The refrigeration applications may be grouped into the following categories:
- Domestic refrigeration
- Commercial refrigeration
- Industrial refrigeration
- Marine and transportation refrigeration
- Food refrigeration
- Preservation by refrigeration
- Comfort air-conditioning
- Industrial air-conditioning
Preservation by refrigeration
This is the most and most commonly used method for the preservation of almost all types of foods. The activity of spoilage agents is more rapid in the fresh liquid content of food. The freezing preservation method keeps this liquid content of food into minute ice crystals. By this method, the activity of the microorganisms and enzymes is considerably retarded and stopped. The most outstanding feature of this method is, that it maintains the tastes of the food.
The process of removing heat from the liquid substance/food/other commodities to change its state to a solid state is called freezing.
Low-temperature refrigerated storage for freezing perishables is normally maintained below zero degrees (0 C).
- A sharp freezing or a sharp freezer is a refrigerated space maintained at a low temperature. The heat from the product is absorbed by the still air and transformed into the refrigeration coils by natural convection currents.
- The temperature maintained in sharp freezers ranges from – 17 C to -40 C.
- Since the air circulation is usually by natural convection, the time required for freezing ranges from 3 hours to 3 days.
- Typical items that are sharply frozen are beef and pork, boxed poultry, fish, fruits in barrels, eggs in cans, etc.
- This method is not satisfactory for consumers’ food because of the slow rate of cooling.
- In commercial terms, quick freezing is a refrigeration process in which a product is frozen so rapidly by the forced circulation of cold air over the product.
- The difference between quick freezing and slow freezing is only in the size of ice crystals formed within the food during freezing.
- The solidification is done quickly: it may be from 30 minutes or less.
The advantages of quick freezing over slow freezing are:
- The rapid rate of crystal formation prevents the loss of valuable nutritious fluids.
- No chance for the separation of water in the form of pure ice.
- No diffusion of salt from the food surface as the time required for freezing is very less.
- Decomposition during freezing is not possible as the period of freezing is very less.
- No chance for the growth of yeast, or mold (fungus appearance) during freezing.
Types of quick freezing:
- Air blast freezing
- Immersion freezing
- Contact freezing
- Direct contact freezing
- Indirect contact freezing
- Air blast freezing:
- Air blast freezing utilizes the combined effects of low temperature and high air velocity to produce a high rate of heat transfer from the product.
- Packaged blast freezers are available in both suspended and floor-mounted models.
- Blast freezing is frequently carried out in insulated tunnels, particularly where large quantities of product are to be frozen.
- The temperature commonly used in this ranges from -20 C to 40 C.
- The velocity of air varies from 30 m/min to 150 m/min according to the type of food to be frozen.
- The product may be moved through the tunnel on racks or by a conveyor.
- Some of the products frozen in air blast freezers are fish fillets, steaks, chicken portions, pizza, and a variety of packaged products.
- The refrigeration coils are located overhead and the fan circulates the air over the product at a velocity of 50 to 150 m/min. The product is moved through the tunnel or racks or by a conveyor.
2. Immersion freezing:
- Immersion freezing is accomplished by immersing the product in a low – temperature brine solution (secondary refrigerant liquid).
- The refrigerated liquid must be a good conductor of heat and non-toxic so that transfer is rapid and the product is completely frozen in a very short time.
- The commonly used refrigerated liquids are;
- Sodium chloride
- Brine solutions
- Sugar brine
- Propylene glycol liquid.
- The cold liquid may be pumped over the product or the product may be conveyed through the liquid.
- The direct immersion type of food freezing, where the packed or unpacked commonly can be used for freezing.
- The product most frequently frozen by immersion are fish and shrimp.
- In addition, immersion freezing produces “Glaze” (thin coating of ice) on the surface of the product, which helps to prevent dehydration of unpackaged products during the storage period.
3. Contact freezing:
- The food product is frozen by direct or indirect contact with a metal surface cooled by cold brine or refrigerants.
- The various plate-type contact freezers consisting of metal plates with food products placed on them and a refrigerant (R12, R22) circulating through them, are used for this purpose.
- This type of freezer is particularly useful when products are frozen in small quantities.
- Some of the products frozen in contact freezers are fruits, vegetables, meat, and fish.
- The microstructures of the foods formed by different freezing methods are the air-blast freezing gives smooth structure.