In common usage,salt is amineral composed primarily ofsodium chloride (NaCl). When used in food, especially in granulated form, it is more formally calledtable salt. In the form of a naturalcrystalline mineral, salt is also known asrock salt orhalite. Salt is essential for life in general (being the source of the essentialdietary mineralssodium andchlorine), andsaltiness is one of thebasic human tastes. Salt is one of the oldest and most ubiquitous foodseasonings, and is known to uniformly improve the taste perception of food, including otherwise unpalatable food.[1]Salting,brining, andpickling are ancient and important methods offood preservation.
Some of the earliest evidence of salt processing dates to around 6000 BC, when people living in the area of present-day Romania boiledspring water to extract salts; asalt works in China dates to approximately the same period.[2] Salt was prized by the ancientHebrews,Greeks,Romans,Byzantines,Hittites,Egyptians, andIndians. Salt became an importantarticle of trade and was transported by boat across theMediterranean Sea, along specially builtsalt roads, and across theSahara oncamel caravans. The scarcity and universal need for salt have led nations to go to war over it and use it to raisetax revenues. For example, theEl Paso Salt War which took place in El Paso in the late 1860.[3] Salt is used in religious ceremonies and has other cultural and traditional significance.
Salt is processed fromsalt mines, and by theevaporation of seawater (sea salt) and mineral-rich spring water in shallow pools. The greatest single use for salt (sodium chloride) is as afeedstock for the production of chemicals.[4] It is used to producecaustic soda andchlorine, and in the manufacture of products such aspolyvinyl chloride,plastics, andpaper pulp. Of the annual global production of around three hundred milliontonnes, only a small percentage is used for human consumption. Other uses include water conditioning processes,de-icing highways, and agricultural use.[5][6] Edible salt is sold in forms such as sea salt and table salt, the latter of which usually contains ananti-caking agent and may beiodised to preventiodine deficiency. As well as its use in cooking and at the table, salt is present in many processed foods.
All through history, the availability of salt has been pivotal to civilization. What is now thought to have been the first city in Europe isSolnitsata, inBulgaria, which was a salt mine, providing the area now known as theBalkans with salt since 5400 BC.[13] Salt was the best-known food preservative, especially for meat, for many thousands of years.[14] A very ancient salt-works operation has been discovered at the Poiana Slatinei archaeological site next to a salt spring inLunca,Neamț County, Romania. Evidence indicates thatNeolithic people of thePrecucuteni Culture were boiling the salt-laden spring water through the process ofbriquetage to extract salt as far back as 6050 BC.[15] The salt extracted from this operation may have directly correlated with the rapid growth of this society's population soon after production began.[16] The harvest of salt from the surface ofXiechi Lake nearYuncheng inShanxi, China, dates back to at least 6000 BC, making it one of the oldest verifiable saltworks.[17]
There is more salt in animal tissues, such as meat, blood, and milk, than in plant tissues.[18]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.[19] 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.[20] An ancient practice in time of war wassalting the earth: scattering salt around in a defeated city to symbolically prevent plant growth. TheBible tells the story of KingAbimelech who was ordered by God to do this atShechem,[21] and various texts claim that theRoman generalScipio Aemilianus Africanus ploughed over and sowed the city ofCarthage with salt after it was defeated in theThird Punic War (146 BC),[22] although this story is now considered to be entirely apocryphal.
Salt may have been used forbarter in connection with theobsidian trade inAnatolia in theNeolithic Era.[23] Salt was included among funeral offerings found inancient Egyptian tombs from the third millennium BC, as were salted birds, and salt fish.[24] From about 2800 BC, the Egyptians began exporting salt fish to thePhoenicians in return forLebanon cedar, glass, and the dyeTyrian purple; the Phoenicians traded Egyptian salted fish and salt fromNorth Africa throughout their Mediterranean trade empire.[25]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 atOstia to the capital.[26]
In Africa, salt was used as currency south of the Sahara, and slabs of rock salt were used as coins inAbyssinia.[19] TheTuareg have traditionally maintained routes across theSahara especially for the transportation of salt byAzalai (salt caravans). The caravans still cross the desert from southern Niger toBilma, although much of the trade now takes place by truck. Each camel takes two bales offodder and two of trade goods northwards and returns laden with salt pillars and dates.[27] 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.[28]
Salzburg,Hallstatt, andHallein 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 theGerman wordSalz, salt. Hallstatt was the site of the world's firstsalt mine.[29] The town gave its name to theHallstatt culture that began mining for salt in the area in about 800 BC. Around 400 BC, the townsfolk, who had previously usedpickaxes andshovels, beganopen pan salt making. During the first millennium BC, Celtic communities grew rich trading salt andsalted meat toAncient Greece andAncient Rome in exchange for wine and other luxuries.[14]
The wordsalary comes from theLatin word for salt. The reason for this is unknown; a persistent modern claim that theRoman Legions were sometimes paid in salt is baseless.[30][31] The wordsalad literally means "salted", and comes from the ancient Roman practice of saltingleaf vegetables.[32]
Salt is mostlysodium chloride (NaCl).Sea salt and mined salt may containtrace 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.[37] From cold solutions, salt crystallises as thedihydrate NaCl·2H2O. Solutions of sodium chloride have very different properties from those of pure water; thefreezing point is −21.12 °C (−6.02 °F) for 23.31 wt% of salt, and theboiling point of saturated salt solution is around 108.7 °C (227.7 °F).[4]
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 adeficiency of the element can cause lowered production ofthyroxine (hypothyroidism) and enlargement of the thyroid gland (endemic goitre) in adults orcretinism in children.[44] Iodized salt has been used to correct these conditions since 1924[45] and consists of table salt mixed with a minute amount ofpotassium iodide,sodium iodide, orsodium iodate. A small amount ofdextrose may be added to stabilize the iodine.[46] Iodine deficiency affects about two billion people around the world and is the leading preventable cause ofintellectual disabilities.[47] Iodized table salt has significantly reduced disorders of iodine deficiency in countries where it is used.[48]
The amount of iodine and the specific iodine compound added to salt varies. In theUnited States, theFood and Drug Administration (FDA) recommends 150micrograms of iodine per day for both men and women.[49] 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.[50]
In "doubly fortified salt", both iodide and iron salts are added. The latter alleviatesiron deficiency anaemia, which interferes with the mental development of an estimated 40% of infants in the developing world. A typical iron source isferrous fumarate.[4] Another additive, especially important forpregnant women, isfolic acid (vitamin B9), which gives the table salt a yellow colour. Folic acid helps preventneural tube defects and anaemia, which affect young mothers, especially in developing countries.[4]
A lack offluoride in the diet is the cause of a greatly increased incidence ofdental caries.[54]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 wherewater fluoridation is not carried out. InFrance, 35% of the table salt sold contains addedsodium fluoride.[4]
Unrefinedsea salt contains small amounts ofmagnesium andcalciumhalides andsulfates, traces ofalgal products, salt-resistant bacteria and sediment particles. The calcium and magnesium salts confer a faintly bitter overtone, and they make unrefined sea salthygroscopic (i.e., it gradually absorbs moisture from air if stored uncovered). Algal products contribute a mildly "fishy" or "sea-air" odour, the latter fromorganobromine 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.[55] The refined salt industry cites scientific studies saying that raw sea and rock salts do not contain enoughiodine salts to preventiodine deficiency diseases.[56]
Salts have diversemineralities 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 traditionalKorean cuisine, so-called "bamboo salt" is prepared by roasting salt[57] in abamboo container plugged with mud at both ends. This product absorbs minerals from the bamboo and the mud, and has been claimed to increase theanticlastogenic andantimutagenic properties ofdoenjang (a fermented bean paste).[58]Kosher or kitchen salt has a larger grain size than table salt and is used in cooking. It can be useful forbrining, in bread orpretzel making, and as a scrubbing agent when combined with oil.[59]
Salt in food
Salt is present in mostfoods, 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 ascanned foods and especiallysalted foods,pickled foods, andsnack foods or otherconvenience foods), where it functions as both apreservative and aflavouring.Dairy salt is used in the preparation of butter and cheese products.[60] As a flavouring, salt enhances the taste of other foods by suppressing the bitterness of those foods making them more palatable and relatively sweeter.[61]
Before the advent of electrically poweredrefrigeration, salting was one of the main methods offood preservation. Thus,herring contains 67 mg sodium per 100 g, whilekipper, its preserved form, contains 990 mg. Similarly,pork typically contains 63 mg whilebacon contains 1,480 mg, andpotatoes contain 7 mg butpotato crisps 800 mg per 100 g.[18] Salt is used extensively incooking as a flavouring, and in cooking techniques such as withsalt crusts andbrining. The main sources of salt in the Western diet, apart from direct use, are bread and cereals, meat, and dairy products.[18]
In many East Asian cultures, salt is not traditionally used as a condiment.[62] In its place, condiments such assoy sauce,fish sauce andoyster 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.[63]
Table salt is made up of just under 40% sodium by weight, so a 6g serving (1teaspoon) contains about 2,400mg of sodium.[66] 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 theosmotic regulation of water content in body organs (fluid balance).[67] Most of the sodium in theWestern diet comes from salt.[9] 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.[68] The high level of sodium in many processed foods has a major impact on the total amount consumed.[69] 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.[70]
Because consuming too much sodium increases risk ofcardiovascular diseases,[9] health organizations generally recommend that people reduce their dietary intake of salt.[9][11][71][72] High sodium intake is associated with a greater risk ofstroke, total cardiovascular disease andkidney disease.[8][68] A reduction in sodium intake by 1,000 mg per day may reduce cardiovascular disease by about 30 percent.[7][9] In adults and children with no acute illness, a decrease in the intake of sodium from the typical high levels reduces blood pressure.[71][73] A low sodium diet results in a greater improvement in blood pressure in people withhypertension.[74][75]
TheWorld Health Organization recommends that adults should consume less than 2,000 mg of sodium (which is contained in 5g of salt) per day.[11] 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.[9][76]
While reduction of sodium intake to less than 2,300 mg per day is recommended by developed countries,[9] one review recommended that sodium intake be reduced to at least 1,200 mg (contained in 3g 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.[71] Another review indicated that there is inconsistent/insufficient evidence to conclude that reducing sodium intake to lower than 2,300 mg per day is either beneficial or harmful.[77]
Evidence shows a more complicated relationship between salt and cardiovascular disease. "The association between sodium consumption and cardiovascular disease or mortality is U-shaped, with increased risk at both high and low sodium intake."[78] The findings showed that increased mortality from excessive salt intake was primarily associated with individuals with hypertension. The levels of increased mortality among those with restricted salt intake appeared to be similar regardless of blood pressure. This evidence shows that while those with hypertension should primarily focus on reducing sodium to recommended levels, all groups should seek to maintain a healthy level of sodium intake of between 4 and 5 grams (equivalent to 10-13 g salt) a day.[78]
One of the two most prominent dietary risks for disability in the world are diets high in sodium.[79]
Only a small percentage of the salt manufactured in the world is used in food. The remainder is used in agriculture, water treatment, chemical production, de-icing, and other industrial use cases.[4] In the practice of watering plants with salt as a fertilizer, applying a moderate concentration helps avoid potential toxicity; typically, 1–3 grams (0.035–0.106 oz) per liter is considered safe and effective for most plants.[80][81][82] Sodium chloride is one of the largest volume inorganic raw materials. It is afeedstock in the production ofcaustic soda andchlorine. These are used in the manufacture ofPVC,paper pulp and many other inorganic and organic compounds. Salt is used as aflux in the production ofaluminium. For this purpose, a layer of melted salt floats on top of the molten metal and removes iron and other metal contaminants. It is used in the manufacture ofsoaps andglycerine, where it is used tosaponify fats. As an emulsifier, salt is used in the manufacture ofsynthetic rubber, and another use is in the firing ofpottery, when salt added to the furnace vaporises before condensing onto the surface of the ceramic material, forming a strongglaze.[83]
When drilling through loose materials such as sand or gravel, salt may be added to thedrilling fluid to provide a stable "wall" to prevent the hole collapsing. There are many other processes in which salt is involved. These include its use as amordant intextile dying, to regenerateresins in water softening, for thetanning of hides, the preservation of meat and fish and thecanning of meat and vegetables.[83][84][85]
Sifto Canada salt mine and processing plant at the harbor inGoderich, Ontario, Canada
Food-grade salt accounts for only a small part of salt production inindustrialized countries (7% in Europe),[86] although worldwide, food uses account for 17.5% of total production.[87]In 2018, total world production of salt was 300 milliontonnes, the top six producers being China (68 million), the United States (42 million), India (29 million), Germany (13 million), Canada (13 million) and Australia (12 million).[88]
The manufacture of salt is one of the oldest chemical industries.[89] A major source of salt is seawater, which has asalinity of approximately 3.5%.[4] This means that there are about 35 g (1.2 oz) ofdissolved salts, predominantlysodium (Na+ ) andchloride (Cl− )ions, per kilogram (2.2 lbs) of water.[90] The world's oceans are a virtually inexhaustible source of salt, and this abundance of supply means that reserves have not been calculated.[84] The evaporation of seawater is the production method of choice in marine countries with high evaporation and low precipitation rates.Salt evaporation ponds are filled from the ocean and salt crystals can be harvested as the water dries up. Sometimes these ponds have vivid colours, as some species of algae and other micro-organisms thrive in conditions of high salinity.[91]
Away from the sea, salt is extracted from the vast sedimentary deposits which have been laid down over the millennia from the evaporation of seas and lakes. These sources are eithermined directly, producing rock salt, or are extracted by pumping water into the deposit. In either case, the salt may be purified by mechanical evaporation of brine. Traditionally, purification was achieved inshallow open pans that were heated to accelerate evaporation.Vacuum-based methods are also employed.[85] The raw salt is refined by treatment with chemicals that precipitate most impurities (largely magnesium and calcium salts). Multiple stages of evaporation are then applied.[92] Some salt is produced using theAlberger process, which involves vacuum pan evaporation combined with the seeding of the solution with cubic crystals, and produces a grainy-type flake.[93] TheAyoreo, an indigenous group from the ParaguayanChaco, obtain their salt from the ash produced by burning the timber of the Indian salt tree (Maytenus vitis-idaea) and other trees.[94]
The largest mine operated by underground workings in the world is the Sifto mine, located mostly 550 meters below Lake Huron, in Goderich, Ontario (Canada). About seven million tons of salt are extracted from it annually.[95] TheKhewra Salt Mine in Pakistan has nineteen storeys, eleven of which are underground, and 400 km (250 mi) of passages. The salt is dug out by theroom and pillar method, where about half the material is left in place to support the upper levels. Extraction ofHimalayan salt is expected to last 350 years at the present rate of extraction of around 385,000 tons per annum.The mine is also a major tourist attraction, receiving around a quarter of a million visitors a year.[96]
Salt has long held an important place in religion and culture. At the time ofBrahmanic sacrifices, inHittite rituals and during festivals held bySemites and Greeks at the time of thenew moon, salt was thrown into a fire where it produced crackling noises.[97] The ancient Egyptians, Greeks and Romans invoked their gods with offerings of salt and water and some people think this to be the origin ofHoly Water in the Christian faith.[98] InJudaism, it is recommended to have either a salty bread or to add salt to the bread if this bread is unsalted when doingKiddush forShabbat. It is customary to spread some salt over the bread or dip it in a little salt when passing it around the table after the Kiddush.[99] To preserve the covenant between their people and God, Jews dip theSabbath bread in salt.[98] Salt plays a role within different Christian traditions. It is mandatory in the rite of theTridentine Mass.[100] Salt is used in the third item (which includes anExorcism) of the Celtic Consecration (cf.Gallican Rite) that is employed in the consecration of a church, and it is permitted to be added to the water "where it is customary" in the Roman Catholic rite of Holy water.[100] TheBible makesmultiple mentions of salt, both of the mineral itself and as a metaphor. Uses include the tale of howLot's wife is turned into a pillar of salt when looking back at the cities ofSodom and Gomorrah as they are destroyed. In theNew Testament,Jesus refers to his followers as the "salt of the earth".[101]
InAztec mythology,Huixtocihuatl was a fertility goddess who presided over salt and salt water.[102] Salt is an auspicious substance inHinduism and is used in ceremonies like house-warmings and weddings.[103] InJainism, devotees lay an offering of raw rice with a pinch of salt before a deity to signify their devotion and salt is sprinkled on a person's cremated remains before the ashes are buried.[104] Salt is believed to ward off evil spirits inMahayana Buddhist tradition. When returning home from afuneral, a pinch of salt is thrown over the left shoulder as this prevents evil spirits from entering the house.[105] InShinto,Shio (塩, lit. "salt") is used forritual purification of locations and people (harae, specificallyshubatsu), and small piles of salt are placed in dishes by entrances to ward off evil and to attract patrons.[106]
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^abWood, Frank Osborne."Salt (NaCl)". Encyclopædia Britannica online.Archived from the original on 2 May 2015. Retrieved9 October 2013.
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^Gevirtz, Stanley (1963). "Jericho and Shechem: A Religio-Literary Aspect of City Destruction".Vetus Testamentum.13 (1):52–62.doi:10.2307/1516752.JSTOR1516752.
^"Tesco Table Salt 750g". Tesco.Archived from the original on 11 May 2009. Retrieved5 December 2010.Nutritional analysis provided with Tesco Table Salt states 38.9 percent sodium by weight which equals 97.3 percent sodium chloride
^Pieters, A.J.; Flint, D.; Garriott, E.B.; Wickson, E.J.; Lamson-Scribner, F.; et al. (1899).Experiment Station Work. Bread and the Principles of Bread Making. U.S. Department of Agriculture. pp. 28–30.Archived from the original on 16 June 2016. Retrieved17 November 2015.
^Dietary Guidelines for Americans(PDF). U.S. Department of Agriculture & U.S. Department of Health and Human Services. 2010. p. 24.Archived(PDF) from the original on 1 September 2016. Retrieved29 April 2015.African Americans, individuals with hypertension, diabetes, or chronic kidney disease and individuals ages 51 and older, comprise about half of the U.S. population ages 2 and older. While nearly everyone benefits from reducing their sodium intake, the blood pressure of these individuals tends to be even more responsive to the blood pressure-raising effects of sodium than others; therefore, they should reduce their intake to 1,500 mg per day.
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