as a means of preserving food (see also preservation), has been practised since antiquity. The salt most used is common salt, sodium chloride, but saltpetre (see nitrates and nitrites), which consists of potassium nitrate and sodium nitrate, has similar effects. For some purposes ‘dry-salting’ is appropriate, for others the use of brine, which is salt in solution.
In its role in preservation of foods, salt operates mainly by its effect on osmosis, which is the passage of water through ‘semipermeable membranes’ such as the cell walls of plant or animal tissue, living or dead. A semipermeable membrane lets water through but blocks the passage of the bigger molecules of substances dissolved in it. When such a membrane has a strong solution on one side and a weak one on the other, water is drawn through it in one direction only, from the weak solution to the strong one, which it dilutes. This pull is called ‘osmotic pressure’.
Plant and animal cells contain relatively weak solutions of natural salts, sugars, and other dissolved substances. Bacteria and other micro-organisms live comfortably in weak solutions of this sort, drawing in nutrients through the cell walls. If, however, these micro-organisms are exposed to a strong solution, such as one containing a lot of salt, the outward osmotic pressure created by the strong solution prevents them from feeding, and thus from reproducing. They may remain alive, and can return to normal functioning if the outside solution is diluted; but while the strong salt solution is present their activity is inhibited and the decay which they would otherwise cause is thus arrested.
However, decay is caused by enzymes naturally present in foodstuffs as well as by living micro-organisms. Salt also stops the working of enzymes by upsetting the electrical balance of the liquid in which they act. So a salt solution, if strong enough, will also prevent decay due to enzymes, and this can be observed when sliced apple is put in brine to stop it from browning.
The strength of the salt solution is important. Some micro-organisms can tolerate quite strong solutions. Among these are certain lactic acid-producing bacteria which, rather than causing decay, bring about beneficial fermentations. For this reason, only a moderate amount of salt is used in some preparations, allowing these bacteria to grow while inhibiting others which would cause decay and which are less tolerant of salt. Examples are sauerkraut, the kimch'i of Korea, soy sauce, and other similar condiments. The lactic acid produced by the ‘good’ bacteria will itself be a safeguard against the growth of the ‘bad’ ones. Eventually the acid becomes so concentrated that even the ‘good’ bacteria are inhibited: fermentation stops and the food keeps.
Osmosis caused by salt is also exploited in the preparation of dry-salted foods such as dry salt fish and meat. The applied salt draws water out of the cells of the tissue, so that salt both speeds drying and suspends decay while the food is still sufficiently moist to be at risk. (See also drying.) The same effect is used when fresh cucumber is salted to make it a less watery salad ingredient; and to collapse the cells of aubergines before cooking them in oil, so that they will not absorb an excessive amount of oil.
Purity is important. For example, extra pure pickling salt is sold, so that impurities will not cause discoloration in the pickles.
From the Middle Ages until well into the 19th century, the only fully satisfactory salt for curing meat and fish was considered to be bay salt, made by solar evaporation from sea water on the coasts of France and the Iberian peninsula, and valued for its good flavour as well as for its purity. The preference for bay salt, which would now be called sea salt, had another reason. Because the process of solar evaporation is slow, large crystals tend to be formed. These dissolve relatively slowly in any curing process, avoiding the risk of what is called ‘salt burn’, an unwelcome phenomenon which occurs when fine salt is used; this dissolves quickly and produces a sudden high concentration, which at once dehydrates the outer layers of the food, making them relatively impervious and denying the benefit of salting to the inner layers.
Alan Davidson was a distinguished author and publisher, and one of the world's best-known writers on fish and fish cookery. In 1975 he retired early from the diplomatic service—after serving in, among other places, Washington, Egypt, Tunisia, and Laos, where he was British Ambassador—to pursue a fruitful second career as a food historian and food writer extraordinaire. Among his popular books are Seafood of South-East Asia, North Atlantic Seafood, and Mediterranean Seafood. In 2003, shortly before his death, he was awarded the Erasmus Prize for his contribution to European culture.
