Salt has played a prominent part in the development of man's activities, trade, politics and culture from prehistoric times. Part of its overwhelming influence stems from its role as a source of sodium and chloride, two dominant chemical constituents of the human body with important metabolic functions. Also this inexpensive and abundantly available commodity is an important raw material in today's industry for the manufacture of chemicals that are used in the manufacture of plastics, aluminium, paper, soaps and glass. Directly or in the form of derivatives, salt finds application in more than 14,000 ways.
History and culture of salt
Man's primary use of salt is as an essential dietary item for both him and his domestic animals. Salt has also been used from pre-historic times for flavoring, pickling, preserving, and curing meat and fish, and for tanning. These features have made salt an important part of human culture and civilization. As one writer points out 'From cells in our brains and bones to customs that spice our languages, salt penetrates every aspect of our existence'. Expressions such as 'worth his salt,' 'above the salt,' 'old salt,' 'loyal to one's salt,' 'the salt of life,' and 'salary,' are used every day. Different cultures have variously held salt as a symbol of divinity, purity, welcome, hospitality, wit or wisdom. In Sanskrit the word "lavanya" expressing grace, beauty and charm, is derived from the word for salt "lavana."
Salt has been equally important in trade and politics. Many earlier cultures used it as currency. Some primitive tribes gave gold, weight for weight, to purchase salt. The Hanseatic league developed initially on the salt trade. Many countries controlled salt as a state monopoly. The salt tax, among other things, provoked the French Revolution. A similar salt tax was a principal issue in Gandhi's civil disobedience movement against the British that eventually led to India's independence.
Salt for human consumption
In the human body, sodium is essential to muscle movement, including that of the heart, to the peristaltic movement of the digestive tract, and to the transmission of messages by the nerve cells. The chloride ion produces hydrochloric acid required for digestion and is present in salivary amylase. A principal function of salt is to regulate osmotic pressure and the movement of fluid to and from the cell. For normal health, the salt concentration in the body can vary only within narrow limits. Lost salt must be replaced. Most leaves the body as sweat. Salt is also excreted through urine, carefully regulated by the kidneys to maintain bodily levels at the appropriate level. Most salt in the gastric juices and digested food is reabsorbed in the intestines, but increased bowel motility and diarrhea can deplete body salt.
Chronic salt deprivation produces loss of weight and appetite, inertia, nausea and muscular cramps. Excessive heat as in desert summers depletes body salt, leading to possible vascular collapse and death. On the other hand, excessive sodium in salt and other foods can contribute to hypertension, and to heart, liver and kidney diseases.
While salt is perhaps the only food item other than water that is universally consumed, its intake varies considerably with factors such as climate, culinary habits and occupation. Hot tropical summers and heavy manual work with excessive sweating increase salt requirements. The rice-eating populations of the world also consume more salt (15-20 g/day) than others because rice is very deficient in salt. In temperate climates consumption is much lower (5-8 g/day).
Salt for animal consumption
Animals need salt as much as humans do. Insufficient salt stunts the growth of young animals and produces lassitude, lowered production of milk and loss of weight in adults. Since fodder and plant life have little salt, domestic animals must receive salt with their feed. In modern farms, salt also provides a vehicle for vitamin and mineral supplements that are essential to the good health of livestock.
Salt for industrial consumption
The uses and importance of salt have multiplied with the advent of industrial civilization. Today only 6% of the world's annual salt production goes directly for human consumption. Most of the rest is used industrially as one of the "Big Five" basic ingredients of the chemical industry, together with sulfur, coal, limestone and petroleum. The largest industrial consumer is the chlor-alkali industry, which produces chlorine, caustic soda and soda ash for manufacture of plastics, paper, soaps, detergents, insecticides, and glass. The food industry needs salt for canning, baking, processing of flour and other foods, meat packing, fish curing, dairying and food flavoring. Other direct industrial uses are leather tanning, highway de-icing, oil well drilling and manufacture of dyestuffs and textiles.
These are only some of the main applications of salt. As the frontiers of the chemical industry grow, new applications for salt and its derivatives are constantly being discovered and are leading to an ever growing role for salt in the future.
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