The following points highlight the top four properties of triglycerols. The properties are: 1. Hydrolysis 2. Saponification 3. Rancidity 4. Lipid peroxidation in vivo.
Triglycerols: Property # 1. Hydrolysis:
Triglycerols undergo stepwise enzymatic hydrolysis to finally liberate free fatty acids and glycerol. The process of hydrolysis, catalyzed by lipase is important for digestion of fat.
Triglycerols: Property # 2. Saponification:
The hydrolysis of triglycerols by alkali to produce glycerols and soaps is known as saponification.
Triglycerol + 3NaOH → Glycerol + 3R—COONa (soaps).
(a) Determination of Saponification Value:
Alkali hydrolysis of fatty acids in triglycerides or fats is called saponification. It is the amount in mg of alkali required to saponify 1 g of fat or oil. This value is useful for a comparative study of the fatty acid chain-length in oils.
(b) Principle:
A known quantity of oil is refluxed with an excess amount of alcoholic KOH. After saponification, the remaining KOH is estimated by titrating it against a standard acid.
(i) Hydrochloric acid 0.5 N, accurately standardized.
(ii) Alcoholic KOH — any amount like 40 g of KOH is dissolved in 1 litre of neutral alcohol. This need not be standardized.
(iii) Phenolphthalein indicator—I % in 95% ethanol.
(iv) Air condenser.
(d) Procedure:
4-5 g of sample is taken into a standard joint flask to which 50 ml of alcoholic KOH is added. A blank is also prepared by taking 50 ml of alcoholic KOH only. Air condenser is connected to the flasks and these are gently boiled for about one hour. After the flask and condenser get cooled, the condenser is removed.
About 1 ml of indicator is added to each flask and the content titrated against 0.5 N HCI until the pink colour just disappears.
Saponification value = 28 x (Titre value of blank – Titre value of sample)/Weight of sample (g)
Here the titre value of 0.5 N HCI will be equivalent to an equal volume of 0.5 N KOH. Since 1 ml of 0.5 N KOH contains 28 mg of KOH, the titre value is multiplied by 28 to get the saponification value.
Triglycerols: Property # 3. Rancidity:
Rancidity is the term used to represent the deterioration of fats and oils resulting in an unpleasant taste. Fats containing unsaturated fatty acids are more susceptible to rancidity. It occurs when fats and oils are exposed to air, moisture, light, bacteria etc. Hydrolytic rancidity occurs due to partial hydrolysis of triglycerols by bacterial enzymes.
Oxidative rancidity is due to oxidation of unsaturated fatty acids. This results in the formation of unpleasant products such as dicarboxylic acids, aldehydes, ketones, etc. Oxygen is required for oxidative rancidity through the formation of peroxides.
(a) Antioxidants:
The substances which can prevent the occurrence of oxidative rancidity are known as antioxidants. Trace amounts of antioxidants such as tocopherols (vitamin E), hydroquinone, gallic acid and α-naphthol are added to the commercial preparations of fats and oils to prevent rancidity. Propyl gallate, butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are antioxidants used in food preservation.
Triglycerols: Property # 4. Lipid Peroxidation in Vivo:
In the living cells, lipids undergo oxidation to produce peroxides and free radicals which can damage the tissue. The free radicals are believed to cause inflammatory diseases, ageing, cancer, etc. The cells possess antioxidants like vitamine E, urate and superoxide dismutase to prevent in vivo lipid peroxidation.