Freezing is the removal of heat from the packaged or whole foods resulting in the temperatures between slightly below the freezing point of food to -18°C. Frozen foods last many months without spoiling however, some quality loss may occur.
Some microorganisms grow even at sub-freezing temperatures as long as water is available. Conversion of water to ice increases the concentration of dissolved solutes in unfrozen water and leads to low water activity. Freezing prevents the growth of microorganisms due to reduced water activity. The concerted effect of low temperatures, reduced water activity, and pre-treatment of blanching prior to freezing of products yield longer shelf life.
Different types of freezing systems are available for foods. No single freezing system can satisfy all freezing needs, because of the wide variety of food products and process characteristics. The selection criteria of a freezing system will depend on the type of the product, reliable and economic operation, easy cleanability, hygienic design and desired product quality.
Although all commercial freezing processes are operated at atmospheric conditions, there are potential applications of high-pressure assisted freezing and thawing of foods. The pressure–induced freezing point and melting point depression enables the sample to be super cooled to low temperature (e.g. -22°C at 207.5 M pa) resulting in rapid and uniform nucleation and growth of ice crystals on release of pressure.
Freezing systems based on time taken for freezing
Freezing systems based on time required to freeze foods can be classified into two types i.e. slow and quick freezing. Rate of freezing affects the quality of frozen food.
Slow freezing occurs when food is directly placed in freezing rooms called sharp freezers. It is also known as sharp freezing. This method involves freezing by circulation of air by convection i.e. through a specially insulated tunnel, either naturally or with aid of fans. The relatively still air is a poor conductor of heat and that is the reason for long time required to freeze the food. The temperature ranges from -15 to -29°C and freezing may take from 3 to 72 hours. The ice crystals formed are large and found in between cells i.e. extra-cellular spaces because of which the structure of food is disrupted. The structure of food is not maintained and thawed food cannot regain its original water content. Large ice crystals create quality problems like mushiness in vegetables etc.
Vigorous circulation of cold air enables freezing to proceed at a moderately rapid rate. In this process, the temperature is kept between -32°C to -40°C and the food attains the stage of maximum ice crystal formation in 30 minutes or less. Small ice crystals are formed within the cells and therefore, it does not damage the structure of food. On thawing, the structure of original food is maintained.
Slow vs. quick freezing
The difference between sharp and quick freezing has been given in Table 1 and .
Table 1. Difference between sharp and quick freezing
| Sharp freezing
- Rates of cooling of less than 1°C/min. Ice crystals form in extra-cellular locations
- Large ice crystals formation
- Maximum dislocation of water
- Shrinkage (shrunk appearance of cells in frozen state)
- Less than maximum attainable food quality
- Produces both extra-cellular and intracellular (mostly) locations of ice crystals
- Small but numerous ice crystals
- Minimum dislocation of ice crystals
- Frozen appearance similar to the unfrozen state
- Food quality usually superior to that attained by slow freezing
Freezing Systems Based On Mode of Operation