Freezing Technology

2.1.1. Introduction to Icing

Icing is the most prevalent method of preserving fish. Ideal icing involves packing crushed ice and fish alternatively in insulated boxes, in the fish to ice ratio of 1:1 (w/w). By this, the temperature of the fish is lowered to near 1 to 2°C in about 2-3 hrs (the melting of the ice needs 80 calories of heat/g and this heat is removed from the fish in contact with ice and hence, the fish get cooled). This lowering of temperature brings about

1. arrest of almost all enzymatic changes,
2. killing of about 50-60% of the mesophilic bacteria and
3. slowing down of the activities and growth of all other bacteria, which are cold-loving (psychrophilic) and cold-tolerant (psychrotrophic). 

As a combined effect of all these three factors, the spoilage of fish is delayed to a considerable length of time in ice. During iced storage of fish, there is an initial drop of bacterial count due to the death of the cold sensitive mesophiles. The surviving cold tolerant bacteria, however, get adapted to growth in low temperature. Consequently, there is a gradual increase in population, which takes about 6 to 8 days to reach a count of one million per gram or above. By that time, the fish has reached the stage of incipient spoilage.

 Qualitatively, there is a selection of bacterial flora during iced storage of fish. Irrespective of the composition of the initial flora, the Pseudomonas/ Alteromonas group emerges as the predominant group of bacteria at the time of spoilage. This is because most of the psychrotrophic bacteria capable of spoilage belong to these genera.

 In the case of tropical fishes, it is not the psychrophiles, but the psychrotrophs, which are the actual spoilers during iced storage. These psychrotrophs, whose population is very low in the fresh tropical fish, easily adopt to grow at low temperature during iced storage and grow very rapidly and spoil the fish. Further, psychrotrophs have a shorter generation time compared with psychrophiles.