Hydrolysis, Hydrogenation, Saponification and Rancidity of oils, Drying
oils of Fatty acids
Session Objectives
By the end of this session, students will be able to:
Ø Discuss
the process of Hydrolysis, Hydrogenation, Saponification and Rancidity of oils,
Drying oils with examples for fatty acids.
Hydrolysis
Fats and oils contain ester groups which can be hydrolyzed
with aqueous acid, aqueous base (saponification) or enzymes
Triacylglycerols undergo stepwise enzymatic hydrolysis to
finally liberate free fatty acids and glycerol.
The process of hydrolysis, catalyzed by lipases is important
for digestion of fat in the gastrointestinal tract and fat mobilization from
the adipose tissues.
Industrially glycerol and some acids are prepared by the
hydrolysis of oils and fats with water under pressure.
Then glycerol is recovered from the aqueous solution.
Free fatty acids are used in the manufacture of candles.
Saponification
The hydrolysis of triacylglycerols by alkali to produce
glycerol and soaps is known as saponification
Any metallic salt of a fatty acid is a soap, but the term
soap is usually applied to water-soluble salts, Since only these have detergent
properties
NaOH is used with saturated fats to produce hard soaps
KOH is used with unsaturated fats to produce softer, more
liquid soaps
Ordinary soap is a mixture of sodium salts of even fatty
acids from octanoic to stearic acid
Hydrogenation
• Hydrogenation
converts alkenes to alkanes
• So,
hydrogenation of unsaturated oils produces saturated fats
• Hydrogenation
is typically carried out by bubbling H2 gas through the heated oil,
in the presence of a metal catalyst (such as nickel or platinum)
• Unsaturated
oils are usually only partially hydrogenated, so that the product is not
completely saturated, giving a soft semisolid fat such as margarine
Rancidity
·
Rancidity is the term used to represent the
deterioration of fats and oils resulting in an unpleasant taste and changes in
their texture and appearance.
·
Fats containing unsaturated fatty acids are more
susceptible to rancidity
·
Rancidity occurs when fats and oils are exposed
to air, moisture, light, bacteria etc.
·
Two types of rancidity:
§
Hydrolytic rancidity
§
Oxidative rancidity (auto-oxidation)
·
Rancid fats and oils are unsuitable for human
consumption
Drying oils
·
Oils which on exposure to air, change into hard
solids e.g., linseed oil
·
All drying oils contain a large proportion of
unsaturated acids linoleic and linolenic
·
This drying property makes these oils valuable
in paint industry
·
Mechanism of drying appears to be a complicated
process involving oxidation, polymerization and colloidal gel formation.
·
It has found to be catalyzed by various metallic
oxides, particularly lead monoxide
·
These are esters of higher homologues of both
the fatty acids and monohydric alcohols.
E.g., beeswax
myricyl palmitate
spermaceti cetyl
palmitate
carnauba
wax myricyl ceroate
·
The drying of paint doesn’t not involve merely
evaporation of a solvent
·
But chemically a tough organic film is formed
·
Aside from the color due to pigments present,
protection of a surface by this film is chief purpose of paint
·
The film is formed by polymerization of the
unsaturated oils that is brought about by oxygen
·
The polymerization process and the structure of
the polymer are extremely complicated and are not well understood
Summary
• Triacylglycerols
undergo stepwise enzymatic hydrolysis to finally liberate free fatty acids and
glycerol
• Hydrolysis
of triacylglycerols by alkali to produce glycerol and soaps is known as
saponification
• Rancidity
is the term used to represent the deterioration of fats and oils resulting in
an unpleasant taste
• Oils
which on exposure to air, change into hard solids e.g., linseed oil
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