Salt, also known as table salt or rock salt (halite), is a crystalline mineral that is composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of ionic salts. It is essential for animal life, but can be harmful to animals and plants in excess. Salt is one of the oldest, most ubiquitous food seasonings and salting is an important method of food preservation. The taste of salt (saltiness) is one of the basic human tastes.
Salt for human consumption is produced in different forms: unrefined salt (such as sea salt), refined salt (table salt), and iodized salt. It is a crystalline solid, white, pale pink or light gray in color, normally obtained from sea water or rock deposits. Edible rock salts may be slightly grayish in color because of mineral content.
Because of its importance to survival, salt has often been considered a valuable commodity during human history. However, as salt consumption has increased during modern times, scientists have become aware of the health risks associated with high salt intake, including high blood pressure in sensitive individuals. Therefore, some health authorities have recommended limitations of dietary sodium, although others state the risk is minimal for typical western diets.
While people have used canning and artificial refrigeration to preserve food for the last hundred years or so, salt has been the best-known food preservative, especially for meat, for many thousands of years. A very ancient saltworks operation has been discovered at the Poiana Slatinei archaeological site next to a salt spring in Lunca, Neamţ County, Romania. Evidence indicates that Neolithic people of the Precucuteni Culture were boiling the salt-laden spring water through the process of briquetage to extract the salt as far back as 6050 BC. The salt extracted from this operation may have had a direct correlation to the rapid growth of this society’s population soon after its initial production began. The harvest of salt from the surface of Xiechi Lake near Yuncheng in Shanxi, China dates back to at least 6000 BC, making it one of the oldest verifiable saltworks.
Salt was included among funeral offerings found in ancient Egyptian tombs from the third millennium BC, as were salted birds and salt fish. From about 2800 BC, the Egyptians began exporting salt fish to the Phoenicians in return for Lebanon cedar, glass, and the dye Tyrian purple; the Phoenicians traded Egyptian salt fish and salt from North Africa throughout their Mediterranean trade empire.
In Africa, the Tuareg have traditionally maintained routes across the Sahara especially for the transportation of salt by Azalai (salt caravans). In 1960, the caravans still carried some 15,000 tons of salt a year but this trade has now declined to roughly a third of that figure.
Salzburg, Hallstatt, and Hallein lie within 17 kilometers (11 mi) of each other on the river Salzach in central Austria in an area with extensive salt deposits. Salzach literally means “salt water” and Salzburg “salt city”, both taking their names from the German word Salz meaning salt. The equivalent Celtic word was Hall, and Hallstatt was the site of the world’s first salt mine. The town gave its name to the Hallstatt culture that began mining for salt in the area in about 800 BC. Around 400 BC, the townsfolk, who had previously used pickaxes and shovels, began open pan salt making. During the first millennium BC, Celtic communities grew rich trading salt and salted meat to Ancient Greece and Ancient Rome in exchange for wine and other luxuries. The word salary originates from Latin: salarium which referred to the money paid to the Roman Army’s soldiers for the purchase of salt.The word salad literally means “salted”, and comes from the ancient Roman practice of salting leaf vegetables.
In 1930, Mahatma Gandhi led at least 100,000 people on the “Dandi March” or “Salt Satyagraha”, in which protesters made their own salt from the sea, which was illegal under British rule, as it avoided paying the “salt tax”. This civil disobedience inspired millions of common people, and elevated the Indian independence movement from an elitist struggle to a national struggle.
Different natural salts have different mineralities depending on their source, giving each one a unique flavor. Fleur de sel, a natural sea salt from the surface of evaporating brine in salt pans, has a unique flavor varying with the region from which it is produced. In traditional Korean cuisine, so-called “bamboo salt” is prepared by roasting salt in a bamboo container plugged with mud at both ends. This product absorbs minerals from the bamboo and the mud, and has been claimed to increase the anticlastogenic and antimutagenic properties of doenjang (a fermented bean paste).
Unrefined sea salt contains small amounts of magnesium and calcium halides and sulfates, traces of algal products, salt-resistant bacteria, and sediment particles. The calcium/magnesium salts make unrefined sea salt hygroscopic (it gradually absorbs moisture from air if stored uncovered) and confer a faintly bitter overtone. Algal products contribute a mildly “fishy” or “sea-air” odor, the latter from organobromine compounds. Sediments, the proportion of which varies with the source, give the salt a dull gray appearance. Since taste and aroma compounds are often detectable by humans in minute concentrations, sea salt may have a more complex flavor than pure sodium chloride when sprinkled on top of food. When salt is added during cooking however, these flavors would likely be overwhelmed by those of the food ingredients.
The refined salt industry cites scientific studies saying that raw sea and rock salts do not contain enough iodine salts to prevent iodine deficiency diseases.
Refined salt, the most widely used form, is mainly composed of sodium chloride. Food grade salt accounts for only a small part of salt production in industrialized countries (3 percent in Europe) although worldwide, food uses account for 17.5 percent of salt production. The main bulk is sold for industrial use where it has great commercial value as a necessary ingredient in many manufacturing processes. A few common examples include the production of pulp and paper, the use as a mordant in the dyeing of textiles and the making of soaps and detergents.
The manufacture and use of salt is one of the oldest chemical industries. Salt can be obtained by evaporation of sea water, usually in shallow basins warmed by sunlight; salt so obtained was formerly called bay salt, and is now often called sea salt or solar salt. Rock salt deposits are formed by the evaporation of ancient salt lakes, and may be mined conventionally or through the injection of water. Injected water dissolves the salt, and the brine solution can be pumped to the surface where the salt is collected. The raw salt is then refined to purify it and improve its storage and handling characteristics. Purification usually involves recrystallization. In this process, a brine solution is treated with chemicals that precipitate most impurities (largely magnesium and calcium salts). Multiple stages of evaporation are then used to collect pure sodium chloride crystals, which are kiln-dried.
In many cuisines around the world, salt is used in cooking, and is often found in salt shakers on diners’ eating tables for their personal use on food. Table salt is refined salt, which contains about 97 to 99 percent sodium chloride. It usually contains additives that make it free-flowing, anticaking agents such as sodium aluminosilicate or magnesium carbonate. Some people put a desiccant, such as a few grains of uncooked rice or a saltine cracker, in their salt shakers to absorb extra moisture and help break up salt clumps that may otherwise form. Table salt has a particle density of 2.165 g/cm3, and a bulk density (dry, ASTM D 632 gradation) of about 1.154 g/cm3.
Chloride and sodium ions, the two major components of salt, are needed by all known living creatures in small quantities. Salt is involved in regulating the water content (fluid balance) of the body. The sodium ion itself is used for electrical signaling in the nervous system.
Salt consumption has increased during modern times and scientists have become aware of the health risks associated with high salt intake, including high blood pressure in sensitive individuals. Therefore, some health authorities have recommended limitations of dietary sodium, although others state the risk is minimal for typical western diets. The United States Department of Health and Human Services recommends that individuals consume no more than 1500–2300 mg of sodium (3750–5750 mg of salt) per day depending on age.
Salt is sometimes used as a health aid, such as a high salt diet being used in the treatment of dysautonomia.
Apart from its use in the human diet, sodium chloride is widely used in industry and is one of the largest inorganic raw materials used by volume. Its major chemical products are caustic soda and chlorine and these are used in the manufacture of PVC, plastics, paper pulp and many other inorganic and organic compounds. Salt itself is used as a food preservative, flavouring agent, in minerals for livestock, for de-icing, for snow control, as a water softening agent and in many industrial processes.