3. Chilled fish

The chilling of fish is a process by which the temperature of fish is reduced close to (but not below) freezing point. It delays the biochemical and bacteriological processes in fish and consequently prolongs the storage life of fish products.

Fresh fish or fish products that have been exposed to cold until they are near freezing point are called chilled. The biochemical and bacteriological processes in them are merely delayed, and not stopped; their storage life is therefore prolonged and will last so long as spoilage has not become sufficiently far advanced to impair their value as food. The storage life of a chilled product depends mainly on the quality of the raw fish, on the method and duration of chilling, and the conditions of storage.

The freezing point for fish is between –0.6°C and -2°C, depending on the species and on the concentration of intercellular fluid. It is usually taken as –1°C. During chilling the heat transfer between the food product and the coolant is often is accompanied by weight transfer (e.g. through evaporation from the surface with air chilling); chilling must therefore be regarded as a complex process of heat and mass transfer.

The important considerations in preparing good quality chilled fish products are the biochemical and physical changes in the fish during chilling time, the temperature conditions, the chilling time, and the equipment and technology used.

Before chilling the fish are graded for species and size, their quality must conform to the technical requirements and is determined both by organoleptic signs and by physical and chemical information derived from laboratory analysis of specimens or by other means. Fish to be processed into frozen fillets or to be delivered in ice to the consumer may be live or dead, but must be absolutely fresh.

Changes occurring during chilling. During chilling important physical and biochemical changes take place in the product. The physical changes generally entail an increase in the density of tissues and in the viscosity of tissue juices and blood, and a loss in the weight due to partial evaporation of moisture from the surface desiccation during air chilling. The extent of desiccation depends on the properties of the product and coolant and on the conditions of chilling. The main properties of the product on which desiccation depends are its specific gravity, the size of the individual fish, and the type of packaging during chilling and storage.

The more water there is in the product, the more moisture will evaporate from the fish; lean fish consequently is more subject to dehydration than fatty, which contains relatively less water. The subcutaneous layer of fat offers protection against evaporation from the tissues. Products with a higher specific gravity contain more organic matter and therefore relatively less water. The evaporation surface depends on the size of the individual fish. Depending on its properties the packaging may offer complete protection against evaporation, or may impede and delay the process; but if it is hydroscopic, it will absorb moisture from the fish.

Chilling time. The chilling time of fish and other marine products depends on their properties of the coolant, and the conditions under which the process takes place (cross-section of the product, thermal efficiency, specific gravity, temperature of the coolant, type of motion and velocity of the coolant, humidity, heat transfer coefficient from product to coolant).

Fish chilling practice. The methods employed for chilling fish and other fishery products are very varied, but may be divided into two groups according to the type of coolant: a) methods of chilling in a homogeneous coolant (as when fish are chilled in cold liquid, for example), and b) methods of chilling in heterogeneous coolant (for instance in ice).

Fish are very seldom air-cooled at a temperature of -2° to – 3°C, because they chill slowly in air and deteriorate in appearance both while being chilled and during subsequent storage. The most used commercial methods of chilling are by immersion in a cold liquid, by spraying with cold brine, or by packing in crushed ice. Of these three methods the least used is brine-spraying, and the most used is chilling in crushed ice. The method of chilling in liquid is to immerse the raw fish in the cold liquid (fresh water, brine or sea water). fresh water, which freezes at 0°C, can only be used if the fish are to be lightly chilled. To chill fish thoroughly down to 0°C or –1°C, it is necessary to use either brine (for example a 2 per cent solution of sodium chloride) or sea water, both of which have a freezing point below 0°C, and can be kept at a temperature within – 3° and - 4°C throughout the chilling process. It goes without saying that, since the fish are in direct contact with the liquid during chilling, the solution must be perfectly harmless to man.