Frozen food industry is growing now. Foods are frozen by using different freezing/refrigeration methods. In frozen foods, microbial contamination is less and also food enzyme activity is retarded. The lower the storage temperature, the slower will be rate of chemical or enzymatic reaction.
Different methods of freezing are: Sharp Freezing, Quick Freezing, Freezing by Direct Immersion, Indirect/Slow Freezing, Cryogenic Freezing, Dehydro-Freezing, and Freeze-Drying.
Sharp Freezing: Sharp freezing involves freezing by circulation of air. Air is circulated either naturally or with the aid of fans. Temperature varies from -15 to -290 C. Freezing may vary from 3 to 72 hours. Products suitable for sharp freezing: meat and butter.
Disadvantages of Sharp Freezing: The ice crystals formed are large and it may rupture the cells. The thawed tissue cannot regain its original water content.
Quick Freezing: Maximum ice crystal formation happens quickly within minutes. Most foods are quick frozen by direct immersion in a liquid solution because liquids are good heat conductors. Quick freezing can be done by immersing fruits/vegetables either in a brine solution or in a sugar solution at low temperature. Examples are: Berries in sugar solution; packed fruit juices and concentrates etc.
Disadvantages of Quick Freezing: Though brine is a good refrigerating medium, it cannot be used for fruit freezing. It is difficult to make sugar syrup that will not become viscous at low temperature. It is very difficult to maintain the medium at a definite concentration. It is difficult to keep it free from dirt and contamination.
Indirect, Slow Freezing: Indirect freezing may be defined as freezing by contact of fruits/vegetables with a metal surface which is it cooled by freezing brine or other refrigerating media.
Advantages of Slow Freezing over Quick Freezing are: Smaller ice crystals are formed; hence, food cells are not destroyed. Period for ice formation is shorter in slow freezing.
Cryogenic Freezing: Cryogenic freezing is defined as freezing at very low temperature. The refrigerants used for cryogenic freezing are liquid nitrogen and liquid carbon dioxide.
Dehydro-Freezing: When freezing is done by partial dehydration of fruits and vegetables, it is known as ‘dehydro-freezing’. In some cases of dehydro-freezing, up to 50 percent of the moisture can be removed through dehydration. Dehydro-freezing improves the quality of the food. Because dehydration does not cause food deterioration. Dehydrate-frozen foods are relatively more stable also.
Freeze-Drying: In ‘Freeze-Drying’ process, foods are first frozen at 180 C on trays in the lower chamber of freezing equipment. Then the frozen material is dried initially at 300 C for 24 hours and then at 200 C under high vacuum in the upper chamber. Direct sublimation of the ice takes place without passing through the intermediate liquid stage. The biggest advantage of freeze-drying is that the product is highly hygroscopic, excellent in taste and flavour and can be reconstituted readily.
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