Freezing Technology

3.4.2. Advantages of brine treatment

1. Treatment with brine destroys the microorganisms present on the fish fillet to a great extent. However, salt should not contain Ca⁺⁺ and Mg⁺⁺ impurities which are responsible for salt odour and bitter taste.
   
2. Prevention of belly bursting by brine treatment: During chilled storage and during freezing and thawing, belly bursting is observed in sardines (poor appearance, viscera comes out) which is a major problem in small fish such as sardine. If sardine is dipped in 15% brine for 30 min and then frozen, the problem of belly bursting can be minimized. By this method, due to coagulation of proteins the belly wall becomes tough. However the treatment promotes oxidation of fat and hence the shelf life in frozen storage gets reduced.

Phosphate treatment : PolyPO₄ treatment minimizes drip loss in frozen fish and fishery products. PolyPO₄ (Polyphosphate) treatment may be given separately or along with brine treatment. In cod fillets drip treatment in 0.0844 < (4%) polyPO₄ solution minimized significant amount of drip loss.

Role of Polyphosphate in minimizing drip loss :

In frozen fish, water holding capacity of tissues comes down due aggregation and protein denaturation and hence results in increased drip loss. The important reasons for drip loss are given below.

1. Freezing changes ionic environment of fish muscle.
2. Reduction in pH due to production of lactic acid.
3. Degradation of nucleotide which contains active phosphorus.
4. Formation of actomyosin from actin and myosin.

The role of polyphosphate in minimizing driploss and preservation of quality of fish is explained as follows.

1. Balanced action of tripoly PO₄, organic phosphate such as ATP and other phosphates leads to break down of the complex protein actomyosin to constituent proteins actin and myosin resulting in increased water holding capacity of muscle.
   
2. Cell fluids of fish (salted) dipped in brine require -22^OC for complete freezing due to increase in salt concentration. Therefore liquid present in such cell fluids remain unfrozen and atmospheric oxygen dissolve in such a liquid and penetrates inside the tissues. This accelerates oxidation process. However, if 12% tripolyPO₄ is mixed with brine, the freezing point of tissue fluids will only be -3^OC. Therefore proportion of unfrozen liquid will be greatly reduced and also concomitant oxidation. This is very much useful in fatty fishes.
   
3. Polyphosphate acts as a chelating agent and chelate proxidant metal ions, thereby minimizing catatytic action of metal ions in fat oxidation.