Freezing basically consists in removing heat from a substance until it attains sub zero temperatures and assumes a hard consistency by the solidification of its water content, both free and combined and hardening of the organic tissues.

1. Quick Freezing

It is the process of rapidly freezing products.  In fast freezing, small ice crystals form and good quality is maintained.  When meat is frozen slowly, large ice crystals form that break the fibers and cells.  Upon defrosting, these meats lose more juice and the texture is not good.  Although the meat loses quality, it is safe to eat.  Temperature just below 00 C is the critical zone for spoilage by protein denaturation.  Quick freezing recommended that all the fish should be reduced from a temperature of 00 C to -50 C in 2h or less. 

2. Factors which affect freezing time:

The following 10 factors affect freezing time of products.

  1. Freezer type
  2. Freezer operating temperature
  3. Refrigeration system and operating condition
  4. Air speed in an air blast freezer
  5. Product temperature
  6. Product thickness
  7. Product shape
  8. Product contact area and density
  9. Product packing
  10. Species of fish

Types of Freezers:

Air Blast Freezers : Blowing a continuous stream of cold air over the fish
Plate Freezers : Direct contact between the fish and a refrigerated surface
Immersion or Spray Freezers: Immersion in or spraying with a refrigerated liquid

a. Air Blast Freezers

 In an air blast freezer, fish is frozen in a stream of high velocity cold air either in a batch or continuously, typically in a duct or tunnel in which a stream of cold air is guided over the product on shelves (batch) or on a conveyor (continuous air blast freezer); also called blast freezer, freezing tunnel, tunnel freezer.  The advantage of the blast freezer is its versatility.  It can cope with  a variety of irregularly shaped products and whenever there is a wide range of shapes and sizes can be frozen. 

Continuous air blast freezers:  In this type of air blast freezer, the fish are conveyed through the freezer (trolleys or they may be loaded on a continuously moving belt or conveyor) usually entering at one and leaving at the other.

Batch air blast freezers  Batch air blast freezers use pallets, trolleys or shelf arrangements for loading the product.  The freezer is fully loaded, and when freezing is complete, the freezer is emptied and reloaded for a further batch freeze.

b. Plate freezers

 In a contact freezer or plate freezer the fish is frozen by direct contact with a refrigerated surface, typically between two hollow metal plates cooled by a refrigerant, such that the distance between the plates can be varied up to 100 mm or more.  One type in which fish, especially in flat packs such as laminated blocks, is frozen between two or more hollow, horizontal, parallel plates through which refrigerant passes is known as a horizontal plate freezer.  In a vertical plate freezer, the refrigerated, parallel plates are vertical and it is used mainly at sea or onshore for freezing large 25 or 50 kg blocks of whole, gutted, or headed and gutted fish.

1.  Horizontal plate freezers

The two main uses for this type of freezer are the freezing of pre packed cartons of fish and fish products for retain sale and the formation of homogeneousrectangular blocks.  An HPE will only operate correctly if good contact is made on both the top and bottoms surfaces of the pack or tray to be frozen. 

2.  Vertical plate freezers

The main advantage of this type of freezer is that fish can be frozen in bulk without the requirement to package or arrange on trays.  The plates form what is in effect in bin with an open top and fish are loaded directly into this space.  This type of freezer is therefore particularly suitable for bulk freezing and it has also been extensively used for freezing whole fish at sea in factory vessels.  In most cases, fish can be loaded between the plates without wrappers and water need not be added either to strengthen the frozen block or improve the contact with the plates.

3. Immersion freezer or spray freezer

It is mostly used for IQF products of small size frozen within minutes.  The fish product to be frozen is in direct contact with the refrigerated liquid (freezant).  One type is brine freezing by immersing fish such as tuna in a tank of re-circulating cold sodium chloride brine at -150 C in a brine immersion freezer that may take about 3 days to freeze a large tuna completely.