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Freezing
Inhalt
Objective
Freezing is a method for preservation. In freezing, the temperature of a food is reduced below the freezing point and a proportion of the water undergoes a change in state to form ice crystals. The freezing rate is very important for the type of ice crystals formed. Slow freezing gives big ice crystals, they stick to structures and cells and destroy the structure of the product. Rapid freezing gives many small ice crystals, they leave the structure of the product intact.Freezing gives a product a longer shelf life. Microorganism can not grow at these low temperatures. Enzymatic spoilage can still occur. To inactivate the enzymes vegetables are blanched, they are given a short heat treatment.
Industry standards for the freezing of food is to reduce the temperature to below -18ºC. Normally a product is frozen to -20 − -24ºC.
Field of application
Several types of food can be frozen, e.g. fruits, vegetables, fish, meat, baked goods and prepared foods (ice-cream, pizzas, etc.).Techniques, methods and equipment
During the freezing process, the sensible heat is first removed to lower the temperature of the food to the freezing point; this includes heat produced by respiration (as in fresh foods). The latent heat of crystallization is then removed and ice crystals are formed. A whole range of methods and equipment for freezing foods is available. Most common are:Blast freezers, batch and discontinues
Cold air at -30 to -40ºC is circulated over the food at a velocity of 1,5 to 6 m/s. The air is cooled by the refrigerants ammonia or freon. Cooling to the air temperatures is possible, but this takes a lot of time. In batch blast freezers, the food is stacked on trays in rooms or cabinets. In continues equipment, trays with food are stacked on trolleys or the food is moved through a freezing tunnel by conveyor belts. Sometimes multiphase tunnels are applied with a number of belts. The product falls from one belt onto another, this also breaks up clumps of frozen food. The thickness of the food layer on the belts can vary from 25 to 125 mm. In blast freezers, large volumes of air are recycled, this can cause freezer burn and oxidative changes to unpackaged food. Moisture from the food is transferred, via the air, to the refrigeration coils, which makes frequent defrosting necessary. Depending on the size of the food and package freezing takes several hours.Belt (spiral) freezers, continues
The principle of belt freezer operation is the same as that of a blast freezer, the product is freezed by cold air. A continues flexible mesh belt is used in a linear line or formed into spiral tiers. Packed food is brought up through a refrigerated chamber on the belt. Disadvantage of the spiral form is that the output of the product is at a higher point than the input. With a double spiral this problem can be dissolved. In the first spiral the product is carried up, in the second spiral the product is carried down again.Co-current airflow is used, or after active cold air or liquid nitrogen may be directed down through the belt stack (countercurrent flow). This reduces the evaporation of water from the food.Fluidised-bed freezers (IQF) The food is fluidised with air of -25 to -40ºC by passing the air vertically upwards through a perforated tray or belt and through a bed of free food 2-20 cm thick. The shape and size of the food pieces determine the thickness of the fluidised bed and the air velocity for fluidisation. In this system food comes in more extensive contact with the air than in blast freezers, so that all surfaces are frozen simultaneously and uniformly. This results in a faster freezing and less dehydration, which also results in less frequent defrosting. Fluidised-bed freezing is restricted to small particulate foods (peas, sweet corn kernels, shrimps, strawberries, etc.).