|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
Methods of salting
Salt is applied to fish by the following basic methods:
- the fish are immersed in a solution of salt in water.
- granular salt is rubbed into the surface of the fish.
- granular salt is rubbed into the surface of split fish and the fish are stacked with a sprinkling of salt between each layer of fish. The liquid (pickle) which forms is allowed to drain away.
Pickle salting - fish are covered with salt and then packed in water-tight containers in layers with salt sprinkled between each layer. The pickle which forms covers the fish; if the fish are not completely covered in 3 - 4 hours, saturated brine is normally added to completely immerse them. A cover should be placed on top of the fish to hold them below the surface of the pickle.
With most brine salting techniques, a saturated brine solution is used. The presence of impurities may reduce the actual concentration of sodium chloride in solution and, in practice, the brine strength ranges between 80 and 100 per cent, which corresponds to 270 - 360 grams of salt to each litre of water. When fish are placed in saturated brine, the concentration of the brine begins to fall as soon as salt begins to penetrate the fish and water is removed. Unless plenty of brine is used and the fish are stirred frequently, the rate of salt penetration and water removal may be seriously reduced.
During pickle curing, the fish are surrounded by granular salt which, initially, dissolves in the surface moisture of the fish. Sufficient salt is then available to go into solution and maintain the pickle at saturation point as salt penetrates the fish and water is removed. The water extracted from the fish also contains blood and other compounds that help to reduce the rate at which fat in the fish is oxidised.
Dry or kench salting cannot be recommended for general use in the tropics as the fish are not covered by the brine or pickle and are, therefore, more susceptible to spoilage and insect attack. Exposure to the air and the presence of salt also encourages the rate of fat oxidation which gives rise to discoloration and the characteristic rancid flavours. Fish should be covered with a saturated brine or pickle as rapidly as possible and kept covered until salting is completed.
The various chemical and physical effects of using salt on fish were discussed earlier. Several of these are apparently contradictory and in commercial salted fish production a compromise may have to be reached to resolve the various factors. The rate of salt penetration of the flesh increases as the temperature rises; increasing the temperature also increases the rate of spoilage. If fish are salted at a reduced temperature, e.g., +5°C, although the rate of salt penetration is reduced, the rate of spoilage is more drastically reduced and it may be possible to salt the fish to the centre before any serious spoilage occurs. Similarly, salt penetration is slower in fresh fish than it is in partly spoiled fish but it is impossible to make a good salt fish product from spoiled fish. If fish spoil in the centre before the salt can penetrate, it produces in cod (Gadus sp.) what has been termed 'putty fish', where the centre is very soft and the texture is destroyed. In many fisheries, large fish are split before salting; this increases the surface area and also reduces the depth of flesh that the salt has to penetrate.
Wooden and plastic barrels are suitable for brine or pickle curing fish; the container should be of a size and shape which allows the largest fish normally handled to be laid flat. Cement-lined vats or tanks are suitable for larger quantities of fish and the vats should be able to hold one days' catch with an internal depth of one metre. Wooden lids fitting internally to the tanks which can be weighted down to hold the fish beneath the brine should be provided. Vats and tubs should be situated in the shade to keep the fish as cool as possible.
The quantity of salt used depends upon the type of cure required, the type of fish and the method used. For a strongly cured product, approximately 30 kg of salt per 100 kg of fish is required.
Spoilage of salted fish
Although salt prevents the growth of spoilage bacteria, other micro-organisms are not so affected by the presence of salt. Micro-organisms can be conveniently divided into three groups by their sensitivity to salt:
(i) Low tolerance - growth is stopped, or the organism is killed, by the presence of low concentrations of salt. Most of the normal spoilage organisms fall within this group and a salt content of a few per cent will prevent growth.
(ii) High tolerance - organisms which can tolerate high concentrations of salt although the rate of growth is usually reduced, or stopped, at very high salt concentrations.
(iii) Halophiles - those organisms which cannot grow without salt.
With dry salted fish, the salt-tolerant and halophilic organisms can continue to grow but they cannot do so in pickle-cured products: most of them are aerobic organisms and the fish and brine of pickle-cured fish contains very little, or no, oxygen.
Most enzymic activity is stopped in heavily salted fish but, with lighter cures, the fish may develop characteristic flavours as a result of enzymic activity and the growth of certain salt-tolerant organisms. If the salt levels and fermentations are not carefully controlled, putrefactive spoilage may occur.
1. BURGESS, G. H. O.et al. (Eds) (1965) Fish handling and processing. HM Stationery office, Edinburgh. 390 pp.
2. COLE, R. C. and GREENWOOD-BARTON, L. H. (1965) Problems associated with the development of fisheries in tropical countries l l l: The preservation of the catch by simple processes. Tropical Science 7, 165 - 183.
3. SHEWAN, J. (1951) Common salt: its varieties and their suitability for fish processing. In: World Fisheries Yearbook, 1951. London: British Continental Trade Press Ltd.