Preservation of Foods with Low Temperature
Use of low temperature reduces the microbial
activity and enzyme activity thus prolongs shelf life of foods. Two dif-ferent
temperatures are employed in low temperature namely chilling temperature and
Chill temperature is just above the
refriger-ated temperature. In chilling fish, the tem-perature is reduced to
freezing point of water. Chill temperature delays both bio-chemical and
bacteriological changes. The deteriora-tive changes are retarded when low
temper-ature 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
sta-bility in the presence of food preservation.
Freezing 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 enzy-matic 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. In this method, the food is frozen under tem
peratures ranging from −4oC to −29oC.
Freezing may require three to seventy-two hours under such conditions. Home freez-ing is done by this
The temperatures used in the quick freez-ing process range from −32oC to −40oC. It freezes food so rapidly that fine crys-tals
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 tem-perature 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 of fruits and
vegeta-bles 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 pre-liminary drying. The cost is marginally less because of
weight and volume savings in packing, freezing, storing and shipping.
Blanching (dipping the products
in boil-ing water for two to three minutes) veg-etables before freezing reduces
the num-ber 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 dete-rioration in palatability, colour and ascor-bic acid content during
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 ani-mal, the pork, meat and poultry is chilled promptly to avoid spoilage.
The ten-dency of the fat of the pork and poultry to become rancid during
storage in a freezer is aggravated by storage before freezing.