Freezing and Cooling

Freezing and cooling are preservation techniques with minimal impact on food products quality. In these unit operations the temperature is reduced, therefore slowing down deteriorative chemical and enzymatic reactions and microorganisms growth.


Chilling food to the correct temperature stops bacteria from growing and multiplying. Some foods need to be kept chilled to keep them safe - for example food with a 'use by' date, food that have been cooked and won't serve immediately, or other ready-to-eat food such as prepared salads.

raw watercress

frozen watercress



left figure: raw watercress; right figure: frozen watercress;



Water is a facilitator of biochemical deterioration of foods. Freezing removes water from the food matrix by forming ice crystals. Although the ice crystals remain in the food, the remaining water which is in contact with the food matrix becomes concentrated with solutes, and water activity lowers. Freezing is unquestionably the most satisfactory method currently available for long-term preservation of foods. Freezing is also one of the most commonly used processes commercially and domestically for preserving a very wide range of food including prepared food stuffs,  The nutrient content is largely retained and the product resembles the fresh material more closely than thermally processed foods. However, some changes may occur: texture is strongly affected and, in a smaller extension, nutrients and colour degraded. All these alterations depend on the type of food, the freezing rate and temperature.

This text was prepared by Cristina L.M. Silva, Universidade Católica Portuguesa, Escola Superior de Biotecnologia, Porto - Portugal.

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