Freezing is done to preserve food by reducing the product temperature, thereby slowing the quality deterioration processes. But still some changes take place on long storage of frozen foods like oxidation of fat, growth of microorganisms, enzymatic reactions and loss of surface moisture (dehydration) which needs attention.
Chemical and physical changes
Many changes, both chemical and physical may take place during freezing and subsequent frozen storage. Enzymes may catalyze chemical change detrimental to the quality of the frozen food. The action of lipase is more likely to be a problem than the activity of proteases during frozen storage.
The treatment of the food prior to storage influences the extent of enzyme activity subsequently. Enzymes, which cause the loss of colour, loss of nutrients and flavour changes in frozen food especially vegetables, must be inactivated to prevent such reactions from taking place. Since fruits are not blanched, enzymes in fruits to be frozen should be controlled by using chemical compounds such as ascorbic acid (vitamin C), which interfere with deteriorative chemical reactions. Other temporary measures include soaking the fruit in dilute vinegar solutions or coating the fruit with sugar and lemon juice to control enzyme-activated browning. But these are not as effective as ascorbic acid.
Oxidative rancidity may develop in frozen products through contact of the frozen product with air. To prevent this problem, proper packaging which does not permit air to pass into the product should be used. Physical changes include clumping of egg yolk.
Flavour may be modified during frozen storage as a result of enzymatic or oxidative changes in the food. Vegetables and food containing fats are most susceptible to the detrimental changes in flavour during frozen storage. The extent of flavour modification is determined by treatment prior to freezing and by storage condition during storage. Vegetables need to be blanched prior to being frozen to aid in minimizing flavour changes. Blanching helps to inactivate lipoxidase, the enzyme which causes development of rancidity in fats in vegetables. Damage to vegetables results in development of off flavours if these vegetables are held for a storage period prior to processing.
Texture of frozen foods is modified by the freezing process. Whether the change is desirable or not will depend on the food itself. When the water freezes, it expands and the ice crystals cause the cell walls to rupture. As a result, the texture of the produce after thawing will be markedly much softer than it was when raw. Therefore, frozen foods to be eaten should be partially thawed. Textural changes due to freezing are less noticeable in high starch vegetables, such as peas and corn etc.
Pigment changes can be noted in frozen beef. The iron in the myoglobin can undergo an oxidative reaction to form metmyoglobin, a reddish brown pigment with less visual appearance than the original myoglobin or oxymyoglobin. Some of the reddish fruits and vegetables become less attractive during frozen storage because the loss of some of their anthocyanin pigments into the medium surrounding them. In peaches, enzymatic action can produce colour changes in storage. The polyphenol oxidase enzyme in peaches catalyses a chemical change of the flavonoid pigments with molecular oxygen resulting in formation of quinones which in turn are converted to brown coloured polymers yielding a familiar brownish colour of frozen peaches.
Fast cooling and freezing can prevent moisture loss or dehydration by minimizing the water evaporation rate and time.
The grainy, brownish spot formed where the tissues become dry and tough in frozen storage is known as freezer burn. This imparts off flavours to the product. Freezer burn can be prevented by using thick moisture proof packaging.
Nutritional value of frozen foods
Freezing, when properly done, may potentially preserve the greatest quantity of nutrients. Nutritional merits of frozen foods are influenced by the preparation required prior to freezing. Blanching is necessary to inactivate enzymes but it also destroys Vitamin C to the extent ranges of 40 per cent. B-vitamins are also destroyed. Water soluble vitamins are lost in the drip while thawing frozen meats. This loss ranges between 3-10 per cent.