Hydrolysis, Hydrogenation, Saponification and Rancidity of oils, Drying oils of Fatty acids
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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