11. Low-Temperature Preservation

Freezing has long been used to preserve high-value food products such as meat; fish; particular foods where the quality of the frozen product is significantly better than the alternative, such as peas; and increasingly for other convenience foods ranging from chipped potatoes to complete ready meals. Lowering the temperature of foodstuffs reduces microbiological and biochemical spoilage by decreasing microbial growth rates and by removing liquid water which then becomes unavailable to support microbial growth. Freezing refers to the storage of food at temperatures between −18 and −30°C. In general lower storage temperatures give a longer shelf life. For example soft fruits may be stored for between 3 and 6 months at −12°C but for 24 months and beyond at −24°C. Most meat has a shelf life of 6–9 months at −12°C and this increases to between 15 and 24 months at temperatures down to −24°C. In contrast, chilling is defined by a storage temperature range between −1 and 8°C and is used for meat, fish, dairy products and chilled recipe dishes prior to consumption. It has little or no effect on the nutritional content or organoleptic properties of food. Similarly, the freezing process itself has little or no effect on the nutritional value of frozen foods. Conversely the quality of the initial raw material cannot be improved by freezing and only high-quality raw materials should be selected for freezing. Thus the quality and nutrient content of any food at the point of consumption are dependent upon the quality of the original raw material, the length of storage and the storage conditions and the extent and nature of the freezing process.