deep freezing

Method of preserving food by lowering its temperature to −18°/0°F or below; see food technology. It stops almost all spoilage processes, although there may be some residual enzyme activity in uncooked vegetables, which is why these are blanched (dipped in hot water to destroy the enzymes) before freezing. Micro-organisms cannot grow or divide while frozen, but most remain alive and can resume activity once defrosted.

Commercial freezing is usually done by one of the following methods: blast, the circulation of air at −40°C/−40°F; contact, in which a refrigerant is circulated through hollow shelves; immersion, for example, fruit in a solution of sugar and glycerol; or cryogenic means, for example, by liquid nitrogen spray.

Accelerated freeze-drying (AFD) involves rapid freezing followed by heat drying in a vacuum, for example, prawns for later rehydration. The product does not have to be stored in frozen conditions.

Rapid freezing avoids structural change that would affect the taste or appearance of the food, as in the shrinkage and distortion of cells by formation of enlarged ice crystals in the extracellular spaces. Some ‘quick-frozen’ foods require thawing before use, and cooking must then be prompt.

Freezing was developed in the late 19th century and found early commercial application in the transportation of large quantities of meat on long sea voyages.