Freezing is a method of preservation. During freezing, the temperature of food is lowered to below the freezing point and a part of the water undergoes a change in state to form ice crystals. The freezing speed is very important for the type of ice crystals that are formed. Slow freezing gives large ice crystals. These remain stuck to structures and cells and destroy the structure of the product. Fast freezing gives many small ice crystals that leave the product intact.
Freezing gives a product a longer shelf life. Microorganisms can not grow at these low temperatures. Enzymatic spoilage can still occur. To inactivate the enzymes, vegetables are blanched (a short heat treatment).
Industry standards for freezing food require a temperature below -18 ° C. Normally, a product is frozen to -20 to -24 ° C.
Field of Application
Various types of food products can be frozen, eg fruit, vegetables, fish, meat, fried food and prepared food products (ice cream, pizzas, etc).
Techniques, methods and installation
During the freezing process, the observable heat is first removed to lower the temperature of the product to the freezing point; this also includes heat produced by respiration (as in fresh food products). The latent heat of crystallization is then removed and ice crystals are formed. There is a wide range of methods and installations for freezing products. The most famous are:
a) Blast freezers, batch and continuously
Cold air from -30 to -40 ° C is circulated over the food at a rate of 1.5 to 6 m / s. The air is cooled by the refrigerants ammonia or freon. Cooling to the air temperature is possible, but this takes a lot of time. In batch blast freezers, the food is stacked in bins or cabinets in bins. In continuous installations, food containers are placed on trolleys or the food is transported through a freezing tunnel on a conveyor belt. Sometimes multi-phase tunnels are used with a number of tires. The product falls from one tire to another; this also breaks up clogs of frozen food. The thickness of the food layer on the belt can vary from 25 to 125 mm. In blast freezers, large amounts of air are recycled which can cause freeze burn and oxidative changes in unwrapped food. Moisture from the food is transported through the air to the cooling coils, which necessitates regular freezing. Depending on the size of the food product and the packaging, freezing takes a number of hours.
b) Belt freezer (spiral freezers), continuous
The principle of the belt freezer is the same as that of a blast freezer: the product is frozen by cold air. Continuous flexible mesh is used in a linear line or becomes spiral-shaped rows. Packaged food is brought up by a chilled room on the belt. A disadvantage of the spiral shape is that the output of the product is at a higher point than the input. This problem can be solved with a double spiral. In the first spiral the product is raised, while in the second spiral the product is brought down again. Parallel air flow is used or cold air or liquid nitrogen can be brought down through the tire stack (counterflow). This reduces evaporation of the water from the food.
c) Fluidized bed freezers (IQF)
The food is fluidized with air from -25 to -40 ° C by raising the air vertically through a perforated plate or band and through a bed of 2-20 cm of free food. The shape and size of the food parts determines the thickness of the fluidized bed and the air velocity for fluidization. In this system, food is brought into contact with the air more extensively than with blast freezers, so that all surfaces freeze simultaneously and evenly. This has a faster freezing and less dewatering to result, which also leads to less defrosting. Fluidised bed freezing is limited to small fine foods (peas, sweet corn kernels, shrimps, strawberries, etc.).
d) Refrigerated plate freezers
Vertical or horizontal plates are cooled with a coolant of about 40 ° C. The food is applied to the surface in thin layers. Sometimes the plates are lightly compressed. This improves the contact between the food and the freezer plates. An 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 plate freezer. This consists of a freezing cylinder that contains rotating knives that remove the frozen material from the freezing surface. This type of installation is used, for example, in ice production.
e) Submerger freezer, batch and continuous
Packaged food is placed on a flooded mesh belt through a bath with refrigerated glycol, brine, glycerol or calcium chloride solution. In this method, the food is in indirect contact with the refrigerant. High freezing speeds are achieved with this method, the intensive contact heat is easily moved. The product can be frozen in a few minutes. For example, it is used for freezing concentrated orange juice in packs and for pre-freezing poultry wrapped in foil before it goes to the blast freezer.
f) Cryogenic freezers, batch and continuous
In cryogenic freezing, the food is brought into direct contact with the refrigerant, which may be solid or liquid carbon dioxide (-80 ° C) or liquid nitrogen (-200 ° C). The refrigerant evaporates or sublimates, removing the heat from the food and causing rapid freezing. The product has a temperature of -40 ° C or lower within minutes. Cryogenic freezing is also called shock freezing. Liquid nitrogen and carbon dioxide refrigerants are colorless, odorless and inert. Cryogenic freezing is an expensive freezing method. The production costs are twice as high as when cooling with cold air, but the investment costs are low and the freezing speed is very high.