Explain the process of Freezing

Freezing, as a method, is the easiest, most  convenient and least time-consuming way of preserving foods. Freezing does not sterilize foods or destroy the organisms that cause spoilage. How then does it preserve foods? Well, the  extreme cold simply slows the growth of microorganisms and the chemical changes that affect quality or cause spoilage. You already know that all microbial and biochemical activities are temperature dependant and slow down as the temperature is reduced. As a rule of thumb, for every 10°C temperature change, the rate of reaction changes by a factor of 2 to 3. 
 
Further, freezing helps to preserve food by its action on enzymes. Enzymes, as you already know are complex proteins, present in all living tissue, that help organisms ripen and mature. During freezing, enzyme action is slowed but not stopped. If not inactivated, these enzymes can cause color and flavor changes and loss of nutrients during freezer storage. 
 
We are all familiar with chilling and refrigerated storage. At home or at the industry level, refrigerated storage of food is generally practiced. In the unit operation of chilling, the temperature of a food is reduced generally to between -1°C and 7 °C and thus subsequent storage at refrigerated temperature extends the shelf life of both the fresh and processed foods. It is also used as an adjunct process to extend the storage life of mildly processed (e.g. pasteurized, fermented and irradiated) and low-acid foods. In the United States, refrigerated storage of food is mandated by regulations at temperatures at or below 7.2°C.  Such foods are also marketed under refrigeration and labeled as 'needing refrigeration'. Commercially sterilized and processed foods that may become contaminated after opening should also be labeled for refrigerated storage.
 
Chilling and refrigerated storage prevents the growth of bacteria, particularly the thermophiles (organisms that grow at high temperature) and mesophiles (organisms that grow at medium temperature). Psychrophiles (organisms that grow at low temperatures), however, can and do cause food spoilage during low temperatures  storage, but there are some psychrophillic pathogens.