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2. Nature and properties of ice
In order to understand why ice is so useful for chilling fish, it is first necessary to consider the nature and properties of ice, as well as to understand the simple principles and technical terms detailed in chapters 11 and 12.
When water freezes at a temperature of 0°C it experiences a phase change from a liquid to a solid, familiar to all as ice. A quantity of heat has to be removed from the water to turn it into ice, and the same amount of heat has to be added to melt it again. The temperature of a mixture of water and ice will not rise above 0°C until all the ice has melted (Fig 3). A given amount of ice always requires the same amount of heat to melt it; 1 kg of ice needs 80 kcal to change it into water, thus the latent heat of fusion of ice is 80 kcal/kg (Fig 4). This amount of heat is always the same for ice made from pure water, and is very little different for ice made from fresh water from almost any commercial source. Ice needs a large amount of heat to melt - it has a large reserve of "cold" - and this is one reason why it is widely used in the fishing industry as a means of chilling fish.
Fig. 4 The quantity of heat needed to melt ice
Ice is ice is ice!
There is often argument about whether ice made at one port is better than another, whether natural ice is better than artificial ice, freshwater ice is better than seawater ice, or new ice is better than old ice. In addition there are arguments about the merits of the type of ice, crushed, flake, tube, slush and so on.
The differences between freshwater ices of different origin are so small, that they are of no significance to those using ice for chilling fish. Ice made from tap water has the same cooling power as ice made from distilled water, and ice three months old is as effective as newly-made ice.
There is, however, one very important thing to remember; if some of the ice has turned to water, much of its value will have been lost, and a slushy mixture of ice and water should never be compared with an equal weight of ice alone. Remember too, that comparisons between different types of ice should be made between equal weight; what may appear to be the same amount of ice may be an equal volume, but 1 m³ of flake ice, for instance, has far less cooling capacity than say 1 m³ of crushed block ice (Fig. 5).
Within limits, when ice is intimately mixed with fish, the size of particle in any one type of ice makes little difference to either the rate at which it melts or the speed at which it chills. If block ice is used in the form of larger lumps, cooling times are likely to be longer due to the poor contact between fish and ice. Ice which has different shaped pieces due to the method of manufacture can also have somewhat different characteristics. Differences in the properties of crushed block ice are discussed later.
Ice made from hard water has the same cooling properties as ice made from soft water, although particles of ice made from hard water sometimes tend to stick together more during melting than pieces made from soft water.
Fig. 5 Equal weights of ice give equal cooling
Seawater ice
The effectiveness of seawater ice, in comparison with freshwater ice, is a little more in dispute. Depending on the method of manufacture, seawater ice may be less homogeneous than freshwater ice when newly made. Brine will also leach out of seawater ice during storage, so that the ice does not have a precisely fixed melting point. For this reason, fish kept in seawater ice may sometimes be at too low a temperature and become partially frozen, or the fish may possibly take up some salt from the ice.
However, where the choice is between not enough ice and plenty of seawater ice, then there is little doubt that seawater ice can, and should, be used for chilling fish that would otherwise spoil more quickly. Manufacture of seawater ice is of particular advantage on board ship for augmenting port supplies on a long voyage, or in coastal communities where fresh water is so scarce and expensive that to make ice from it would be prohibitive. It is important to remember, however, that seawater for making ice must be uncontaminated; all too often the quality of coastal or harbour water makes it unsafe for use with food.
To sum up, equal weights of ice, not equal volumes, have equal cooling capacity, no matter what the source. No single type of ice is significantly better than another when it comes to the amount required to chill a box of fish.
