Three types of freezing processes are:
Immersion freezing:
The foods to be frozen are plunged directly into a very cold solution. At one time brine was employed, particularly for freezing whole fish. Alternative freezing liquids that have more recently come into use are invert sugar solutions, particularly for fruits & vegetables. A variation of immersion freezing is the indirect contact system in which the food is placed in a metal container & the container then immersed in a bath of refrigerated brine or some other freezing solution.
Convection freezing:
This system involves the use of a blast of cold air which, when directed on the foods, quickly freezes them.
For example: fluidized-bed freezing. Here, a blast of cold air is blown upwards through a bed of the food to be frozen & the portions of food bounce up & down in the air flow & the entire mass behaves almost as if it were a liquid.
Cryogenic freezing:
This method involves the exposure of food to a cooling medium at a very low temperature. Liquid nitrogen at a temperature of -195.80 C has been used successfully on a commercial scale. The principle may be used in a number of ways.
For example: strawberries immersed in liquid nitrogen for 30 seconds can be cooled from 230C to -120C. Alternatively, the food to be frozen can be exposed to a spray of liquid nitrogen or to a blast of the very cold N2 gas. The rapid freezing by these processes can give the products good flavor.
