Freezing Time:

The time occupied to get lower the temperature of any product from its initial temperature to a specified temperature at its thermal centre is called the freezing time. Generally the ultimate temperature is the intended storage temperature of the product. For instance, in the case of fish, the suggested storage temperature is - 30°C. To ensure rapid freezing, the freezer temperature ought to be below this temperature. This is desired that after freezing, the temperature of the thermal centre must be reduced to at least - 20°C therefore the average temperature of the fish is close the storage temperature of - 30°C. Therefore the freezing time shall be the time needed to decrease the thermal centre from its initial temperature to - 20°C. Therefore the residence time of the product in the freezer is equal to its freezing time.

An exact calculation of the freezing time for irregular-shaped product is complex. But for consistently-shaped products like rectangular blocks, appropriate relations have been proposed. Although, they frequently do not take into account the pre cooling from the initial temperature to the final temperature. They suppose that the product has been cool initially, and that all of the extraction of heat is at the freezing temperature. In the presence of other factors like packing, etc., might also give wrong results. Nevertheless, calculations, by using a computer, by the finite-difference method give very good results.

A solution for the calculation of temperature sharing during a mass, in which a change of state is occur, has been proposed through Neuman for freeze- drying. The similar can be applied for freezing. The equations which  expressing the temperature like a function of the position and time in an infinite slab along a change of state are provided but are not given in this text since they involved mathematics for which reader might not be prepared. In estimating freezing time it is supposed that surface comes to temperature of freezing media instantly.