Elementary plant Biochemistry and Biotechnology (2+1)

Chemical Properties of fat

• The most important chemical reaction of neutral fat is their hydrolysis to yield
• three molecules Alkali hydrolysis (saponification)The process of alkali hydrolysis is called 'saponification'
• The alkali salt of fatty acid resulting from saponification is soap.
• The soaps we use for washing consists of Na or K salts of fatty acids like palmitic, stearic and oleic acid.
• The potassium soaps are soft and soluble whereas the sodium soaps are hard and less soluble in water.
• Enzyme hydrolysis
• Hydrolysis of triacylglycerol may be accomplished enzymatically through the action of lipases.
• Lipases are widespread in both plants and animals.

Rancidity

• Development of disagreeable odour and taste in fat or oil upon storage is called rancidity.
• Rancidity reactions may be due to hydrolysis of ester bonds (hydrolytic rancidity) or due to oxidation of unsaturated fatty acids (oxidative rancidity).

Hydrolytic rancidity

• This involves partial hydrolysis of the triacylglycerol to mono and diacylglycerol.
• The hydrolysis is hastened by the presence of moisture, warmth and lipases present in fats or air.
• In fats like which contains a high percentage of volatile fatty acids,hydrolytic rancidity produces disagreeable odour and taste due to the liberation of the volatile butyric acid.
• Butter becomes rancid more easily in summer.

Oxidative rancidity

• The unsaturated fatty acids are oxidised at the double bonds to form peroxides, which then decompose to form aldehydes and acids of objectionable odour and taste.

Hydrogenation

• The degree of unsaturation of the fatty acids present in triacylglycerol determines whether a fat is liquid or solid at room temperature.
• The presence of more unsaturated fatty acids lower the melting point.
• The presence of highly unsaturated fatty acids makes the oil more susceptible to oxidative deterioration.
• The objective of hydrogenation is to reduce the degree of unsaturation and to increase the melting point of the oil.
• The oil can be selectively hydrogenated by careful choice of catalyst and temperature.
• Hydrogenation of unsaturated fats in the presence of a catalyst is known as hardening.
• Normally the process of hydrogenation is partial so as to get desired characteristics and to avoid products with high melting points.
• Hydrogenation is carried out in a closed container in the presence of finely powdered catalyst (0.05 - 0.2% of nickel) at temperature as high as 180oC.
• The catalyst is usually removed by filtration.
• During hydrogenation process a proportion of the cis double bonds are isomerized to trans double bonds and there is also migration of double bonds.
• The hydrogenation process has made it possible to extend the food uses of a number of vegetable oils and marine oils whose melting points are too low.