In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as rock salt or halite. Salt is essential for life in general (being the source of the essential dietary minerals sodium and chlorine), and saltiness is one of the basic human tastes. Salt is one of the oldest and most ubiquitous food seasonings, and is known to uniformly improve the taste perception of food, including otherwise unpalatable food.[1] Salting, brining, and pickling are ancient and important methods of food preservation.
Some of the earliest evidence of salt processing dates to around 6000 BC, when people living in the area of present-day Romania boiled spring water to extract salts; a salt works in China dates to approximately the same period.[2] Salt was prized by the ancient Hebrews, Greeks, Romans, Byzantines, Hittites, Egyptians, and Indians. Salt became an important article of trade and was transported by boat across the Mediterranean Sea, along specially built salt roads, and across the Sahara on camel caravans. The scarcity and universal need for salt have led nations to go to war over it and use it to raise tax revenues. Salt is used in religious ceremonies and has other cultural and traditional significance.
Salt is processed from salt mines, and by the evaporation of seawater (sea salt) and mineral-rich spring water in shallow pools. The greatest single use for salt (sodium chloride) is as a feedstock for the production of chemicals.[3] It is used to produce caustic soda and chlorine, and in the manufacture of products such as polyvinyl chloride, plastics, and paper pulp. Of the annual global production of around three hundred million tonnes, only a small percentage is used for human consumption. Other uses include water conditioning processes, de-icing highways, and agricultural use. Edible salt is sold in forms such as sea salt and table salt, the latter of which usually contains an anti-caking agent and may be iodised to prevent iodine deficiency. As well as its use in cooking and at the table, salt is present in many processed foods.
Sodium is an essential element for human health via its role as an electrolyte and osmotic solute.[4][5][6] However, excessive salt consumption increases the risk of cardiovascular diseases such as hypertension. Such health effects of salt have long been studied. Accordingly, numerous world health associations and experts in developed countries recommend reducing consumption of popular salty foods.[6][7] The World Health Organization recommends that adults consume less than 2,000 mg of sodium, equivalent to 5 grams of salt, per day.[8][9]
History
Main article: History of salt
Salt production in Halle, Saxony-Anhalt (1670)
Ponds near Maras, Peru, fed from a mineral spring and used for salt production since pre-Inca times
All through history, the availability of salt has been pivotal to civilization. What is now thought to have been the first city in Europe is Solnitsata, in Bulgaria, which was a salt mine, providing the area now known as the Balkans with salt since 5400 BC.[10] Salt was the best-known food preservative, especially for meat, for many thousands of years.[11] A very ancient salt-works operation has been discovered at the Poiana Slatinei archaeological site next to a salt spring in Lunca, Neamț 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 salt as far back as 6050 BC.[12] The salt extracted from this operation may have directly correlated with the rapid growth of this society's population soon after production began.[13] 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.[14]
There is more salt in animal tissues, such as meat, blood, and milk, than in plant tissues.[15] Nomads who subsist on their flocks and herds do not eat salt with their food, but agriculturalists, feeding mainly on cereals and vegetable matter, need to supplement their diet with salt.[16] With the spread of civilization, salt became one of the world's main trading commodities. It was of high value to the ancient Hebrews, the Greeks, the Romans, the Byzantines, the Hittites and other peoples of antiquity. In the Middle East, salt was used to seal an agreement ceremonially, and the ancient Hebrews made a "covenant of salt" with God and sprinkled salt on their offerings to show their trust in him.[17] An ancient practice in time of war was salting the earth: scattering salt around in a defeated city to prevent plant growth. The Bible tells the story of King Abimelech who was ordered by God to do this at Shechem,[18] and various texts claim that the Roman general Scipio Aemilianus Africanus ploughed over and sowed the city of Carthage with salt after it was defeated in the Third Punic War (146 BC).[19]
Salt may have been used for barter in connection with the obsidian trade in Anatolia in the Neolithic Era.[20] Salt was included among funeral offerings found in ancient Egyptian tombs from the third millennium BC, as were salted birds, and salt fish.[21] 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 salted fish and salt from North Africa throughout their Mediterranean trade empire.[22] Herodotus described salt trading routes across Libya back in the 5th century BC. In the early years of the Roman Empire, roads were built for the transportation of salt from the salt imported at Ostia to the capital.[23]
In Africa, salt was used as currency south of the Sahara, and slabs of rock salt were used as coins in Abyssinia.[16] The Tuareg have traditionally maintained routes across the Sahara especially for the transportation of salt by Azalai (salt caravans). The caravans still cross the desert from southern Niger to Bilma, although much of the trade now takes place by truck. Each camel takes two bales of fodder and two of trade goods northwards and returns laden with salt pillars and dates.[24] In Gabon, before the arrival of Europeans, the coast people carried on a remunerative trade with those of the interior by the medium of sea salt. This was gradually displaced by the salt that Europeans brought in sacks, so that the coast natives lost their previous profits; as of the late 1950s, sea salt was still the currency best appreciated in the interior.[25]
Salzburg, Hallstatt, and Hallein lie within 17 km (11 mi) of each other on the river Salzach in central Austria in an area with extensive salt deposits. Salzach means "salt river" while Salzburg means "salt castle", both taking their names from the German word Salz, salt. Hallstatt was the site of the world's first salt mine.[26] 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.[11]
The word salary comes from the Latin word for salt. The reason for this is unknown; a persistent modern claim that the Roman Legions were sometimes paid in salt is baseless.[27][28] The word salad literally means "salted", and comes from the ancient Roman practice of salting leaf vegetables.[29]
Wars have been fought over salt. Venice won a war with Genoa over the product, and it played a role in the American Revolution. Cities on overland trade routes grew rich by levying duties,[30] and towns like Liverpool flourished on the export of salt extracted from the salt mines of Cheshire.[31] Various governments have at different times imposed salt taxes on their peoples. The voyages of Christopher Columbus are said to have been financed from salt production in southern Spain, and the oppressive salt tax in France was one of the causes of the French Revolution. After being repealed, this tax was reimposed by Napoleon when he became emperor to pay for his foreign wars, and was not finally abolished until 1946.[30] In 1930, Mahatma Gandhi led a crowd of 100,000 protestors on the "Dandi March" or "Salt Satyagraha", during which they made their own salt from the sea as a demonstration of their opposition to the colonial salt tax. This act of civil disobedience inspired numerous Indians and transformed the Indian independence movement into a national struggle.[32]
Physical properties
Main article: Sodium chloride
SEM image of a grain of table salt
Salt is mostly sodium chloride (NaCl). Sea salt and mined salt may contain trace elements. Mined salt is often refined. Salt crystals are translucent and cubic in shape; they normally appear white but impurities may give them a blue or purple tinge. When dissolved in water sodium chloride separates into Na+ and Cl− ions, and the solubility is 359 grams per litre.[33] From cold solutions, salt crystallises as the dihydrate NaCl·2H2O. Solutions of sodium chloride have very different properties from those of pure water; the freezing point is −21.12 °C (−6.02 °F) for 23.31 wt% of salt, and the boiling point of saturated salt solution is around 108.7 °C (227.7 °F).[3]
Edible salt
See also: List of edible salts and Salt substitute
Comparison of table salt with kitchen salt. Shows a typical salt shaker and salt bowl with salt spread before each on a black background.
Salt is essential to the health of humans and other animals, and it is one of the five basic taste sensations.[34] Salt is used in many cuisines, and it is often found in salt shakers on diners' eating tables for their personal use on food. Salt is also an ingredient in many manufactured foodstuffs. Table salt is a refined salt containing about 97 to 99 percent sodium chloride.[35][36][37] Usually, anticaking agents such as sodium aluminosilicate or magnesium carbonate are added to make it free-flowing. Iodized salt, containing potassium iodide, is widely available. Some people put a desiccant, such as a few grains of uncooked rice[38] or a saltine cracker, in their salt shakers to absorb extra moisture and help break up salt clumps that may otherwise form.[39]
Fortified table salt
Some table salt sold for consumption contains additives that address a variety of health concerns, especially in the developing world. The identities and amounts of additives vary from country to country. Iodine is an important micronutrient for humans, and a deficiency of the element can cause lowered production of thyroxine (hypothyroidism) and enlargement of the thyroid gland (endemic goitre) in adults or cretinism in children.[40] Iodized salt has been used to correct these conditions since 1924[41] and consists of table salt mixed with a minute amount of potassium iodide, sodium iodide, or sodium iodate. A small amount of dextrose may be added to stabilize the iodine.[42] Iodine deficiency affects about two billion people around the world and is the leading preventable cause of intellectual disabilities.[43] Iodized table salt has significantly reduced disorders of iodine deficiency in countries where it is used.[44]
The amount of iodine and the specific iodine compound added to salt varies. In the United States, the Food and Drug Administration (FDA) recommends 150 micrograms of iodine per day for both men and women.[45] US iodized salt contains 46–77 ppm (parts per million), whereas in the UK the recommended iodine content of iodized salt is 10–22 ppm.[46]
Sodium ferrocyanide, yellow prussiate of soda, is sometimes added to salt as an anticaking agent.[47] Such anticaking agents have been added since at least 1911 when magnesium carbonate was first added to salt to make it flow more freely.[48] The safety of sodium ferrocyanide as a food additive was found to be provisionally acceptable by the Committee on Toxicity in 1988.[47] Other anticaking agents sometimes used include tricalcium phosphate, calcium or magnesium carbonates, fatty acid salts (acid salts), magnesium oxide, silicon dioxide, calcium silicate, sodium aluminosilicate and calcium aluminosilicate. Both the European Union and the United States Food and Drug Administration permitted the use of aluminium in the latter two compounds.[49]
In "doubly fortified salt", both iodide and iron salts are added. The latter alleviates iron deficiency anaemia, which interferes with the mental development of an estimated 40% of infants in the developing world. A typical iron source is ferrous fumarate.[3] Another additive, especially important for pregnant women, is folic acid (vitamin B9), which gives the table salt a yellow colour. Folic acid helps prevent neural tube defects and anaemia, which affect young mothers, especially in developing countries.[3]
A lack of fluoride in the diet is the cause of a greatly increased incidence of dental caries.[50] Fluoride salts can be added to table salt with the goal of reducing tooth decay, especially in countries that have not benefited from fluoridated toothpastes and fluoridated water. The practice is more common in some European countries where water fluoridation is not carried out. In France, 35% of the table salt sold contains added sodium fluoride.[3]
Other kinds
Himalayan salt is halite with a distinct pink colour.
Salt deposits beside the Dead Sea, Israel
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 and magnesium salts confer a faintly bitter overtone, and they make unrefined sea salt hygroscopic (i.e., it gradually absorbs moisture from air if stored uncovered). Algal products contribute a mildly "fishy" or "sea-air" odour, the latter from organobromine compounds. Sediments, the proportion of which varies with the source, give the salt a dull grey appearance. Since taste and aroma compounds are often detectable by humans in minute concentrations, sea salt may have a more complex flavour than pure sodium chloride when sprinkled on top of food. When salt is added during cooking however, these flavours would likely be overwhelmed by those of the food ingredients.[51] 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.[52]
Salts have diverse mineralities depending on their source, giving each a unique flavour. Fleur de sel, a natural sea salt from the surface of evaporating brine in salt pans, has a distinctive flavour varying with its source. In traditional Korean cuisine, so-called "bamboo salt" is prepared by roasting salt[53] 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).[54] Kosher or kitchen salt has a larger grain size than table salt and is used in cooking. It can be useful for brining, in bread or pretzel making, and as a scrubbing agent when combined with oil.[55]
Salt in food
Salt is present in most foods, but in naturally occurring foodstuffs such as meats, vegetables and fruit, it is present in very small quantities. It is often added to processed foods (such as canned foods and especially salted foods, pickled foods, and snack foods or other convenience foods), where it functions as both a preservative and a flavouring. Dairy salt is used in the preparation of butter and cheese products.[56] As a flavouring, salt enhances the taste of other foods by suppressing the bitterness of those foods making them more palatable and relatively sweeter.[57]
Before the advent of electrically powered refrigeration, salting was one of the main methods of food preservation. Thus, herring contains 67 mg sodium per 100 g, while kipper, its preserved form, contains 990 mg. Similarly, pork typically contains 63 mg while bacon contains 1,480 mg, and potatoes contain 7 mg but potato crisps 800 mg per 100 g.[15] Salt is used extensively in cooking as a flavouring, and in cooking techniques such as with salt crusts and brining. The main sources of salt in the Western diet, apart from direct use, are bread and cereals, meat, and dairy products.[15]
In many East Asian cultures, salt is not traditionally used as a condiment.[58] In its place, condiments such as soy sauce, fish sauce and oyster sauce tend to have a high sodium content and fill a similar role to table salt in western cultures. They are most often used for cooking rather than as table condiments.[59]
Biology of salt taste
See also: Taste receptor § Salt, and Taste § Saltiness
Human salt taste is detected by sodium taste receptors present in taste bud cells on the tongue.[60] Human sensory taste testing studies have shown that proteolyzed forms of epithelial sodium channel (ENaC) function as the human salt taste receptor.[61]
Sodium consumption and health
Main article: Health effects of salt
Table salt is made up of just under 40% sodium by weight, so a 6 g serving (1 teaspoon) contains about 2,400 mg of sodium.[62] Sodium serves a vital purpose in the human body: via its role as an electrolyte, it helps nerves and muscles to function correctly, and it is one factor involved in the osmotic regulation of water content in body organs (fluid balance).[63] Most of the sodium in the Western diet comes from salt.[6] The habitual salt intake in many Western countries is about 10 g per day, and it is higher than that in many countries in Eastern Europe and Asia.[64] The high level of sodium in many processed foods has a major impact on the total amount consumed.[65] In the United States, 75% of the sodium eaten comes from processed and restaurant foods, 11% from cooking and table use and the rest from what is found naturally in foodstuffs.[66]
Because consuming too much sodium increases risk of cardiovascular diseases,[6] health organizations generally recommend that people reduce their dietary intake of salt.[6][67][68][69] High sodium intake is associated with a greater risk of stroke, total cardiovascular disease and kidney disease.[5][64] A reduction in sodium intake by 1,000 mg per day may reduce cardiovascular disease by about 30 percent.[4][6] In adults and children with no acute illness, a decrease in the intake of sodium from the typical high levels reduces blood pressure.[68][70] A low sodium diet results in a greater improvement in blood pressure in people with hypertension.[71][72]
The World Health Organization recommends that adults should consume less than 2,000 mg of sodium (which is contained in 5 g of salt) per day.[67] Guidelines by the United States recommend that people with hypertension, African Americans, and middle-aged and older adults should limit consumption to no more than 1,500 mg of sodium per day and meet the potassium recommendation of 4,700 mg/day with a healthy diet of fruits and vegetables.[6][73]
While reduction of sodium intake to less than 2,300 mg per day is recommended by developed countries,[6] one review recommended that sodium intake be reduced to at least 1,200 mg (contained in 3 g of salt) per day, as a further reduction in salt intake led to a greater fall in systolic blood pressure for all age groups and ethnicities.[68]