Freezing

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.).

Cooled surface freezers

Vertical or horizontal hollow plates are cooled with a refrigerant of about -40ºC. The food is put on the surface in thin layers. Sometimes the plates are pressed slightly together. This improves the contact between the food and the freezing plates. One advantage of such freezers is that little dehydration of the food takes place, which reduces the frequency of defrosting. A special form of a plate freezer is the scraped surface freezer. This consists of a freezing cylinder containing rotating knives which remove the frozen material from the freezing surface. This type of equipment is used, for example, in ice-cream production.

Immersion freezers, batch & continues

Packaged food is passed through a bath of refrigerated glycol, brine, glycerol or calcium chloride solution on a submerged mesh conveyor. By this method the food has an indirect contact with the refrigerant. High freezing rates are obtained with this method, with the intensive contact heat is transferred easily. The product can be frozen in a few minutes. It is used, for example, for freezing concentrated orange juice in cans and for prefreezing film-wrapped poultry prior to blast freezing.

Cryogenic freezers, batch & continues

In cryogenic freezing, the food is in direct contact with the refrigerant, which can be solid or liquid carbon dioxide (-80ºC) or liquid nitrogen (-200ºC). The refrigerant evaporates or sublimates away removing the heat from the food and causing rapid freezing. The product has a temperature of -40ºC or lower in a few minutes. Cryogenic freezing is also called shock freezing. Liquid nitrogen and carbon dioxide refrigerants are colourless, odourless and inert. Cryogenic freezing is an expensive freezing method. The production costs are twice as high as cooling with cold air, but the investment costs are low and the freezing rate is very high.

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