Freezing Technology

5.1.1.4 Fat oxidation during freezing

The changes in lipids are directly and indirectly responsible for the quality deterioration in frozen seafoods. They involve lipolysis, lipid oxidation, and interactions of the products of these processes with non-lipid components. The endogenous fish lipases are relatively resistant to low temperatures and retain much of their activity in the frozen tissues. Some of them may even be activated in the freezing process eg. by release from lysosomes. Fish lipids undergo two main types of changes during storage of fish viz, hydrolytic changes and oxidative changes. These changes result in rancidity.

Lipid hydrolysis

Lipases bring about hydrolysis of lipids producing free fatty acids and resulting in hydrolytic rancidity. Free fatty acids trigger protein insolubilization and texture degradation in frozen stored fish.

Lipid oxidation

It is a very series problem in fish, in view of the highly unsaturated nature of fish lipids. The double bonds of unsaturated fatty acids are highly susceptible to oxidation and this leads to the production of various carbonyls and other secondary oxidation products, which impart the characteristic rancid off flavour to the product. These products, besides producing off flavour reduce the shelf life and nutritional value of the product also. Some of them are toxic in nature. These reactions are initiated by free radicals generated from unsaturated bonds, which start chain reactions resulting in the production of various undesirable compounds like peroxides, hydroperoxides, aldehydes, ketones etc. Finally, the free radicals form non-radical polymers, which terminate the chain reaction.

Oxidised lipids interact with proteins reducing the nutritive value of the proteins considerably. Melonaldehyde is one of the major oxidation products and estimation of this compound by forming the thiobarbituric acid (TBA) complex is the accepted method for monitoring the extent of lipid oxidation. In lipid oxidation, the first step leads to formation of hydroperoxides, which are tasteless but can cause brown and yellow discolouration of the fish tissue. The degradation of hydroperoxides gives rise to formation of aldehydes and ketones. These compounds have a strong rancid flavour. Lipid oxidation primarily non enzymatic in nature, recently the involvement of microsomal enzymes and lipoxygenase has been reported. This lipid oxidation takes place in fishes having more than 2% of the lipids eg. fatty fishes.

Factors affecting the oxidation

1. Content and composition of unsaturated fatty acid
2. Oxygen availability
3. Light radiation
4. pH
5. Temperature
6. Moisture content
7. Content of pro and anti oxidant.