|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
Salting of fish is a traditional processing method in most countries of the world. Very often salting is used in combination with drying and smoking; the following lectures on salting, drying and smoking outline the basic principles and discuss the practical application of the various methods that are in common use.
The presence of sufficient quantities of common salt (sodium chloride) in fish can prevent, or drastically reduce, bacterial action. When fish are placed in a strong solution of salt (brine) which is stronger than the solution of salt in the fish tissue, water will pass from the tissue into the brine until the strength of the two solutions is equal. At the same time, salt will penetrate into the tissue. This phenomenon is known as osmosis. A concentration of between 6 and 10 per cent salt in the tissue will prevent the activity of most spoilage bacteria; the removal of some water from the tissue during the salting process will also reduce the activity of the spoilage bacteria.
If fish are salted before drying, less water needs to be removed to achieve preservation. A water content of 35 - 45 per cent, depending on the amount of salt present, will often prevent, or drastically reduce, the action of bacteria.
Salt: sources, composition and properties
Pure common salt is sodium chloride (NaCl) but almost all commercial salts contain varying levels of impurities depending on the source and method of production.
Commercial salt can be classified into three main groups depending on the source and the method of manufacture:
(i) Solar salt - prepared by the evaporation of sea or salt lake waters by the action of sun and wind. Major centres of production tend, therefore, to be found in tropical or sub-tropical countries.
(ii) Brine evaporated salts - underground salt deposits are brought to the surface in solution (a brine) and this is evaporated, usually by heating.
(iii) Rock salt - natural deposits of salt are ground to varying degrees of fineness without any purification.
The suitability of salt for any particular application depends upon several factors, the most important of which are:
(i) the chemical composition;
(ii) the microbiological purity;
(iii) the physical properties.
Commercial salts vary widely in their composition; high quality salt may contain 99.9 per cent sodium chloride, whereas low quality salt may only contain 80 per cent sodium chloride. Apart from contaminants such as dust, sand and water, the main chemical impurities of commercial salts are calcium and magnesium chlorides and sulphates, sodium sulphate and carbonate, and traces of copper and iron. Solar salts tend to be less pure than mine-evaporated salts.
Calcium and magnesium chlorides, even when present in small quantities, tend to slow down the penetration of salt into the flesh; the presence of these salts may also increase the rate of spoilage. Magnesium chloride is hygroscopic and tends to absorb water, making the fish more difficult to dry and to keep dry.
Fish salted in pure sodium chloride may be soft and yellow in colour. Calcium and magnesium salts give a whiter colour but tend to impart a bitter taste. Very often the consumer demands a whitish colour in salted fish products and small quantities of calcium and magnesium compounds in the salt are usually considered desirable. Excessive quantities, however, lead to a bitter flavour and the dried product tends to be brittle which can cause problems during packaging and distribution.
Trace quantities of copper can cause the surface of salted fish to turn brown; this does not reduce the eating quality but it does make the fish look like a spoiled or poor quality product.
Many commercial salts, particularly solar salts, contain large numbers of salt tolerant bacteria (halophiles) and counts of up to 105/g have been recorded. One group of halophiles, the red or pink bacteria, can be a problem in commercial fish curing operations as they cause a reddening of wet or partly dried salt fish. They do not grow when the fish are fully immersed in brine or when they are fully dried. Halophilic moulds can grow on fully dried fish and cause the formation of dark patches, which is called 'dun'. Halophilic moulds tend to occur more frequently in rock salt.
It is possible to sterilise or add preservative agents to salt to control the growth of halophilic organisms but this is very often too expensive for commercial use. Most salt used in fisheries contains appreciable numbers of halophiles.
Fine grain salt dissolves more rapidly in water and is preferred for making brines. If fine grain salt is used directly on a fish, it may cause a rapid removal of water from the surface which becomes hard and prevents the penetration of salt to the inside of the fish; this condition is called 'salt burn'. For dry salting, a mixture of large and small grain sizes is recommended.
Uptake of salt by fish
Several factors which affect the rate at which salt is taken up and water is replaced in fish are:
(i) the higher the fat content, the slower the salt uptake;
(ii) the thicker the fish, the slower the penetration of salt to the centre;
(iii) the fresher the fish, the more slowly salt will be taken up;
(iv) the higher the temperature, the more rapid the salt uptake.
During subsequent drying the presence of salt has the following effects:
(i) the higher the salt concentration, the greater the replacement of water and, therefore, the less water that remains to be removed during drying;
(ii) the higher the salt concentration, the less water that needs to be removed to produce a satisfactorily preserved product;
(iii) the higher the salt concentration, the more slowly the fish dries;
(iv) salt tends to absorb moisture from the air and at relative humidities of more than about 75 per cent during the drying process or during subsequent storage, fish will not dry further; they may even absorb more moisture.