In this low temperature preservation methods post we have briefly explained about preservation of food by low temperature definition, chill storage, freezing, and points to be considered during preservation of food by low temperature.
Low Temperature Preservation Methods
Many foods cannot be stored as such and need to undergo a treatment or a technique which helps to prevent spoilage. The techniques adopted to preserve the foods are grouped into traditional techniques and modern industrial techniques. Traditional techniques include curing, freezing, boiling, heating, sugaring, pickling, canning, smoking, salting and fermentation. Modern industrial technique involve pasteurization, vacuum packing, artificial food additives and irradiation.
Use of low temperature reduces the microbial activity and enzyme activity thus prolongs shelf life of foods. Two different temperatures are employed in low temperature namely chilling temperature and freezer temperature.
Preservation of Food by Low Temperature
Chill temperature an important preservation of food by low temperature technique is just above the refrigerated temperature. In chilling fish, the temperature is reduced to freezing point of water. Chill temperature delays both bio-chemical and bacteriological changes.
The deteriorative changes are retarded when low temperature is maintained. Hence the shelf life of food is improved and this ensures preserving natural and functional properties of food. Storage at −1°C and −4 °C can provide stability in the presence of preservation of food by low temperature.
Freezing an important preservation of food by low temperature technique is a means of preserving food through the application and maintenance of extreme cold temperature (−4°C to −40°C). It is effective because most of the water of the food tissue is changed from the liquid to the solid state. This change in the physical state of water retards enzymatic action and stops microbial growth, the cause of food spoilage, thus preserving food. Many foods can be frozen for twelve months or more without major changes in size, shape, texture, colour and flavour.
It is also known as sharp freezing an important preservation of food by low temperature technique. In this method, the food is frozen under temperatures ranging from −4C to −29C. Freezing may require three to seventy-two hours under such conditions. Home freezing is done by this method.
The temperatures used in the quick freezing process range an important preservation of food by low temperature technique from −32oC to −40oC. It freezes food so rapidly that fine crystals are formed. The time taken for quick freezing is significantly lower than that of slow freezing.
In quick freezing, large quantities of food can be frozen in a short period of time. The use of very low temperature for both freezing and holding frozen products adds to the cost but of desirable for many products in terms of retention of palatability and nutritive value.
Dehydro freezing an important preservation of food by low temperature technique, Dehydro freezing of fruits and vegetables is the drying of the food to about 50 percent of its original weight and volume and then freezing the food to preserve it. The quality of dehydro frozen fruits and vegetables is equal to that of fruits and vegetables which are frozen without preliminary drying. The cost is marginally less because of weight and volume savings in packing, freezing, storing and shipping.
Points to be considered
Blanching (dipping the products in boiling water for two to three minutes) vegetables before freezing reduces the number of microorganisms, removes some air from the tissues, makes them more compact and enhances their colour. Its most important function is to inactivate enzymes otherwise that would cause deterioration in palatability, colour and ascorbic acid content during storage.
The enzymes of fruits can be inactivated by blanching but it is not done as it gives the fruit a cooked flavour and soft texture. Rather fruits are cut directly into sugar syrup or sugar to prevent oxidation. Sugar not only increases the sweetness but helps to retain volatile aroma.
Meat and poultry require only wrapping for freezing. After slaughtering the animal, the pork, meat and poultry is chilled promptly to avoid spoilage. The tendency of the fat of the pork and poultry to become rancid during storage in a freezer is aggravated by storage before freezing.
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