The document discusses lipids, their classifications, properties, and extraction methods. It details the chemical reactions involved in lipid processing, including hydrolysis, saponification, hydrogenation, and inter-esterification, as well as the technologies used in refining and extracting fats. Additionally, it examines the effects of frying on fats, highlighting the significance of oxidative reactions and changes in flavor, quality, and nutritional value of fried foods.

1. LIPIDS
Jacent K. Asiimwe (PhD)

2. Importance
• Fuel
• Insulation & protection
• Constituent of cell-wall
• Absorption of fat soluble
vitamins.
Food:
Palatability
Satiety
Flavour etc.

3. Source
Edible fats traditionally classified according to food source.
1. Milk fats : Derived from the milk of ruminants and in particular dairy
cows. Major fatty acids in milk fat are palmitic, oleic and stearic, milk fats.
Also appreciable amounts of the shorter chain acids C4 to C12, small
amounts of branched and odd-numbered acids.
2. Lauric acids :Derived from species of palm, e.g. coconut. 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.
•

4. Edible Fat classification
3. Vegetable Butters :Derived from the seeds of various tropical plants and
are distinguished by their narrow melting range which is due to mainly to
the arrangement of fatty acids in the tri-acyl glycerol molecules.
4. Animal fats: Fats from animals. Contains large amounts of C16 and C18
fatty acids, medium amounts of unsaturated acid, e.g. oleic and linoleic
& small amounts of odd-numbered acids.
5. Marine Oils; Typically contain large amounts of long chain omega-3-
polyunsatured fatty acids, with up to six double bonds, & are usually rich in
vitamins A and D. Less resistant to oxidation than other animal or vegetable
oils due to their high degree of unsaturation.

5. Physical Properties
• Colorless, odorless and tasteless. Any color or taste is due to
association with other substances eg. yellow color of animal fat is due
to carotenoids.
• Have specific gravity of <1 (float on H2O)
• Insoluble in water but soluble in organic solvents (benzene, ether)
• Low melting points but higher than solidification points

6. Chemical Reactions
1.Hydrolysis (Lipolysis); Hydrolysed to constituents (glycerol & FAs) due to
action of heat and moisture, acids, alkali or enzymes.
In deep-fat frying hydrolysis ocuurs due to large amounts of water released
from food and the relatively high temperature used.
FFA released during frying causes decrease in surface tension of the oil &
the reduction in quality of fried food (FFA more susceptible to oxidation)
• Hydrolytic rancidity is responsible for the development of an undesirable
rancid flavor in raw milk .
• certain typical cheese flavours are produced by deliberate addition of
mild lipases.
• Controlled and selective lipolysis is used in manufacture of yogurt and
bread.
• Phospholipid hydrolysis in most species of fish during frozen storage is
associated with deterioration in quality.

7. Saponification
• Hydrolysis is by the action of alkali usually KOH or NaOH to glycerol
and corresponding salts of fatty acids(soaps)

8. Halogenation
• The addition of halogens (hydrogen & iodine) at the point of
unsaturation in fat. Important in determining the degree of
unsaturation of a fat hence its biological value

9. Hydrogenation
• A subset of halogenation: addition of H-atoms at point of
unsaturation in fat to harden and stabilize the fat.
• Done under high pressure of hydrogen & catalyzed by finely divided
nickel/copper and heat.
• Mainly used to make margarines .

10. Auto-Oxidation (Rancidity)
• Physico-chemical change in the natural properties of fat leading to
devt of off-odor, taste or color esp, during exposure to atmospheric
oxygen, light, moisture, bacteria or fungal contamination and/or heat
known as rancidity
• Saturated fats are more resistant to rancidity than unsaturated .
• Auto-oxidation follows 3 major steps
• Initiation where free radicals are formed
• Propagation : Free radicle chain reactions occur
• Termination: Formation of non-radical products.

11. Steps
• Initiation
• RH+02→R.+.OH
• R.+O2→+ROO.
• Propagation
• ROO.+RH→R.+ROOH
• ROOH→RO.+.OH
• Termination
• R.+R.→RR
• R.+ROO.→ROOR
• ROO.+ROO.→ROOR+O2
• RH is the unsaturated fatty acid
• R. is the free radical formed by removal of
hydrogen from the carbon atom adjacent
to the double bond in the Fatty acid.
• ROOH is hydrogen peroxide. ROOH
decomposes to form products such as
hexanal, monoaldehyde and pentanal
which are responsible for the off-flavours
and odours during rancidity)
• ROO. is a peroxyradical. These radicals may
attract hydrogen atoms to form more
hydrogen peroxide and the chain reaction
continues or react together to form
nonradicals hence termination of oxidation.

12. • The reaction RH+O2 to form free radicals is thermodynamically
difficult. Production of the first few radicals in the initiation stage
must occur by some catalytic means such as
1. Hydrogen peroxide decomposition
2. By metal catalysis (e.g. CU+)
3. By exposure to light
4. Enzymatic action

13. Factors Affecting Rate of Oxidation
• Fatty Acid Composition: The number, position and geometry of double bonds
affect rate of oxidation. Cis acids oxidize more readily than trans isomers and
conjugated double bonds are more reactive than non- conjugated.
• FFA Vs corresponding acyl glycerols: FFA oxidize at faster than when esterified to
glycerol.
• Oxygen Concetration:. 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 O2 concentration.
• Temperature: In general, the rate of oxidation increases as the temperature is
raised.

14. • Moisture: rate of oxidation depends strongly on water activity. In
dried foods with very low moisture contents, oxidation proceeds
very rapidly. Increase the aw to about 0.3 retards lipid oxidation and
produces a minimum rate.
• This protective effect of small amounts, water is believed to occur by
reducing the catalytic activity of metallic catalysts by quenching free
radicals and by impeding access of oxygen to the lipid. At higher
water activities (aw = 0.55 – 0.85) the rate oxidation increases again
as result of increased mobilization catalysts and oxygen

15. • 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 and catalyse oxidation of lipids
• Light: Visible, U.V and gamma radiation are effective promoters of
oxidation.
• Surface area: The rate of oxidation increases in direct proportion to
the surface area of the lipid exposed to air

16. TECHNOLOGY OF FATS: Extraction.
• Three principle methods are used for extraction of edible fats and oils from the
animal or vegetable tissues in which they occur i.e. Rendering, Pressing & Solvent
extraction.
Rendering :
• Process of fat removal from a tissue by heat. The tissue containing a high
percentage of fat is carefully removed from the animal and chopped and minced
and heated.
• Heat allows the lipid to escape from the cells. If the heat is high the cells
are completely ruptured, a cooked flavor develops and the oil is left floating on
top. Rendering can be carried out either in the presence of water as in wet
rendering or in its absence as in dry Rendering

17. Rendering
Wet Rendering:
• fat is separate by gentle heat in an open kettle or in autoclave in the presence of steam.
The well chopped tissue is introduced into an open kettle along with a charge of water,
stirred gently and heated to about 500C. The fat floats to the top and is carefully
skimmed off.
• It has a bland flavor and requires little deodorization but the process does not remove
all of the fat from the tissue. Nowadays autoclaves and steam in digesters at high
temperature and pressures of 40 to 60 psi is used. In this way the tissue is quite well
disintegrated and separation of fat is efficient.
Dry Rendering :
• The tissue is heated and the fat separates as the protein is denatured and
water is evaporated. Commercially the process is carried out under vacuum steam
jacketed cookers. process used in cooking bacon.

18. Pressing
• It is application of high pressures to the oil bearing tissue to squeeze
out the fat. The oil is removed by pressure from an expeller or screw
press. In some cases, such as pressing olives, virgin oil is the first
pressing of the fruit and is partially bland in flavor. The fruit is then
subjected to subsequent pressing to give other grades of oil.

19. Solvent extraction:
• In this method the oil is removed from tissues which have relatively low
percentage of oil.
• Some times a tissue is subjected to pressing and then the press cake with
its low fat concetration is extracted with a solvent. The solvent must
be removed from oil after extraction of oil.
• The method is quite efficient but is expensive because of the inevitable loss
of the solvent through evaporation. Common solvents are petroleum
ether, Benzene, chlorinated hydrocarbon and carbon disulfide. The
residue obtained after pressing or extraction is high in protein and is
valuable feed for cattle.

20. Soxhlet Extraction
• Method of isolating compounds,
such as lipids, from a large amount
of solid material with a relatively
small volume of solvent.
• The solid to be extracted is placed
in a thimble in the central chamber
of the apparatus. The extraction is
aided by the addition of energy in
the form of heat, known as
refluxing.
A typical assembly uses a
round-bottomed flask, a cold
water condenser, and the
Soxhlet apparatus.

21. • The solvent vapor rises through the distillation path in the Soxhlet
apparatus to the condenser.
• Upon condensing, the solvent collects in the chamber, dissolving
some of the organic material in the thimble.
• As the chamber fills, the siphon fills as well. When the siphon is full,
the solution flows back into the flask. The solution level never
exceeds the top of the thimble, so no solid enters the flask.

22. • The lipid extract continually collects in the flask, whereas the solvent becomes part of
the next extraction cycle. Thus, the cycle can repeat indefinitely without loss of solvent.
• After extraction solvent is removed by use a rotary evaporator leaving the extracted
compound.
• Non soluble portion of extracted solid remains in the thimble and is usually discarded.
Advantages.
• The conservation of the solvent, the continuous nature of the extraction, and the ability
to accommodate large sample sizes makes Soxhlet extraction ideal for isolating organic
compounds from large portions of insoluble material.
• State the Characteristics of a good Solvent

23. Technology of fats: Hydrogenation and inter
esterification.
Refining:
• After extraction, the crude oils extracted from the tissue often contains
materials that must be removed. Including;
1. Cellular material derivatives like protein and CHO’s
2. Free fatty acids and phospholipids
3. Pigments
4. Odour compounds such as aldehydes, ketones and essential oils
5. Glycerides with high melting point.
• The first step in refining most oils and fats is the removal of cell debris.
• Free fatty acids present in some crude oils can be fairly completely removed by
steam refining.

24. Steam refining:
• Process is used for the deodorization of fats & consists of blowing steam through hot
oil through a vacuum.
• Oil with high concentration of free fatty acids is usually first subjected to steam
refining which is then followed by alkali refining.
• Oils with low fatty acids may directly undergo alkali refining.
• In alkali refining hot oil is treated with a solution of alkali usually sodium hydroxide.
Sometimes with sodium carbonate.
• The free fatty acids react with alkali and forms soaps which are dispensable in water.
The process is called saponification. The oil is then mixed with hot water and passed
through a centrifuge that discharges it out.
•

25. Bleaching
• Bleaching involves removal of pigments by adsorption through the addition of small
amount of charcoal (activated). The oil is heated to 220– 240oF and then agitated and
filtered until the color of filtered oil is sufficiently light. Chemical agents like oxidizing or
reducing agents are not used for bleaching of edible oils, but used for waxes.
• Steam deodorization: The last step of the refining process of oils and fats to remove
unwanted odours and taste. This is achieved by applying low pressures and small
quantities of steam. The fat is heated as steam is injected into the bottom of the vessel.
The oils are rapidly deodorized at 425 to 475oF. If lower temperatures are used longer
periods are required then oil is rapidly cooled.
• By steam deodorization fats and oils are rendered, so bland that it is difficult to detect
their origin by taste or smell.

26. Hydrogenation :
• Hydrogenation of fats involves the addition of hydrogen double to bonds in the
fatty acid chains. The process is of major importance in the fats and oils
industry & accomplishes two major objectives.
- First it allows the conversion of liquid oils into semisolid or plastic fats more
suitable for specific applications, such as in shortenings and margarine
- second, it improves the oxidative stability of the oil.
• In practice, the oil is first mixed with a suitable catalyst heated to a desired
temperature (140-2250C), then exposed, while stirring to hydrogen at pressures
up to 60 psi.
• Agitation is necessary to aid dissolving the hydrogen to achieve uniform mixing.

27. Inter-esterification & Intra-esterification
• Processes used to improve the consistency of lipids
to improve their usefulness. This process involves
rearranging the fatty acids within a triglyceride
molecule. Generally done to modify the melting
point, slow down rancidity or making the oil more
suitable for deep frying.
• Note: This is not the same as hydrogenation.
Principle :
• If a lipid contains two fatty acids A & B, 8 possible
TAG species are possible according to rule of
chance.
A A A A B B B B
A A B B A A B B
B A A B A B A B

28. • Regardless of the distribution of the two acids
in the original fat (eg:
• AAA & BBB or ABB,ABA,BBA), intra-
esterification results in the “shuffling” of fatty
acids within a single molecule and among TAG
molecules until an equilibrium is achieved in
which all possible combinations are formed.
• Inter-esterification follows the same principle
but this time rearrangement is between
molecules and not within molecules.

29. Chemistry of Frying
• Foods fried in fat make significant contributions to the calories in the
average global population diet. In the course of deep fat frying, food
contacts oil at about 1800C and is partially exposed to air for various
periods of time.
• Thus frying more than any other standard food process and handling
method, has the great potential for causing chemical changes in fat,
and sizeable amount of this fat are carried with food.

30. Behavior of the frying oil:
• The following classes of compounds are produced from the oil during
frying.
1. Volatiles:
• During frying, oxidative reactions involving the formation and
decomposition of hydroperoxides lead to formation of compounds such as
saturated and unsaturated aldehydes, ketones, hydrocarbons, alcohols,
acids and esters.
• After oil is heated for 30 min at 1800c in the presence of air, the primary
volatile oxidation products can be detected by gas chromatography.
Although the amounts of volatiles produced vary widely, depending on oil
type, food type and the heat treatment.

31. 2. Non polymeric polar compounds of moderate volatility. The
compounds are produced according to the various oxidative
pathways involving the radicals.
3. Dimeric and polymeric acids, and dimeric and polymeric
glyceride. These compounds occur as a result of thermal and oxidative
combinations of free radicals. Polymerization results in a substantial
increase in the viscosity of the frying oil.
4. Free fatty acids :
• These compounds arise from hydrolysis of triacylglycerols in the
presence of heat and water.

32. What Happens During Frying
• During frying, water is continuously released from the food into the
hot oil. This produces a steam distillation effect, sweeping volatile
oxidative products from the oil.
• The released moisture also agitates the oil and hastens hydrolysis.
The blanket of steam formed above the surface of the oil tends to
reduce the amount of oxygen available for oxidation.
• Volatiles may develop in the food itself or from the interactions
between the food and oil.

33. • Food absorbs varying amounts of oil during deep fat frying, resulting
in the need for frequent or continuous addition of fresh oil.
• In continuous fryers, this result in rapid attainment of a steady state
condition for oil properties. The food itself can release some of its
endogenous lipids into the frying fat and consequently the oxidative
stability of the new mixture may be different from that of the original
fat. The presence of food causes the oil to darken at an accelerated
rate.

34. • The changes that occur in the oil and food during frying should not be automatically
construed as undesirable or harmful. Infact some of these changes are necessary
to provide the sensory qualities typical of fried food. On the other hand,
extensive decomposition, resulting from lack of adequate control of the frying
operation, can be potential source of damage not only to sensory quality of the fried
food but also to nutritional value.
• The chemical and physical changes in the frying fat are influenced by a number of
frying parameters. Obviously, the compounds formed depend on the composition of
both the oil and the food being fried. High temperature, long frying times, and
metal contaminants favour volume ratios.