) a body of heated frying fat, which is (3)
contained in a metal vessel, and (4) maintaining the food in the fat at the appropriate
frying temperature for (5) the duration required to cook the product. Going into the
cooking utensil are (a) frying fat, (b) heat, and (c) the food prepared for frying. Emerging
from the utensil are (a) steam and steam- entraining frying fat, (b) volatile by-products of
heating and frying, (c) the finished product, and (d) with filtering, the crumbs or foreign
solid by-products of the frying operation.
As you read the next section, you will realize all these factors mentioned above, have a
role to play in the deep fat frying process. Let us get to know them.

3.5.1 Factors affecting the process of deep fat frying.

The common factors influencing the process of deep frying include:
1. Heat- Frying temperatures ranging from 150 –190°C are necessary to properly
prepare the different fried food products. Unfortunately, exposure to high
temperatures accelerates all of the breakdown reactions of fats and oils.
2. Air- Oxygen from the air is necessary to sustain human life, but it also reacts with the
double bonds in the frying oils to oxidize the unsaturated fatty acids, which results in
offensive odors and flavours and promotes gum formation or polymerization.
3. Moisture- All food products contain moisture, which causes hydrolysis of fats and
oils, resulting in an increased fat absorption in most foods.
4. Contamination- Any material associated with the frying process that causes the frying
media to deteriorate or accelerate the process is a contaminant. Some examples of
frying fat contaminants are:
• Trace metals – Most metals are pro-oxidants that exert a marked catalytic effect
to accelerate fat breakdown, but some metals are much more active than others.
These pro-oxidants can be picked up during processing or storage, from frying
equipment, the food fried, or some other contact with a metal. Two metals that
promote more rapid breakdown of frying than others are brass and copper.
• Soap or detergent- Residue of these materials from cleaning storage tanks, fryers,
or utensils which will catalyze fat breakdown.
• Gums or polymerized fats- Addition of polymerized fats or oils to fresh oils act as
catalysts to accelerate the formation of more gums, which contribute to foaming
and darkening.
• Burnt food particles- Food particles allowed to remain in the frying fat impart a
bitter, caramelized and / or burnt taste along with an unappealing appearance to
the food fried and accelerate frying oil breakdown.
5. Time – The extent of the frying oil’s exposure to the effects of the above factors
determines the degree of product deterioration.
So now you realize, the simple process of deep frying is not actually so simple. Utmost
care needs to be taken while using this method to ensure that the quality of the frying oil
is maintained. The next section is devoted to this crucial practical aspect i.e., maintaining
the quality of fried oils.

3.5.2 Maintaining the quality of frying oil

As frying continues, the level of oil in the fryer depletes. There are two beneficial frying
fat quality factors affected during the frying operation. These include:
• the steam released during frying, and
• the addition of fresh oil to replace the fat absorbed by the food fried.

Steam formed from the moisture released from the food mixes intimately with the fat,
and when given off, it carries with it the odor- and flavour-bearing volatile by-products of
frying that would otherwise accumulate in the frying fat to adversely affect the flavour
and odor of the fried food. This steam continually scrubs or purges the frying fat of the
potential off – flavours and odors each time the food is fried, even though it is the same
moisture that causes hydrolysis. Fresh oil must be added to the fryer to compensate for
the fat removed by the fried product. This addition helps to overcome the changes to the
frying fat brought about by the heat and other frying fat enemies. Obviously, the frying
fat will remain in better condition when higher replacement oil quantities are required.
The ratio of the fryer’s capacity to the rate at which the fresh oil is added to replenish the
fryer is referred to as turnover rate, or the number of hours required for the addition of
fresh frying oil equal to the amount of fat maintained in the fryer. Because oxidative
changes occur continuously in heated fats, turnover must be related to the total period
that the fat is heated, rather than only the actual time the product is fried. Obviously, the
quality and, especially, the flavour of the frying fat will be maintained at a more desirable
level with the highest turnover rate. In general, an operation with a turnover less than a
day should never have to discard used frying oil because of breakdown, except in the case
of product abuse or a contaminant. Operations with a slower turnover rates need to
include this product quality and economic factor in their frying oil selection criteria.

Before we move on further, let us recapitulate what we have learnt so far. The salient
points are listed in points to remember given herewith. Read them carefully.

POINTS TO REMEMBER
1. Natural fats and oils vary widely in their physical properties even though they are
composed of the same or similar fatty acids.
2. Physical properties of a oil or a fat are of critical importance in determining its
functional characteristics or use in food products.
3. Performance testing is the means for evaluating the fat or oil’s ability to perform
the desired functions in a food product.
4. Cake batter aeration can be affected by the plasticity, consistency, emulsification
and other properties of fats and oil.
5. Deep fat frying has become one of the most important methods of food
preparation.
6. Factors affecting the frying process are frying temperature, oxygen from air,
moisture content of the food, duration of frying and presence of contaminants.
7. The rate of frying fat deterioration varies with the food fried, the frying fat
utilized, the fryer design and the operating conditions.
8. Fresh oil must be added to the fryer to compensate for the fat removed by the
fried product. This addition helps to overcome the changes in the frying fat.
9. Quality and flavour of the frying fat will be maintained at a desired level with the
highest turnover rate.

Check your progress Exercise 2

1. Name the factors that affect physical properties of fats and oils.
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2. Mention important functional properties of fats and oils
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3. What are the salient features of performance test?
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4. Name the factors affecting deep fat frying.
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5. What is turnover rate of frying oil?
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6. How does turn over rate affect the quality of frying oil?
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3.6 DETERIORATIVE CHANGES IN FATS AND OILS

From our discussion so far it is clear that the food products undergo changes in flavour
due to the chemical changes occurring in fats and oils present in them. The causative
factors responsible for such changes are presence of enzymes, atmospheric oxygen and
application of high temperature. Lipid oxidation is one of the major causes of food
spoilage. It is of great economic concern to the food industry because it leads to the
development of various off-flavours and off odours generally called ‘rancid’ (oxidative
rancidity), in edible oils and fat-containing foods, which render these foods less
acceptable. In addition, oxidative reactions can decrease the nutritional quality of food
and certain oxidation products are potentially toxic. On the other hand, under certain
conditions, a limited degree of lipid oxidation is sometimes desirable, as in aged cheeses
and in some fried foods.

In this section we will look at the oxidative and other changes occurring in fats and oils
causing deterioration. We begin with autoxidation.

3.6.1 Autoxidation
It is generally agreed that “autoxidation”, that is, the reaction with molecular oxygen via
a self – catalytic mechanism, is the main reaction involved in oxidative deterioration of
lipids. Autoxidation reaction can be divided into three parts:
a. initiation,
b. propagation, and
c. termination.

In the initiation part, hydrogen is removed from the fatty acid chain to yield a free radical.
The removal of hydrogen takes place at the carbon atom next to the double bond and can
be brought about by the action of light, metals etc. Let us understand this concept with
the help of an example. For example, in oleic acid, the reaction will proceed by removal
of hydrogen from carbons 8 or 11 resulting in free radical as shown herewith.

8 9 10 11
COOH (CH2)6 CH2 CH=CH CH2_(CH2)6_CH3
_ _ _ _

8 9 10 11 8 9 10 11
_
CH_CH=CH_CH2_ CH2_CH_CH=CH_

Generally, the reaction can be shown as:

RH R’ +H’
(R’ is the free radical)
Once a free radical is formed, it will combine with oxygen to form a peroxy-free
radical which can remove hydrogen from another unsaturated molecule who yield a
peroxide and a new free radical. This is called ‘propagation reaction’, as illustrated
herewith. This reaction may repeat upto several thousand times and has the nature of
a chain reaction.
R” + O2 RO2
RO2 + RH ROOH + R’

The propagation can be followed by termination, if the free radicals react with
themselves to yield non-active products:

R’ + R’ R _R
R’ + RO2 RO2R
nRO2’ (RO2)n

You must remember that the hydro peroxides (ROOH) formed in the propagation part
of the reaction are the primary oxidation products. They are generally unstable and
decompose into secondary oxidation products which include a variety of compounds.
Among the secondary oxidation products, aldehydes and alcohols form an important
group. The volatile aldehydes are mainly responsible for the oxidized flavour (rancid)
of fats.

A general scheme summarizing the overall picture of lipid autoxidation is given in the
figure 3.1.
 RH
O2
Dimers, polymers; cyclic
peroxides;
ROO’
Hydroperoxy compounds
R’

Cleavage

Aldehydes,
Ketones, hydrocarbons,
RH Furans, acids
ROOH

OH
ROOR,
ROR dimers RO’ Keto,
hydroxy
and epoxy
compounds
Cleavage

Aldehydes Alkyl radicals Semialdehydes

Or oxo-esters

O2
Condensation

O2
Hydrocarbons Hydrocarbons
Shorter aldehydes
Acids
epoxides
Terminal
ROOH

Hydrocarbons
Aldehydes,alcohols
 Figure 3.1:Generalised scheme for autoxidation of lipids
There are many factors influencing the lipid autoxidation process you have just learnt
about. Let us get to know them.

3.6.2 Factors Influencing Lipid Oxidation

Food lipids contain a variety of fatty acids that differ in chemical and physical properties
and also in their susceptibility to oxidation. In addition, foods contain numerous non lipid
components that may co-oxidize and / or interact with the oxidizing lipids and their
oxidation products. Oxygen concentration, temperature and moisture are the other factors
influencing autoxidation. Let us learn how.
• Fatty acid composition
We know fats/oils are made up of fatty acids. The number, position and geometry
of double bonds within the fatty acids affect the rate of oxidation. Relative rates
of oxidation for arachidonic, linolenic, linoleic and oleic acids are approximately
40:20:10:1, respectively. Cis acids oxidize more than their trans-isomers, and
conjugated double bonds are more reactive than nonconjugated. Autoxidation of
saturated fatty acids is extremely slow. At room temperature, they remain
practically unchanged when oxidative rancidity of unsaturates becomes
detectable. At high temperatures, however, saturated acids can undergo oxidation
at significant rates.
• Oxygen concentration
When oxygen is abundant, the rate of oxidation is independent of oxygen
concentration, but at very low oxygen concentration, the rate is approximately
proportional to oxygen concentration. However, the effect of oxygen
concentration on rate is also influenced by other factors, such as temperature and
surface area.
• Temperature
In general, the rate of oxidation increases as the temperature is increased.
Temperature also influences the relation between rate and oxygen partial pressure.
As the temperature is increased, changes in oxygen partial pressure have a smaller
influence on the rate because oxygen becomes less soluble in lipids and water, as
the temperature is raised.
• Surface area
The rate of oxidation increases in direct proportion to the surface area of the lipid
exposed to air. Furthermore, as surface – volume ratio is increased; a given
reduction in oxygen partial pressure becomes less effective in decreasing the rate
of oxidation. In oil-in-water emulsions, the rate of oxidation is governed by the
rate at which oxygen diffuses into the oil phase.
• Moisture
In model lipid systems and various fat-containing foods, the rate of oxidation
depends strongly on water activity. In dried foods with very low moisture
contents (aw values of less than about 0.1), oxidation proceeds very rapidly.
Increasing the aw to about 0.3 retards lipid oxidation and often produces a
minimum rate. The protective effect of small amounts of water is believed to
occur by reducing the catalytic activity of metal catalysts, by quenching free
radicals and / or by impeding access of oxygen to the lipid.

At somewhat higher water activities (aw = 0.55 – 0.85), the rate of oxidation
increases again, presumably as a result of increased mobilization of catalysts and
oxygen.
• Pro-Oxidants
Transition metals, particularly those possessing two or more valency states and a
suitable oxidation – reduction potential between them (e.g., cobalt, copper, iron,
manganese and nickel), are effective pro-oxidants. If present, even at
concentrations as low as 0.1 ppm, they can decrease the induction period and
increase the rate of oxidation. Trace amounts of heavy metals are commonly
encountered in edible oils and they originate from the soil in which the oil –
bearing plant was grown, from the animal, or from metallic equipment used in
processing or storage. Trace metals are also naturally occurring components of all
food tissues and of all fluid foods of biological origin (eggs, milk, and fruit juices)
and are present in both free and bound forms.
After autoxidation, we look at the deteriorative changes caused by lipolysis.

3.6.3 Lipolysis
What do we mean by lipolysis? Hydrolysis of ester bonds in lipids is called lipolysis. This
may occur by enzyme action or by heat and moisture, resulting in the liberation of free
fatty acids. Free fatty acids are virtually absent in the fat of living animal tissue. These
can be formed, however, by enzyme action after the animal is killed. Since animal fats
are not usually refined, prompt rendering is of particular importance. The temperatures
commonly used in the rendering process are capable of inactivating the enzymes
responsible for hydrolysis. In contrast to animal fats, oils in mature oil seeds may have
undergone a substantial hydrolysis by the time they have harvested, giving rise to
significant amounts of free fatty acids. Neutralization with alkali is thus required for most
vegetable oils after they are extracted.

Lipolysis is a major action occurring during deep fat frying due to large amounts of water
introduced from the food and the relatively high temperatures used. Development of high
level free fatty acids during frying is usually associated with foaming and a decrease in
the smoke point of the oil and reduction in the quality of the fried food. The release of
short – chain fatty acids by hydrolysis is responsible for the development of an
undesirable rancid flavour (hydrolytic rancidity) in raw milk. Furthermore, free fatty
acids are more susceptible to oxidation than other fatty acids esterified to glycerol.

Lipolysis, therefore, can cause changes in fats and oils which are best avoidable. On the
other hand, you would be surprised to learn that certain typical cheese flavours are
produced by deliberate action of microbial and milk lipases. Controlled and selective
lipolysis is also used in the manufacture of other food items, such as yogurt and bread.

Besides lipolysis, thermal decomposition too can bring about changes in oils and fats
which are deteriorative. Let us learn about these changes.

3.6.4 Thermal Decomposition

Heating of food produces various chemical changes, some of which can be important to
flavour, appearance, nutritive value and toxicity. Not only do the different nutrients in
food undergo decomposition reactions, but these nutrients also interact among themselves
in extremely complex ways to form a very large number of new compounds.

The chemistry of lipid oxidation at high temperatures is complicated by the fact that both
thermolytic and oxidative reactions are simultaneously involved. Both saturated and
unsaturated fatty acids undergo chemical decomposition when exposed to heat in the
presence of oxygen. A schematic summary of these mechanisms is shown in figure 3.2.

Fatty acids, Esters, and Triacylglycerols

Saturated Unsaturated

O2 Thermolytic O2
Thermolytic (α, β, γ,δ- attack ) Reactions
Reactions

Volatile and
Long-chain alkanes, Acyclic and dimeric
Acids aldehydes, ketones and cyclic dimers products of
hydrocarbons lactones autoxidation
propenediol
acrolein
ketones

Figure 3.2: Thermal Decomposition of Fats and Oils

With thermal decomposition, we complete our study on the deteriorative changes in fats
and oils. Now, the next important issue is how to prevent these deteriorative changes?
The answer lies in one word ‘Antioxidants’. Let us learn about what the antioxidants are
and how they play a protective role in the context of fats and oils.

3.7 ANTIOXIDANTS
Antioxidants are the substances that can delay onset, or slow the rate of oxidation of
autoxidizable materials. By virtue of this property, they provide protection against
oxidative changes in fats and oils. They act by reacting with the free radicals and thereby
terminate the propagation of chain reaction. The antioxidant reacts with the fatty acid free
radical or with the peroxy free radical. Literally hundreds of compounds, both natural
(including vitamins C and E, vitamin A, selenium (a mineral) and a group known as the
carotenoids) and synthesized, have been reported to possess antioxidant properties. Their
use in foods, however, is limited by certain obvious requirements not the least of which is
adequate proof of safety. The main lipid soluble antioxidants currently used in food are
monohydric or polyhydric phenols with various ring substitutions. For maximum
efficiency, primary antioxidants are often used in combination with other phenolic
antioxidants or with various metal sequestering agents.

Although the mechanisms by which many antioxidants impart stability to pure oils are
relatively well known, much remains to be learned about their action in complex foods.
Some commonly used/present antioxidants in fats and oils and their characteristics are
discussed herewith.

Characteristics of Some Commonly Used Primary Antioxidants:

• Tocopherols: These are the most widely distributed antioxidants in nature, and
they constitute the principal antioxidants in vegetable oils. A relatively high
proportion of the tocopherols present in crude vegetable oils survives the oil
processing steps and remains in sufficient quantities to provide oxidative
stability in the finished product.
• Butylated hydroxyanisole (BHA): It is commercially available as a mixture of
two isomers and has found wide commercial use in the food industry. It is
highly soluble in oil and exhibits weak antioxidant activity in vegetable oils,
particularly those rich in natural antioxidants. BHA is relatively effective
when used in combination with other primary antioxidants. BHA has a typical
phenolic odor that may become noticeable if the oil is subjected to high heat.
 • Tertiary Butylhydroquinone (TBHQ): TBHQ is moderately soluble in oil and
slightly soluble in water. In many cases, TBHQ is more effective than any
other antioxidant in providing oxidative stability to crude and refined
polyunsaturated oils, without problem of colour or flavour stability. TBHQ is
also reported to exhibit good carry - through characteristics in the frying of
potato chips.

POINTS TO REMEMBER
1. Lipid oxidation is one of the major causes of food spoilage. It leads to the
development of off flavours and off odours generally called rancid.
2. Autoxidation is the reaction of fats and oils with molecular oxygen. It consists of
three steps namely, initiation, propagation and termination.
3. Volatile aldehydes formed during autoxidation are mainly responsible for the
rancid flavour of fats and oils.
4. The number, position and geometry of double bonds in the fatty acid chain affect
the rate of oxidation. As the number of double bonds increase, there is an increase
in the rate of oxidation.
5. Oxygen concentration, temperature, surface area of the lipid exposed to air and
moisture content influence the lipid oxidation.
6. Hydrolysis of ester bonds in lipids can occur by enzyme action, heat and
moisture, resulting in the liberation of free fatty acids.
7. Development of high level of free fatty acids during frying is associated with a
decrease in smoke point and reduction in the quality of fried food.
8. Lipid oxidation at high temperature involves both thermolytic and oxidative
reactions leading to loss of flavour, appearance and nutritive value.
9. Antioxidants can delay the onset, or slow the rate of oxidation of fats and oils.

Check your progress Exercise 3

1. What is autoxidation and mention the three steps involved in it?
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2. What is rancidity? Mention the compounds responsible for it.
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3. List the factors influencing lipid oxidation.
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4. Define lipolysis and name the compound liberated by it.
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5. List the compounds formed by the thermal decomposition of fats and oils.
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6. How antioxidants delay the onset of rancidity?
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7. Name some commonly used antioxidants.
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3.8 LET US SUM UP

Lipids are the major components of oil bearing materials and adipose tissue. They consist
of broad group of compounds that are generally soluble in organic solvents. Largest
source of vegetable oil is the seeds of plants such as peanut, sunflower, cottonseed,
mustard and safflower. Acylglycerols or glycerol esters of fatty acids which make upto
99% of the lipids of plant and animal origin have been traditionally called fats and oils.
Oils of vegetable origin contain large amounts of oleic and linoleic acid. Physical and
chemical properties of oils and fats are important to their functional properties in many
foods. Many essential attributes contributed by fats and oils can be evaluated by
conducting performance tests. These tests are designed for products such as baked goods,
candy, coating snacks and formulated products. Deep fat frying is an important method of
food preparation. Factors affecting the process of deep fat frying are frying temperature,
exposure to air, moisture content of the food being fried, presence of contaminants such
as trace metals, soap or detergent and duration of heating. Fats and oils undergo changes
in flavour or they develop rancid flavour due to the presence of enzymes, atmospheric
oxygen and application of high temperature. Autoxidation is the main reaction involved
in the oxidative deterioration of lipids. Autoxidation occurs through free radical
mechanism consisting of three steps, namely, initiation, propagation and termination.
Lipid oxidation is influenced by the fatty acid composition, oxygen concentration,
temperature, surface area, moisture and pro-oxidants. Lipolysis is the hydrolysis of the
ester bonds in the lipids resulting in the liberation of free fatty acids which are more
susceptible to oxidation than acylglycerols. Lipids undergo chemical decomposition
when exposed to heat in presence of oxygen. Antioxidants are the substances that can
delay the onset or slow the rate of oxidatioin of lipids. The main antioxidants used in
food are monohydric or polyhydric phenols with various ring substitutes. Tocopherols are
the most widely distributed natural antioxidants in vegetable oils. Tertiary butyl
hydroquinone (TBHQ) is more effective than any other antioxidant in providing
oxidative stability to oils and fats.

3.9 GLOSSARY
Acylglycerols : Most abundant; these constitute upto 99% of the lipids of
plant and animal origin. They are esters of fatty acids with
glycerol.
Antioxidants : Substances that can delay the onset, or slow the rate of
oxidative deterioration of oils and fats.
Autoxidation : Reaction of the molecular oxygen with oils and fats
leading to the development of off odour or rancidity.
Cis-trans isomers : Atoms or groups are called cis or trans to one another
when they project respectively on the same or on opposite
sides of a reference plane identifiable as common among
stereoisomers. The compounds in which such relations
occur are termed cis/trans-isomers.
Fatty acids : Aliphatic monocarboxylic acids that can be liberated by
hydrolysis from naturally occurring fats and oils.
Functional property : Properties of fats and oils which have a marked influence
on the preparation and quality of a food product.
Lipids : Broad group of compounds that constitute the principal
structural components of all living cells, and are generally
soluble in organic solvents.
Lipolysis : Hydrolysis of ester bond in lipid caused by enzyme
action, heat and moisture resulting in liberation of free fatty
acids.
Oleic – linoleic group : Most abundant group of fats and oils that contain large
amounts of oleic and linoleic acid.
Oxidation-reduction
Potential (ORP) : ORP is related to the concentration of oxidizers or
reducers in a solution, and their activity or strength. It
provides an indication of the solution's ability to oxidize or
reduce another material. These chemicals have the ability
to oxidize (accept electrons) or reduce (donate electrons)
molecules.
Performance test : A method for evaluating the ability of fat or oil to perform
the desired functions in a food product.
Pro-oxidants : Transition metals, possessing two or more valency states
and a suitable oxidation-reduction potential between them.
Rancidity : Development of-off flavour in fats and oils caused by
autoxidation, lipolysis or thermal decomposition.
Thermal
Decomposition : Chemical decomposition of oils and fats when exposed to
heat in the presence of oxygen.

3.10 ANSWERS TO CHECK YOUR PROGRESS EXERCISES

Check Your Progress Exercise 1

1. Lipids are a broad group of compounds those are generally soluble in organic
solvents but sparingly soluble in water. Main sources of lipids are oil bearing
nuts, seeds and fruits. Example peanuts, sunflower, mustard and soyabean.

2. The role of food lipids in human diet are follows: lipids supply calories, essential
fatty acids, act as fat-soluble vitamin carriers and increase the palatability of food.

3. The major classes of lipids are simple lipids (acylglycerols and waxes);
Compound lipids (glycerophospholipids); Derived lipids (compounds that are not
simple or compound lipids) example, Carotenoids and vitamins. Acylglycerols are
the glycerol esters of fatty acids which make up to 99% of the lipids of plant and
animal origin.

4. Milk fats Example: Cow and Buffalo milk;

Lauric acid group Example: Coconut oil;
Vegetable butters Example: Cocoa butter;
Oleic linoleic acid group Example: Peanut, cotton seed, sunflower oils;
 Linolenic acid group Example: Soyabean, Mustard;
Animal fats Example: Lard, tallow

Check Your Progress Exercise 2

1. The factors that affect physical properties of fats and oils are fatty acid
composition, degree of unsaturation and structure of individual triglycerides.

2. The important functional properties of fats and oils are discussed as follows.
Frying oil is an effective heat exchange medium, to help in development of
texture and flavour of fried foods. Form emulsions in batter and dough.
Contribute to the texture and flavour of baked products.

3. The salient features of performance test are that it helps in evaluating the ability
of fat and oil to perform the desired function in a food product. It is essential
for the development of specific food products and formulations.

4. The factors affecting deep fat frying are temperature of frying; presence of
oxygen; moisture content of food; presence of contaminants such as trace metals
and duration of frying

5. Number of hours required for the addition of fresh frying oil equal to the
amount of oil maintained in the fryer is referred to as the turnover rate of frying
oil.

6. The turn over rate affects the quality of frying oil by maintaining the flavour at a
more desirable level, slowing down oxidative changes occurring in frying fat
and minimizing discard of used frying oil.

Check Your Progress Exercise 3

1. Reaction of fats and oils with molecular oxygen by means of a self catalytic
mechanism is referred to as autooxidation. The three steps involved are initiation,
propagation and termination.

2. Off flavours caused by the chemical changes occurring in fats and oils is referred
to as rancidity. The compounds responsible for it are unsaturated aldehydes,
ketones, alcohols and acids.

3. The factors influencing lipid oxidation are fatty acid composition, oxygen
concentration, temperature, surface area of the lipid, moisture and the presence of
pro-oxidants.

4. Lipolysis is the hydrolysis of ester bonds in lipids caused by the action of enzyme,
heat or moisture. Free fatty acids are liberated as a result of lipolysis.

5. The compounds formed by the thermal decomposition of fats and oils are acyclic
and cyclic dimers, long-chain alkanes, aldehydes, ketones and lactones and
hydrocarbons.

6. Antioxidants delay the onset of rancidity by reacting with the fatty acid free
radical or with the peroxy free radical and thereby terminate the chain reaction of
lipid oxidation.

7. Some commonly used antioxidants are Tocopherols, Butylatedhydroxyanisole and

tertiary Butylhydroquinone.