Freezing Technology

7.1.1.2 Refrigerants and Coolants

Ice should be properly mixed with the product and applied in sufficient quantities to last for the duration of the shipment. When fish are in close contact with each other or with a smooth impervious surface such as the inside of a box, anaerobic bacteria producing offensive odors may develop. The air temperature in the container should be 2-3^ºC to allow the ice to melt slowly. Melt-water keeps the fish well chilled and moist. The melt-water also helps to wash away the products of spoilage. The melt-water should be drained away from the produce.

Alternative refrigerants to wet ice are dry ice and gel ice. Dry ice is solid carbon dioxide. Evaporation of the dry ice gives the cooling effect. Because of its low temperature (-78.9^ºC), dry ice should not be in direct contact with chilled fish, to avoid cold burns. 

Gel ice is made by freezing a water based gel. The advantage of gel ice is that all the water is bound with no danger of water leakage during thawing. It is often claimed that gel packs release the ‘cold’ more slowly and equally than other coolants so that they are superior in maintaining chilled temperatures.

Chilling air cargo     

           With air cargo often preference is given to dry ice or gel packs, as these do not cause leakage. However the use of dry ice is subject to restrictions because it expands from a solid form into a gas and it may ‘drive-out’ oxygen. This obviously poses a hazard to the safety of airplanes. The international identification number for dry ice is UN 1845.