Human Nutrition 3(3+0)

Lesson 15 : Lipids

Rancidity in Fats

The development of off-flavours in fats is known as rancidity. There are three main types of rancidity

 

• Hydrolytic Rancidity
  Hydrolysis of fats by lipase need not always produce off-flavours. In the case of butter fat and coconut oil, butyric acid and other low molecular weight fatty acids are set free on hydrolysis by lipase. The odours of these acids contribute largely to the smell of rancid butter. The saturated fatty acids such as palmitic and stearic acids have little odour.

• Oxidative Rancidity
  This is the common type of rancidity observed in all fats and oils. The oxidation takes place at the unsaturated linkage. Certain metals, such as copper, hasten the onset of oxidative rancidity. The addition of oxygen to the unsaturated linkage results in the formation of a peroxide which, on decomposition, yields aldehydes and ketones having pronounced off odour.

• Ketonic Rancidity
  This type is most frequently encountered as a result of action of fungi such as Aspergillus niger and blue-green mould, penicillium glaucum on coconut or other oil seeds. The tallow odour developed may be due to aldehydes and ketones formed by the action of the enzymes present in the fungi on oil.