Freezing Technology

2.2.1.1. Heat Requirements

            The heat required to change from a solid to a liquid is known as the latent heat; 1 kg of ice requires 80 kilocalories (kcal) of heat to melt it. This figure of 80 kcal/kg is known as the latent heat of fusion. It is this property of requiring a large amount of heat to melt ice that makes it such a good cooling agent.  One kilocaloryis the amount of heat required to raise the temperature of 1kg water by 1^oC.         

            More heat is required to warm water than almost any other substance. This capacity of substances to hold heat, when compared to water, is known as the specific heat. Specific heat of water is 1, for other substances it is less than 1.

Ice	About 0.5
Wet fish	About 0.96 (usually taken as 1)
Frozen fish	About 0.4
Air	About 0.25
Most metals	About 0.1

Specific heat can be used to discover how much heat has to be removed to cool a substance, i.e.

             Heat to be removed = weight of substance x temperature change x the specific heat,

            To cool 60 kg ice from – 5 to 10^oC requires the removal of:

            60 x [ - 5 – (-10)]^oC x 0.5 (sp. Heat of ice) = 150 kcal

 

We can now calculate how much ice is needed theoretically to cool a given weight of fish:

             If we want to cool 10 kg fish from 25 to 0^oC, we would need to remove             

10 x 25 x 1 = 250 kcal

But when ice melts it adsorbs 80kcal/kg.

                                                        250

Thus the weight of ice required = -------- = 3.12 kg.

          80

            This is strictly a theoretical calculation and does not take the following factors consideration

1. Ice is also melted by the surrounding air; thus a lot of ice is lost, particularly at high ambient temperatures, unless the fish and ice are protected from the ambient heat, preferably with insulating materials.
2. How the fish are packed in ice.
3. The length of time that the fish needs to be kept chilled once cooled.
4. How quickly the fish are chilled.