They are constructed for batch, continuous and batch/continuous modes of operation.
Batch freezing This type of freezing is mostly used for small operations. If a variety of products are to be frozen, a batch freezer may be selected over continuous as they are more versatile. Such freezing systems are also likely to be used for products with longer freezing times since with a batch freezer there is better utilization of floor space due to the multi-layer arrangement of loading. It is difficult to choose on an exact line of demarcation but generally freezing times longer than one hour would usually require a batch mode of operation.
Continuous freezing This process of freezing is used in large-scale production lines. Continuous freezers are best used for freezing individual portions, such as small pieces of vegetables. The main advantage in using a continuous freezer for these smaller and/or thinner products is that since they freeze quickly they will also thaw quickly and the delays that occur with a batch-freezing operation may be overcome. Continuous freezing allows quick handling after freezing and a quick transfer to the cold store.
Batch/ continuous freezing These kind of freezers are usually batch type freezers operated with trolleys which are loaded in sequence at fixed-time intervals rather than all at one time as in the truly batch freezer.
Freezing systems Based on contact with the product Based on the contact between food and the freezing medium, the freezing systems can be categorized in two categories viz. direct and indirect contact freezing .
Direct contact freezing In case of direct contact freezers, the product to be frozen is completely surrounded by the freezing medium i.e. the refrigerant, which maximizes the efficiency of heat transfer.
Indirect contact freezing In this type of freezer, food materials to be frozen are separated from the refrigerant by a conducting material, usually a steel plate/ belt. The food product is indirectly exposed to the freezing medium.
Freezing systems based on heat transfer systems Different types of freezing methods based on the heat transferring systems are depicted in the
Air In general, air is used as the freezing medium in the freezing, either as still air or forced air . Packaged or unpackaged solid foods can be frozen in air at temperatures ranging from -18 to -40°C. Different methods are available for freezing by contact with cooled gas.
Cabinet freezing This method involves circulation of cold air in a cabinet where the product is placed in a tray. The moisture pick-up from the product may deposit on the cooling coils as frost, which acts as an insulation.
Air blast freezing This method employs the use of high velocity cold air to reduce the temperature of the food. Air velocities between 2.5 and 5 meter per second give the most economical freezing. Lower air velocities result in slow product freezing and higher velocities increase unit freezing costs considerable. This method can be further divided into fluidized bed freezing, belt freezing, and tunnel freezing, depending on how the air interacts with the product.
Belt freezing The first mechanized air-blast freezers consisted of a wire mesh belt conveyor in a blast room for continuous product flow. Uniform product distribution over the entire belt is required to achieve uniform product contact and effective freezing. Controlled vertical airflow forces cold air up through the product layer, thereby creating good contact with the product particles and increasing the efficiency. The principal current design is the two–stage belt freezer. Temperatures used are usually -10 to 4°C in the pre-cool section and -32 to -40° C in the freezer section.
Tunnel freezing In this process, the products are placed in trays or racks in a long tunnel and cool air is circulated over the product.
Fluidized bed freezing The fluidized bed freezer consists of a perforated bed through which refrigerated air is blown vertically upwards. The air velocity must be greater than the fluidization velocity. A schematic diagram of fluidized-bed freezer is shown in This freezing method is suitable for small particulate food bodies of fairly uniform size, e.g. peas, diced carrots, corns and berry fruits. The high degree of fluidization improves the heat transfer rate and results in good use of floor space. The use of fluidization is advantageous compared with other methods of freezing since the product is individually quick frozen (IQF). The idea of individually quick frozen foods (IQF) started with the first technological developments aimed at quick freezing for the food particles with a tendency to stick together. Small vegetables, prawns, shrimp, french-fried potatoes, diced meat, and fruits are some of the products now frozen with this technology.
Plate freezing In this method, the product is sandwiched between metal plates and pressure is usually applied for good contact. Most familiar types of such freezers include vertical plate freezers and horizontal plate freezers based on loading of food in freezer. Plate freezers are only suitable for regular-shaped food materials such as blocks, packages and cartons. Batch freezing is the most common for plate freezers, however some are available for a continuous operation. When the product has been frozen, hot liquid is circulated to break the ice seal and defrost. Spacers are used between the plates during freezing to prevent crushing or bulging of the package. Temperatures are between -17°C to -45°C.
Immersion freezing In this method, the food is immersed in low-temperature brine to achieve fast temperature reduction through direct heat exchange. The refrigerating fluids usually used are salt solutions (sodium chloride), sugar solutions, glycol and glycerol solutions, and alcohol solutions. These should have low boiling points. Immersion freezing is advantageous as compared with air blast freezing in terms of reduced weight loss by evaporation. Refrigerant must be non-toxic, pure and should not contribute to taste or flavour of food. Covering food with flexible membranes, which allow rapid heat transfer, may be done to prevent contact between food and liquid refrigerants. Immersion freezing with the use of liquids has now only a limited use. Nowadays, liquefied gases have replaced the salt/ sugar solutions and are used mainly for continuous freezing processes.
Cryogenic freezing Cryogenic freezing, is one of the fast, flexible and cheap method of freezing. In this, the liquefied gases are placed in direct contact with the foods. The three liquefied gases most commonly used as refrigerants are nitrogen, carbon dioxide and liquid freon freezant (CCl2F2). Unlike nitrogen and carbon dioxide, most of the refrigerant is recondensed and used again, resulting in reduction of cost. The product can be exposed to a cryogenic medium in three ways:
The cryogenic liquid is directly sprayed on the product in a tunnel freezer.
The cryogenic liquid is vaporized and blown over the food in a spiral freezer or batch freezer.
The product is immersed in the cryogenic liquid in an immersion freezer.
Food is exposed to an atmosphere below -60°C through direct contact with liquid nitrogen or liquid carbon dioxide or their vapour. This is a very fast method of freezing, thus adequate control is necessary for achieving quality products. It also provides flexibility by being compatible with various types of food products and having a low capital cost. The rapid formation of small ice crystals greatly reduces the damage caused by cell rupture, preserving colour, texture, flavour and nutritional value. The rapid freezing also reduces the evaporative weight loss from the products, provides high product throughout and has low floor space requirements. The advantages of liquid nitrogen are that it is colourless, odourless and is chemically inert and boils at -195.8°C. It is usually used for high-value products due to the high capital cost for gas compression.
Dehydro-freezing Dehydro-freezing is a process involving dehydration followed by quick freezing. First of all, food product is removed of its 50-60 per cent of the water by evaporation before freezing. The texture and flavour of food products frozen using such process are claimed to be superior to those resulting from freezing alone, and rehydration is more rapid than with dehydrated products.