Preservation is the treatment of food to prevent or delay spoilage and inhibit growth of pathogenic organisms which would render the food unfit. Preservation may involve:
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Food preservation by the use of low temperatures
This form of preservation is primarily to prevent spoilage by micro-organisms, the enzyme-producing activity of which is slowed down or arrested by low temperatures. Temperatures used may be:
Temperatures above freezing
Refrigerators, operating at between 1°C and 4°C, are suitable for the short-term storage of most perishable foods. Most common pathogenic organisms stop growing below 5°C. Some can continue to grow down to about -2°C, although the growth rate is slow. Certain spoilage bacteria and moulds can also cause spoilage at refrigeration temperatures.
Temperatures below freezing
Freezing reduces the moisture available for bacteria growth and significantly reduce enzyme activity. It also destroys some bacteria, including pathogens, and a gradual reduction occurs during spoilage. Some parasites can also be destroyed by freezing. However, bacterial spores and toxins are generally unaffected.
Moulds and yeasts are more likely to grow on frozen food than bacteria as they are better able to withstand the reduced water availability and the low temperatures. In practice very few organisms grow below -10°C. On thawing, however, surviving bacteria can grow rapidly, compensating for those destroyed, especially if food reaches temperatures of 20°C or higher.
Before vegetables are frozen they must be blanched by dipping in hot water for a short period, approximately one minute. Blanching destroys enzymes which produce off-flavours and odours, and reduces the bacteria load. It also fixes colour, removes trapped air and softens some vegetables, which helps packing. Over blanching will result in excessive loss of vitamin C. During the freezing process, ice crystals are formed; the slower the rate of freezing, the rate if freezing, the larger the ice crystals, which can damage certain foods. To avoid this, quickly freezing techniques are used.
Most foods will keep for prolonged periods in a freezer , although a recommended shelf life is given because of loss of texture, flavour, tenderness, colour and overall nutritional quality. Foods must be properly wrapped to avoid loss of moisture from the surface, i.e. freezer burn. The oxidation of food is slower at -18°C and this also assists in preservation. However, vacuum packing is essential to extend the shelf life of frozen food susceptible to oxidative rancidity, for example, bacon.
There are several commercial systems used to freeze food. Air-blast freezing is the commonest. It uses static tunnels where trolleys of boxed product, such as beef and cakes, are passed through. Solid continuous-belt freezers are used for fish fillets, patties and pizzas. Air circulates around the food at temperatures of -30°C to -40°C. Plate freezing is used for packed in flat cartons, for examples fish blocks and ready-meals. For products like peas, a process called fluidised-bed freezing can be used. The food is moved along a tunnel on a perforated tray, borne along by a cushion of freezing air forced up from below. Each item is individually quick frozen. In cryogenic freezing, food is sprayed with, or dipped into, a refrigerant such as liquid nitrogen. This is very quick, although more expensive than conventional.