Freezing Technology

3.4.3. Use of Polyphosphate in freezing of fish

Dip treatment in phosphate solution prior to freezing gives better results. In prawns, in addition to dip treatment in phosphate solution, it may be mixed with glaze water.

Quantity of polyPO₄ to be used

Fairly high proportion of phosphate ion is required for breakdown of complex acto-l-myosin into its constituent proteins. Among various phosphates, tripolyPO₄ and pyropolyPO₄ gives the best results.

Use of tripolyphosphate solution of 0.16 M ionic strength (8%) and 4.5% pyropolyPO₄ along with salt to increase the ionic concentration to 0.4 M and 0.35 M respectively increases the water holding capacity of muscle tissues effectively. Among the above, tripoly phosphate is extensively used but its effect depends on the species of fish. Though tripolyPO₄ solution and brine can be used separately, fish and fillets are treated with pyropolyPO₄ and brine together very good effect was observed.

Polyphosphate should be used in such a way that it is absorbed quickly into fish muscle. It has been found that the enzymes and digestive juices convert the polyPO₄ in to orthoPO₄ in the body and hence it does not pose any health hazard. However it is better to take proper care while using depending on its desired effect on the product.

Antioxidant treatment

Rancidity is the commonly observed storage problem in fatty fish. To minimize the problem of rancidity, antioxidants which are commonly used in edible oils and fats have to be used in fish. Rancidity in oils and fats is controlled by following methods.

1. Removal of pro-oxidant chemical compounds or inactivation of these factors.
2. Packaging of fish so that oxygen is prevented from coming into contact with fish surface and subsequent penetration inside the tissues.
3. Before or after freezing fish may be treated with antioxidants. Enzymes responsible for fat oxidation may be inhibited.

Phenolic antioxidants, even though very effective, they are not soluble in water. Some of the methods used for application of antioxidants for frozen fish and fillets are as follows.

1. Water soluble antioxidants such as ascorbic acid penetrates the tissues of fish through tissue fluids and helps in arresting oxidation process. It is effective against salmon, trout, herring and tuna during frozen storage. It is also effective against colour change in pomfrets. Ascorbic acid also acts as a synergist when used along with BHA, tocopherol and hydroquinone. Ascorbic acid in glaze increases the shelf life of mackerel and pomfret during chilled and frozen storage.
   
2. Glaze containing ascorbic acid and citric acid mixture is effective against fatty fish and onset of rancidity can be delayed for a long time.
   
3. Oil soluble antioxidants are useful in minimizing rancidity in fatty fishes. This is because the oil of fish is on the surface and the antioxidants dissolve quite easily into such oil and penetrates into the tissues. The antioxidant power of BHA and Ascorbic acid double when used as a mixture.