RANCIDITY OF FATS & OILS, TYPES, FACTORS AND PREVENTIVE MEASURES

Similar to all organic substances, fasts do undergo deterioration called rancidity. It is one of the most important degradative change among fats and oils. Rancidity is defined as  the un favourable changes in fats which leads to development of unpleasant odours, taste. There are different kinds of rancidity, they are:

1. Oxidative rancidity

Oxidative rancidity occurs due to oxidation of fat/oil in the presence of oxygen. it is also named as auto oxidation. The series of reactions involved in oxidation of double bonds, breakage of bonds, rearrangements in fats and results in loss of hydrogen molecules. in this process, the long chain fatty acids degrades to short chain fatty acids and forms end products like are aldehyde epoxides and peroxides leads to off odours and flavours. Unsaturated fatty acids are more likely to undergo oxidative rancidity than saturated fatty acids. 

The oxidative rancidity of fats mainly composed of three steps:

(1) Initiation reaction - The fat molecules generates alkyl free radicals  in the presence of energy source like light, heat. 

(2) Propagation reaction -  In the presence of oxygen the alkyl free radicals forms peroxyl radicals, which further react with lipid molecules to form hydroxy peroxides and alkyl radicals.

(3) Termination reactions - the peroxyl and alkyl radicals interacts and forms non radical products like aldehydes, ketones, acids.

The components of oxidative rancid fats:

• Peroxidised fatty acids - degrades the vitamin A and E components in foods.
• Oxidised sterols - main reason for occurance of atherosclerotic disease. 
• Polymeric material - mainly forms during processing of fats in small amounts.

2. Hydrolytic rancidity

Hydrolysis of fats due to the action of lipase enzyme. The hydrolytic rancidity may occurs in the presence of moisture or high temperature. The hydrolysis splits glycerides and fatty acids from triglycerides and results in formation of glycerols and free fatty acids. These free fatty acids further prone to oxidation and aids in oxidative rancidity. The moisture present in food rise the rate of hydrolysis of fats at high temperatures.

3. Ketonic rancidity

the ketonic rancidity mainly occurs due to contamination of fats with fungi. The short or intermediate fatty acids converts to methyl ketones, the end products of ketonic rancidity are ketones, fatty alcohols. The common fungi are Aspergillus niger,  penicillium glaucum, blue green mold in coconut and other oils.  

4. Flavour reversion

The development of unpleasant flavours from fats prior to rancidity. flavour reversion usually occurs in presence of slight oxygen and mostly found in soyabean oil, marine oils. It occurs mainly in linolenate containing oils due to low oxidation, with peroxide value less than or equal to 10. 

Factors that initiates and elevates rancidity are:

a. Oxygen

Oxygen is more soluble in fats when compared with oxygen thus, exposure to oxygen is a major cause for rancidity. different types of oxygen molecules like singlet oxygen, reactive oxygen species, triplet oxygen has their own role in developing rancidity. The amount of lipid oxidation can be measured by hexanal formation. 

b. Moisture 

Moisture content in food affects the rate of rancidity. Triglycerides in fats reacts with water molecules and forms derivatives of glycerides and fatty acids. 

c. Temperature & Light

Temperature has influence on shelf life of fats and reaction rate of oxygen with fats. the reaction rate doubles with every 10 oC rise in temperature. The acid value and peroxide value of fats were increased during storage of fat at high temperatures. The light elevates the auto oxidation by photo-oxidation. 

d. Microorganisms

Microorganisms releases enzymes like lipase which accelerates breakdown of fats chemical structures and results in hydrolysis of fats. it leads to formation of undesirable volatile compounds by producing undesirable flavors.

e. Type of fat

The fats with more double bonds, carboxyl and hydroxyl groups are more prone for rancidity. The unsaturated fats like oleic, linoleic and  linolenic acids are more susceptible for rancidity. The positioning of double bonds, unsaturated fatty acids and triglycerides also has prominent role in auto oxidation.  

Preventive measures:

• Avoiding fats in contact with oxygen
• Using colored containers to prevent photo-oxidation
• Adding antioxidants to prevent the initiation of rancidity
• Storing at low temperatures