FOOD CHEMISTRY PROPERTIES OF FOODS

2. Presented By : Syed Aasif Mujtaba
14pft012
asifiust@gmail.com

3. Lipids are chemically diverse group of
compounds with different structural
features.
oLipids are a group of naturally occurring
molecules that include fats, waxes, sterols, fat-
soluble vitamins (such as vitamins A, D, E, and
K), monoglycerides, diglycerides, triglycerides,
phospholipids.
o They are widely distributed in all foods in
good quantities. The common property of
lipids is that they are insoluble in water but,
soluble in organic solvents like hexane,
petroleum ether, chloroform, acetone etc.

4. o Food lipids are generally referred as fats and
oils.
oAt ambient temperature (room temperature)
fats are solids, and oils are liquids. This
property may be due to the reason that fats
contain more saturated fatty acids and oils
contain more unsaturated fatty acids.
oFats and oils constitute about 95% of the
plant and animal lipids. Both oils and fats are
made up of triacylglycerols. Triacylglycerols are
glycerol esters of fatty acids.

6. •Lipids undergo various complex chemical
changes during processing and storage of
foods.
• During processing and storage, they also
react with other constituents present in food
and from different compounds, which are
desirable and undesirable for food quality.

7. Occurrence
 Lipids are present in all the animal and plant tissues.
 In plants, they are more concentrated in seeds and
nuts.
 In animals, it is more concentrated in adipose tissue.
Generally, cereals, fruits and vegetables are poor in
lipids while nuts, legumes like soybean, peanut are
rich in lipids.

8. Lipid content in different plant and animal sources

9. Lipids have two important functions in living
organisms:
i) it is a major structural component of
membrane and
ii) it serves as a storage form of energy-rich fuel.
In addition, lipids play an important role as
electron carriers, light absorbing pigments,
hormones and intracellular messengers. They
also facilitate the absorption of fat-soluble
vitamins and provide essential fatty acids. Fats
are added to foods as a method of transferring
heat in frying, for flavour, mouth feel and to
enhance the texture, palatability .

10. They are broadly classified into simple lipids,
compound (or complex) lipids and derived
lipids.
a. Simple lipids (neutral lipids)
Fats and oils, waxes, sterol esters and ether
lipids are categorized in this class of lipids.
Common structural feature of the simple lipids
is that these are esters of fatty acids with
alcohols.
b. Compound lipids
These are also called complex lipids. Like simple
lipids, these are also esters of fatty acids with
alcohols, but these lipids contain other groups
such as phosphate, nitrogenous base etc.

11. Cont…
 Phospholipids glycolipids and lipoproteins are
grouped in this class of lipids.
 c. Derived lipids
As the name suggests, these lipids are obtained
upon the hydrolysis of simple and complex lipids.
This class of lipids contains fatty acids, sterols,
hydrocarbons, alcohols, monoacylglycerol (MAG),
and diacylglycerols (DAG), carotenoids and fat-
soluble vitamins.

12. Schematic representation of
classification

13. o Acylglycerols are the most abundant
group of naturally occurring lipids.
Almost all (99%) of the fatty acids
present in the living system exist in the
form of acylglycerol.
o Acylglycerols are the fatty acids esters
of glycerol.
o They exist in three forms namely,
i) monoacylglycerol,
ii) diacylglycerols and
iii) triacylglycerols.

14. Fatty acids
 Fatty acids contain aliphatic chain (tail) with a
terminal carboxylic acid group (head). Most fatty acids
have even number of carbons in a straight chain.
Majority of fatty acids contain 4 - 24 carbon.
 Although the presence of free fatty acids in foods is
low, they are associated with both major and minor
lipids. Therefore, fatty acids are the major components
of the lipids. Fatty acids present in plants and animals
are esterified to glycerol (example: triacylglycerol).

15. Fatty acids can be classified into
Saturated (i.e. without any double bonds)
unsaturated fatty acids (i.e., with one or more
double bonds). Unsaturated fatty acids are
further classified into mono and
polyunsaturated fatty acids. If fatty acid
contains one double bond, it is called
monounsaturated fatty acid (MUFA). If the
fatty acid contains more than one double bond,
it is called polyunsaturated fatty acid (PUFA).

16. Examples
 Example for Saturated fatty acids: butyric acid
CH3- CH2- CH2- COOH
Example for MUFA: oleic acid
18CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-10CH2 = 9CH2-8CH2-CH2-6CH2-
CH2-4CH2-CH2-2CH2-1COOH
The most common unsaturated fatty acid present in fats is oleic acid.
Example for PUFA: Linoleic acid
18CH3-CH2-CH2-CH2-CH2-CH2=12CH2-CH2-10CH2 = 9CH2-8CH2-CH2-
6CH2-CH2-4CH2-CH2- 2CH2-1COOH

17. Trivial and systematic names of some fatty acids

18. Oxidative and hydrolytic rancidity
of lipids
The word rancidity is a general term
commonly refers to off-flavours resulting
from oxidative deterioration (auto
oxidation) and hydrolytic cleavage
(lipolysis) of triacylglycerols in fats and
oils.

19. Rancidity of fats and oils is
caused due the presence of
prooxidative enzyme like
lipoxygenase and lipolytic
enzyme like lipase, exposure to
light, oxygen and moisture, and
presence of transition metals
like copper and iron.

20. Lipolysis
 Lipolysis is the breakdown of lipids and involves
hydrolysis of triglycerides into glycerol and free fatty
acids
 Hydrolysis of esters in lipids is called lipolysis. For
example, triacylglycerol is hydrolyzed to free fatty acid
and glycerol.
 Free fatty acids released in food by lipolysis produce
off-flavours. This is also termed as ‘hydrolytic
rancidity'. For example, rancidity flavour in raw milk
due to lipolysis of fat in milk.

21. Cont…
 Though free fatty acids cause rancidity of foods, they
are also required for flavour generation in certain
foods. For example, in cheese, bread, yogurt lipases
are added to release free fatty acids.
 Hydrolysis of ester will occur in two different ways
-
 1) Enzymatic or
 2) heat and moisture

22. Enzymatic lipolysis:
During processing and storage of food,
lipases present in the food hydrolyze
the ester bonds of triacylglycerols of fat
and release fatty acids. These free fatty
acids produce off-flavours.

24. During deep fat, frying of foods temperature is
very high and water present in the foods is
released. Thus, lipolysis is a more favourable
reaction during deep fat frying of foods. Presence
of free fatty acids in deep-fat fried foods
decreases the quality of foods.

25. Auto-oxidation
 Lipid oxidation is a major concern to the food industry
because it generates off-flavour and off-odour in edible
oils and foods containing oils. Lipid oxidation is often
referred to as auto-oxidation. In auto-oxidation lipids
get oxidized via a self-catalyzed reaction in presence of
molecular oxygen. Auto-oxidation in fats is a free
radical reaction and it consists of three steps.

26. Initiation
 The lipid auto-oxidation initiates with removal of
hydrogen from the polyunsaturated fatty acid (LH).
This removal of hydrogen from fatty acid results in the
formation of lipid alkyl radical (L•).
 LH → L• + H•

27. Propagation
 Lipid alkyl free radicals (L•) formed reacts with O2
molecule and forms peroxy radical (LOO•). The peroxy
radical can then abstract a H atom from a different
polyunsaturated fatty acid to form lipid hydroperoxide
(LOOH) and another new alkyl radical on another
fatty acid (LH).
 L• + O2 → LOO•
LOO• + LH→LOOH+ L•

28.  The hydroperoxides formed can degrade to volatile
aldehydes and ketones. These volatiles produced cause
strong off-flavour, which is termed as rancidity.
Termination
 The propagation reaction (chain reaction) terminates
due to the combination of two free radicals as shown
below. The termination reaction yields stable non-
reactive products like L-L or LOOL
L• + L•→ L - L
 L•+ LOO• → LOOL

29. Lipid Oxidation

30. Rancidity can be prevented in different ways. They are
i) They should be stored in closed containers to
minimize oxygen exposure.
ii) They should be packed under vacuum to prevent
from oxygen exposure.
iii) They should be stored in coloured jars or wraps to
control exposure to light,
iv) Metal chelating agents like EDTA should be
added to control metal induced oxidation
v) They should be kept away from moisture and heat
to prevent hydrolytic cleavage.vi) Addition of
antioxidants.