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�HYDROLYSIS, HYDROGENATION, SAPONIFICATION AND RANCIDITY OF OILS, DRYING OILS OF FATTY ACIDS

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SESSION OBJECTIVES�

BY THE END OF THIS SESSION, STUDENTS WILL BE ABLE TO:

  • DISCUSS THE PROCESS OF HYDROLYSIS, HYDROGENATION, SAPONIFICATION AND RANCIDITY OF OILS, DRYING OILS WITH EXAMPLES FOR FATTY ACIDS.

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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.

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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

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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

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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

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Hydrolytic rancidity

Oxidative rancidity

Caused by the breaking down of a lipid into its component fatty acids and glycerol.

C-O-CO-R + H2O → C-O-H + HO-CO-R

Occurs due to the oxidation of fatty acid chains, typically by the addition of oxygen across the C=C bond in unsaturated fatty acids.

Occurs more rapidly in the presence of enzymes such as lipase, and with heat and moisture.

The process proceeds by a free radical mechanism catalysed by light in the presence of enzymes or metal ion.

The water present in the food and the high temperature will increase the rate of hydrolysis to fatty acids.

The complex free radical reactions will produce a wide variety of products, many of which have unpleasant odours or tastes.

The free fatty acids have an unpleasant smell giving a rancid smell and taste to milk and butter that have been stored for too long. Longer chain acids are less volatile, so the smell is less noticeable.

In highly unsaturated lipids, such as fish oils, oxidative rancidity can be a major problem.

Rancidity

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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

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Drying oils

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  • 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

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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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