SALT

SALT. Because salt is indispensable to life, acts as a food preservative, and uniquely flavors foods, humans have been preoccupied with it since the beginning of recorded history. The desire to obtain salt politically or militarily has influenced the histories of countries in Asia, Africa, Europe, South America, and the Middle East. Indeed, salt was used as a form of currency and had greater value than gold in some ancient societies. Even religious and magical significance has been attributed to this mineral.

In chemistry, the term "salt" generally refers to any compound that results from the interaction of an acid and a base. In the fields of geology and agriculture, the term "salt" is used as a synonym for the word "mineral." Although numerous salts are essential to human health (for example, potassium chloride, sodium hydroxide), in the following paragraphs the term "salt" will refer specifically to the inorganic, white crystalline substance that is known as sodium chloride (abbreviated NaCl), unless otherwise noted. It is also known as table salt, rock salt, sea salt, and saline. The reader should be aware that some paragraphs below refer to sodium chloride, whereas others refer to sodium, the mineral/ion/electrolyte.

When sodium chloride enters the body, it dissociates almost completely into its constituent particles, the ions sodium and chloride. Sodium chloride is soluble in water and glycerin. Sodium is the most plentiful ion in blood. As electrically charged particles, positively charged sodium (Na⁺) and negatively charged chloride (Cl^-) are classified as electrolytes because they conduct electricity when dissolved in water.

Dietary Salt

Sodium exists in many foods that are commonly consumed in Western diets including processed sandwich meats, cheese, canned vegetables, pickled foods, salty snacks, and soft drinks. Other sources of sodium are not as well recognized: condiments, sauces, baking soda, baking powder, and bread. In restaurant foods, fast-food meals, and Chinese cuisine the sodium levels can be very high. Only about 10 percent of the sodium in Western diets is due to discretionary salt added at the table.

The sodium content of plants and vegetables depends on numerous factors. These include plant maturity, genetics, agricultural practices, soil salinity, soil fertility, soil pH, the rate at which water percolates through soil, as well as meteorological factors such as rainfall, cloud cover, and sunlight.

For most Americans today, eating preserved and processed foods has become a way of life. Sodium chloride is the most common food additive. Approximately 75 percent of sodium in Western diets originates from processed foods. Because salts of all kinds, including sodium chloride, are very stable, it is virtually impossible to remove sodium from foods that have been canned in glass or metal containers. In fact, the addition of sodium may occur during home meal preparation as well as commercial processes. For example, it is possible that a vegetable contains only 2 mg of sodium per 100 g on the vine but may contain 2 to 310 times that amount after canning. Processes such as adding a salt solution to prevent discoloration of vegetables (that is, brining), or the use of sodium salts as processing aids, also result in the addition of sodium to the final product.

Salt in Food Processing

In the late nineteenth and early twentieth centuries, before modern processing techniques existed, food preservation consisted primarily of heat sterilization used in combination with the addition of salts and spices. Salt was used to suppress the growth of unwanted bacteria. Today, sodium is added to processed foods in several forms. Sodium nitrate and sodium nitrite are added to meats as preservatives. Sodium citrate monobasic is added as a pH buffering agent. Both sodium fumarate and malic acid sodium salt are added to foods as buffering agents and flavor enhancers. These salts are used in concert with numerous other food additives in the United States (for example, antioxidants, stabilizers, colors, sweeteners, enzymes, and emulsifiers), under the direction of the U.S. Food and Drug Administration.

Fergus Clydesdale, a professor at the University of Massachusetts at Amherst, explained in 1988 that the loss of sodium during processing is solely due to leaching (that is, extraction, rinsing, or filtration). Canning, boiling, steaming, blanching, and cooking are the processes most likely to cause leaching of sodium and other salts. However, the extent to which these electrolytes are lost varies with the food product, type of processing, and properties of each ion. The amount of water used in a given commercial process also affects mineral losses. Steaming, for example, uses less water than boiling. Further, the total processing time may affect sodium losses from foods. Brief procedures will likely extract less salt than lengthy ones.

Various other salts (for example, potassium chloride, magnesium chloride, sodium nitrate, sodium benzoate, and sodium acetate) are added to foods during commercial processing. They serve to cure meats, provide or intensify the flavor of numerous products, decrease caking of dry products, stabilize pH (that is, when used with jams, gelatins, baked goods, pasteurized cheese), fortify nutrients, and enhance texture. Sodium nitrite, for example, reacts with meat pigments to develop a characteristic pink color. In bread and baked goods, salt serves a variety of functions including the control of the rate of fermentation in yeast-leavened products. Fermented vegetables such as sauerkraut require salt for flavor and to extract water and other nutrients from the plant tissue to form brine, in which desirable organisms flourish and undesirable ones are subdued. The firmness and color of fruits and vegetables are preserved by the calcium salt of lactic acid. In cheese products, salt is added to the curd or applied to the cheese surface to remove whey and to slow the production of acid. Sorbic acid and its salts are antimicrobial agents that work to suppress the growth of bacteria; molds in cheese, sausages, fruits, jellies, bread and cakes; and yeasts in salad dressings, tomato products, syrups, candies, and chocolate syrup.

Biological and Physiological Considerations

The various minerals in the human body serve to maintain acid-base balance, blood volume, and cell membrane permeability, and provide the constituents of bones and teeth. Sodium chloride is important in maintaining the proper concentration of body fluids (that is, osmolality), expediting fluid movement between cells, enhancing glucose absorption, and allowing proper conduction of impulses along nerve and muscle tissues.

Body fluids are distinguished as either intracellular (that is, existing inside muscle and organs) or extracellular (that is, circulating blood plus the interstitial fluid that lies between cells). To accomplish their functions, body tissues maintain intracellular and extracellular ions in different concentrations. This requires considerable energy, approximately one-third of all resting metabolism, and is accomplished by molecules that are embedded in cell membranes throughout the body; these large protein molecules are known as pumps because their action causes an unequal distribution of an ion on the inside and outside of a membrane. In blood, the concentrations for some ions (for example, potassium and calcium) are maintained within narrow limits. Table 1 illustrates these concepts for sodium, chloride, potassium (K⁺), and magnesium (Mg²⁺). Chloride is the most common negative ion that combines with sodium in the extracellular fluid. Sodium and chloride account for more than 80 percent of all particles in the extracellular fluid. Potassium, magnesium, and phosphate are the most abundant intracellular ions. Potassium speeds energy metabolism and is involved in the synthesis of proteins and a storage form of carbohydrate (that is, glycogen). Magnesium allows the body's chemical reactions and biochemical pathways to function efficiently. Approximately 60 percent of the body's magnesium exists in the skeleton, in combination with calcium and phosphorus; in fact, 99 percent of all calcium exists in bones and teeth. The remaining magnesium is present in red blood cells and muscle, supporting the transport and storage of oxygen.

The concentrations of ions in sweat and urine, which constitute the major avenues of loss, may vary markedly between individuals. This large range exists in sweat and urine because diet, acute exercise, chronic physical training, and heat acclimatization alter the loss of these ions—especially sodium and chloride—at the sweat glands and kidneys.

Sodium Metabolism

Sodium is so intimately related to other intracellular ions, extracellular ions, and water that it is difficult to consider the factors that regulate its metabolism independently. Nevertheless, the following text is limited to the regulation of sodium retention and excretion.

At rest, the kidneys filter circulating blood at the rate of 1.0 to 1.5 L/min, causing the kidneys to generate approximately 180 L of fluid during a 24-hour period. Because the average urine volume of normal adults totals 1.3 L/day, almost all of the renal filtrate is reabsorbed and returned to the bloodstream. The amount of sodium excreted into the urine depends upon the body's need for sodium. If excess sodium is consumed without water, the kidney excretes urine with a high concentration of sodium. If dietary sodium is restricted, the kidneys are capable of producing a dilute urine that maintains the concentration of sodium in body fluids at a normal level.

Whole-body sodium balance is maintained over a wide range of dietary and environmental conditions, primarily due to the action of the hormone aldosterone on the kidneys. When dietary sodium is high, urinary sodium increases to excrete the excess. When dietary sodium is low, aldosterone reduces the loss of sodium in urine appropriately. Thus, a sodium deficiency is rare, even among individuals who consume very low-sodium diets (see below). The body may experience a sodium deficiency when sweat losses are large and persistent, or when illness (for example, chronic diarrhea, renal disease) results in inadequate sodium retention by the kidneys. Following major changes in dietary sodium levels, concentrations of the following hormones also adapt, suggesting that they minimize perturbations of extracellular fluid-ion balance: renin, angiotensin II, atrial natriuretic peptide, and nitric oxide. The latter compound plays a pivotal role in blood pressure maintenance by regulating sodium and water excretion at the kidneys. Despite our knowledge of these facts, scientists cannot explain the exact mechanism by which the brain assesses whole-body sodium status.

A predictable sequence of events occurs when a normal individual limits the intake of sodium (for example, 230 mg daily). During the initial days of salt restriction, urinary sodium levels progressively decrease until about the fifth day, when the 24-hour losses become small (for example, 115 mg or less). This individual ordinarily loses 1 or 2 kg of body weight, which is attributable to the loss of sodium and an appropriate volume of water. Initially, the reduced body water comes almost exclusively from the extracellular fluid; as time passes, the intracellular fluid compartment also shrinks. For the next few days, urinary sodium concentration remains low, and the body continues to maximize salt conservation until a reduced whole-body sodium equilibrium is established. Sweat sodium levels decrease in a manner similar to urine during dietary restriction; both are due to the action of the hormone aldosterone.

Toxicity

As is true for virtually all nutrients and compounds, salt can be detrimental or lethal in large quantities. Direct contact with sodium chloride can cause skin irritation, and heating it to high temperatures emits a vapor that irritates the eyes. When heated to the point of decomposition, it emits toxic chloride and disodium oxide (Na₂O) fumes. When consumed in large amounts, sodium chloride can cause stomach irritation. In addition, laboratory experiments have shown the following dose-response effects: 50 mg/24 hr, skin irritation in rabbits; 100 mg/24 hr, moderate eye irritation in rabbits; 125 ml/L, inhibition of DNA synthesis in isolated human cells; 27 mg/kg body weight, abortion of a human fetus; and 3,000 mg/kg body weight, lethal oral dose for 50 percent of the animals tested. Potassium chloride causes physiological responses at the following doses: 500 mg/24 hr, mild eye irritation in rabbits; 125 g/L, lung cell death in hamsters; 2,600 mg/kg body weight, lethal oral dose for 50 percent of the animals tested. Calcium chloride is lethal for 50 percent of the animals tested at a dose of 1,000 mg/kg body weight, when administered orally, and at an intraperitoneal dose of 264 mg/kg body weight. Studies have shown that magnesium chloride is lethal for 50 percent of rats tested at an oral dose of 2,800 mg/kg body weight.

The preservatives known as sulfites (see Sodium and Hypertension, below) can produce deleterious side effects, when consumed in large quantities. Investigations involving laboratory animals have shown that sulfites may inhibit some of the body's biochemical reactions and retard whole-body growth in infants; cause gastrointestinal distress; and induce reversible anemia, nutrient deficiency (for example, thiamine), and gene mutations. A lethal oral dose of sodium bisulfite (50 percent of the animals tested) was 498 mg/kg body weight in rats and 300 mg/kg body weight in mice.

Monosodium glutamate (MSG) is added to foods by chefs to potentiate various flavors. This effect is greatest in meat-and vegetable-based soups, sauces, gravies, and spice blends. The levels of MSG in foods range from approximately 0.3 percent in spinach and tomatoes to about 10 percent in parmesan cheese and 20 percent in dehydrated soup mixes. Some consumers also mix additional MSG into foods in the form of sauces. This may be strongly influenced by cultural food preferences. In Korea and Taiwan, for example, the average adult consumes six to ten times more MSG each day than the average person in the United States. Because sodium is a part of the molecular structure of MSG, it becomes available as free, metabolically active sodium. Therefore, individuals who consume restricted-sodium diets (see Sodium and Hypertension, below) should monitor both the natural levels of MSG in foods as well as the amount that is intentionally added. Monosodium glutamate also produces unwanted side effects in some individuals, including warmth, tingling, tightness, headache, swelling of the liver, and a feeling of pressure in the upper body or face. This phenomenon is often associated with consumption of Chinese food because of its high MSG content. The toxicity of MSG has been studied extensively and it is relatively low, compared to other salts. It has been estimated, for example, that an average adult, weighing 70 kilograms, would have to consume more than 3 pounds of MSG at one time to experience a toxic effect. This does not mean, however, that detrimental effects are nonexistent. A large quantity of MSG has been associated with convulsions, vomiting, and nerve cell damage in research animals, although there are great differences between species. Studies have shown that MSG is lethal, for 50 percent of rats tested, when consumed as an oral dose of 17,300 mg/kg body weight. Thus, when consumed in typical amounts, MSG does not appear to induce illness or toxicity. Because the scope of this article does not allow detailed considerations of the toxicities of other salts, the reader may refer to the book Food Additive Toxicology for further information.

Sodium and Hypertension

Because the kidneys regulate the volume of circulating blood, they are intimately involved in the genesis of high blood pressure (that is, hypertension). This disease often involves excessive retention of extracellular fluid, especially in the bloodstream. For unknown reasons, resistance to blood flow through the kidneys is increased two-to fourfold. And, unfortunately, even though blood pressure may be reduced by prescription medications, the kidneys do not excrete normal amounts of salt and water in urine. This scant urine output causes water and sodium retention until blood pressure rises again to an elevated level. Treatment for this fluid and electrolyte retention often involves diuretics, which increase hourly water and salt losses in urine markedly. Considering these facts, a multiple-stage scientific hypothesis has evolved. This concept proposes that a high dietary sodium intake (1) overloads the kidneys' capacity to excrete sodium and results in fluid retention, (2) increases endocrine gland secretion (that is, natriuretic hormone), (3) inhibits cell membrane function, (4) increases the sodium concentration inside cells and calcium levels in the smooth muscles that encircle blood vessels, which (5) subsequently increases the resistance to blood flow and blood pressure. Interestingly, some research indicates that hypertension may be dependent on the coexistence of sodium and chloride in the diet. Consumption of chloride salts (for example, potassium chloride and calcium chloride) is associated with hypertension, in a way similar to that of sodium.

Forty-three million Americans live with persistently high blood pressure, defined as readings of 140/90 mm Hg or above; this represents 24 percent of the adult population of the United States. This makes it one of today's most prevalent disease conditions. High blood pressure increases the risk of stroke, heart disease, and kidney failure. Individuals with a family history of hypertension, the elderly, middle-aged men, and middle-aged black women are at greatest risk. Yet, everyone is vulnerable because blood pressure typically rises with age.

It is important to acknowledge that heredity plays a critical role in hypertension and that this complex disease is affected by many different genes. Present wisdom states that, without these genes, a person will not develop high blood pressure. Such individuals, whose blood pressure increases with increasing sodium consumption, are salt-sensitive. This explains why there are great differences in human responses to sodium chloride.

Several factors play a role in reducing high blood pressure. In hypertensive adults, for example, a single aerobic exercise session (45 minutes) reduces blood pressure for 12 to 24 hours. A healthy diet (high in fruits, vegetables, low-fat dairy products; low in saturated and total fat) also reduces blood pressure. But salt has received the most attention. There is a large body of evidence, and consensus within the scientific community, that dietary sodium chloride is a risk factor for high blood pressure, independent of other risk factors such as alcohol and obesity. During the last 25 years, numerous professional organizations and advocacy groups have supported reductions of sodium in commercially processed foods, including the American Academy of Pediatrics, American College of Cardiology, Food Research Action Center, American College of Preventive Medicine, American Health Foundation, National Alliance of Senior Citizens, and National Urban Coalition.

In countries where dietary sodium is low, high blood pressure is rare. According to clinical investigations, when hypertensive adults reduce salt consumption their blood pressure usually decreases, although not always to a normal level. Additional evidence suggests that a high-salt diet aggravates other illnesses including asthma, gastric cancer, kidney stones, and osteoporosis. Therefore, consuming a low-salt diet will, for many people, reduce their risk of developing or aggravating a chronic illness such as cardiovascular stroke.

Individuals who are placed on sodium-restricted diets often consume other salts in place of sodium chloride. This increases the daily potassium intake because salt substitutes usually contain a high percentage of potassium chloride. This dietary strategy offers potential health benefits in the form of lowered blood pressure and reduced risk of stroke. For some individuals, however, the use of a potassium-containing salt substitute can cause illness or death. Individuals with a disease, those taking medications, and the elderly should be advised that these salt substitutes ought to be used only to enhance taste, and not for cooking purposes. Sulfites also should be considered. These compounds preserve food by retarding deterioration, rancidity, or discoloration and thus are categorized as antioxidants. At least three sulfites are commonly used as food additives: sodium sulfite (Na₂SO₃), sodium metabisulfite (Na₂S₂O₅), and sodium bisulfite (NaHSO₃). Because these preservatives contain sodium that becomes free and metabolically active in cells, each contributes to the diet's total sodium load.

Unfortunately, reducing the salt content of foods, to restrict sodium consumption, affects the quality and properties of foods. In the meatpacking industry, for example, reducing sodium chloride extremely results in inferior meat cohesion and water retention, and reduces shelf life. These and other unwanted effects explain why commercial food processors usually do not reduce the sodium chloride levels in their products voluntarily.

Managing Dietary Sodium

Compared to the average daily intake in the United States, ranging from 2,300 to 6,900 mg/day, the minimum physiological need for sodium (40 to 300 mg/day) and the intake necessary for good health (500 mg/day) are very small. In fact, the amount of sodium in fresh vegetables alone may be enough to meet an adult's basal requirement. Eight simple procedures make reducing salt intake effective. First, cook with only small amounts of added salt. Second, add little or no salt to food at the dinner table. Third, limit your intake of salty foods such as potato chips, salted nuts, pretzels, popcorn, soy sauce, steak sauce, garlic sauce, pickled foods, and cured meats. Fourth, request that the chef omit salt from your restaurant meal. Fifth, educate yourself about foods that contain large qualities of sodium and seek low-sodium brands when shopping for crackers, pasta sauce, canned vegetables, bread, and other commercial products. Sixth, develop a taste for the unsalted flavor of foods. The taste preference for salty foods can be altered with patience. Seventh, evaluate your diet by reading food labels carefully to determine the sodium content. This can be especially helpful in the aisles of a supermarket because you cannot eat what you do not purchase. Eighth, make a mental list of foods that you will avoid because they contain too much sodium. Here are a few examples, presented in units of milligrams per 100 g of food: fried crisp bacon, 2,400; baking soda, 9,000; beef bouillon cube, 24,000; bologna, 1,300; celery salt, 28,000; cured ham, 1,100; dill pickle, 1,400; frankfurters, 1,100; salt pork, 1,800; green pickled olives, 2,400; and processed cheese, 1,500.

Careful selection of low-sodium food items also will prove to be useful. Table 2 provides a comparison of the sodium content of several vegetables, in fresh and canned forms. Obviously, individuals who desire to reduce their total dietary sodium levels should substitute fresh vegetables for canned varieties, whenever possible. The exception to this recommendation lies in vegetables that lose sodium during processing, due to leaching. This provides the added benefit of ensuring that other dietary nutrients are not lost during commercial packaging (that is, leaching, boiling, blanching).

Another excellent way to lower sodium intake is to alter food preparation practices in the home. Many spices, herbs, and other flavorings do not contribute significant amounts of sodium but may be used to improve the flavor of low-sodium meals. These include allspice, basil, bay leaf, chives, cinnamon, cloves, curry, dill, garlic, ginger, leeks, lemon juice, mint, mustard, nutmeg, orange extract, oregano, paprika, parsley, pepper, peppermint, pimento, poppy seed, saccharin, saffron, sage, sesame, brown and white sugar, tarragon, vanilla extract, and wine.

In determining the amount of sodium that a person consumes, groundwater is often ignored. However, the sodium content of public and private aquifers in the United States varies greatly from one location to another. Although most sources of water include less than 20 mg of sodium per liter, a minor input to daily sodium, certain areas of Arizona, Texas, and Illinois report 325 to

432 mg of sodium per liter of groundwater. Considering the fact that the average adult consumes more than 2 L of fluid each day, this could mean that some Americans receive over 1 g of sodium per day from tap water alone. If a water softener is used to reduce hardness from a local water supply (for example, remove the mineral calcium carbonate), sodium content can be magnified.

Individuals on low-salt diets also should be concerned about the adequacy of other nutrients. It has been estimated that 40 percent of all low-sodium diets lack other essential nutrients, especially protein, the B vitamins, riboflavin, and calcium. These deficiencies result from the removal of food items that contain sodium.

Salt Restriction and Sodium Deficiencies

As noted above, the basal physiologic need for sodium is 40 to 300 mg/day and the amount recommended for good health is 500 mg/day. Field studies, conducted between 1931 and 1962, confirmed that adults can eat low-sodium diets and remain healthy. Interestingly, some of these populations lived in tropical climates, where sweat losses were great, including the vigorous Masai warriors of Africa who consume less than 1,955 mg of sodium per day, the inhabitants of tropical Nigeria who ingest less than 2,760 mg of sodium per 24-hour period, and Galilean naturalists who ingest only 736 mg of sodium per day.

It is difficult to deplete the body of sodium. The action of the hormone aldosterone on the kidneys, and the relatively large per capita daily intake of sodium in Western diets relative to basal physiological needs, are quite adequate to maintain whole-body sodium levels. Thus, sodium deficiencies are rare, but may be experienced in three extraordinary situations. The first involves dietary salt restriction as therapy for disease (for example, hypertension or congestive heart failure). The possibility that sodium depletion may occur in these illnesses does not contraindicate the use of a low-sodium diet when suitable, but it is important that the patient be monitored carefully. Frequent measures of serum sodium concentration are desirable during the first few weeks of a salt-restricted diet. A decline in serum sodium level should prompt a reevaluation. The second circumstance involves diseases of the kidneys or endocrine glands that alter normal sodium balance, such as Addison's disease or diabetes insipidus. The third situation, involving hot environments, is considered in the following section.

Hot Environments Exaggerate Salt Losses

Exercise or labor in cool environments increases the sweat loss and water intake, but the psychological drive to drink and fluid-electrolyte hormones regulate total body water within >0.2 percent (>150 g) of the normal body weight each day. Blood plasma volume is regulated within > 0.7 percent (> 25 g) on consecutive days.

During mild-to-moderate intensity exercise in a hot environment, voluntary water intake does not keep pace with water losses. Most humans produce 0.8 to 1.3 L of sweat per hour, but replace only one-third to three-fourths of this amount by drinking. Thus, if exercise in a hot environment is prolonged and strenuous, a 3 to 5 percent body weight loss can occur. This is significant because, at these levels, both endurance and strength decline.

Table 1 demonstrates that sweat contains sodium, chloride, and other minerals. In fact, sweat contains more than forty distinct organic compounds. Regarding the sodium chloride content, considerable interindividual differences exist among healthy adults. Physically fit athletes who are heat-acclimatized (that is, adapted to exercise in a hot environment) usually lose 400 to 800 mg of sodium chloride per liter of sweat. In contrast, the sweat of unfit, nonacclimatized adults contains from 1,000 to 3,000 mg of sodium chloride per liter. This difference occurs because physical training and heat acclimatization reduce the concentration of salt in sweat.

Salt Balance during Exercise and Labor

Table 3 provides estimates of the amount of fluid and salt lost in sweat, during different activities that are conducted in hot environments. Obviously, water and sodium chloride losses increase in proportion to the duration and the intensity of exercise. As a point of reference, Table 4 describes selected nutrients that are consumed by an adult in the United States. The intake of sodium chloride averages 4,600 to 12,700 mg, and water consumption averages 2.5 L/day. Comparing these two tables, it becomes obvious that 30 minutes of mild gardening produces a small fluid and sodium loss that can be replaced by a normal diet. An ultramarathon, requiring 20 to 30 hours to complete, involves extraordinary salt (14,400 to 70,000 mg NaCl) and water (18.0 to 35.0 L) losses that far exceed normal 24-hour food consumption. Clearly, constant fluid-electrolyte intake is required, during and after an ultramarathon, to replace lost nutrients.

Three fluid-electrolyte disorders involve sodium (that is, heat exhaustion, heat cramps, and exercise-related hyponatremia)

and have become the most common illnesses among athletes and laborers in hot environments. Heat exhaustion, an inability to continue exercise in the heat, is primarily a fluid depletion disorder in which either large sodium, water, or mixed sodium-water losses occur during exercise-heat exposure. Heat cramps occur most often in the abdominal wall and large muscles of the extremities and are due to whole-body sodium depletion. Treatment for these two heat illnesses involves replacing the sodium chloride and water that was lost in sweat and urine. Exertional hyponatremia involves a reduced serum sodium concentration (<130 mEq/L) and represents a marked dilution of the extracellular fluid. This disorder, unlike the previous two, involves overhydration. Athletes

or laborers, who consume and retain a large volume of pure water (for example, 10 L in 5 hr), may experience a life-threatening series of physiological changes that signal water intoxication. The most serious effects are coma, fluid in the lungs (pulmonary edema), and brain swelling (cerebral edema).

Replacing Salt Losses due to Exercise

Individuals who exercise for more than two hours, and who are not hypertensive, should increase their salt intake slightly (see Table 3). Similarly, if a weight loss of 3 percent or more is due to fluid losses during work or exercise, a minor sodium deficit should be expected. The simplest means to replace these deficits after exercise involve adding salt to your meals and selecting saltier foods. Canned soup, for example, contains 1,950 to 2,450 mg of sodium chloride; canned tomato juice contains 1,525 mg. Fluid-electrolyte replacement beverages contain 150 to 300 mg, and 1 percent low-fat milk contains 300 mg sodium chloride It also is wise to eat more fruits, such as bananas and watermelon, to replace lost potassium.

See also Assessment of Nutritional Status ; Body Composition ; Electrolytes ; Fish, Salted ; Meat, Salted ; Microbiology ; Minerals ; Nutrition ; Sodium ; Thirst .

BIBLIOGRAPHY

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Clydesdale, Fergus M. "Minerals: Their Chemistry and Fate in Food." In Trace Minerals in Foods, edited by Kenneth T. Smith, pp. 57–94. New York: Marcel Dekker, 1988.

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Lawrence E. Armstrong

Two Low-Sodium Recipes

The April 1985 issue of FDA Consumer magazine provided two recipes as examples of low-sodium meal items that are easy to prepare. The first describes baked dinner rolls and yields 100 servings: 3¼ounces active dry yeast, 2 quarts water, 7¼ pounds all-purpose flour, 1⅓ cups sugar, 1 tablespoon salt. Normally, a recipe of this size would utilize 4 tablespoons of salt, resulting in a sodium content of 295 mg in each roll. By reducing the amount of salt by 25 percent, each roll contains only 73 mg of sodium.

The second recipe describes low-sodium sausage patties and yields 16 servings. Mix 1 pound ground beef with 1 tablespoon lemon juice, ¼ cup dry bread crumbs, ¼ teaspoon sage, 1⁄₄ teaspoon ginger, 1 teaspoon garlic powder, 1 teaspoon onion power, and ½ teaspoon liquid smoke. Dissolve 1 low-sodium bouillon cube in water and add this solution to the ground beef mixture. Mix thoroughly and let stand for 15 minutes. Form sixteen 1-ounce patties. Brush skillet with vegetable oil and cook the patties for seven to eight minutes on each side, or modify the time as desired. The use of low-sodium bouillon is the key to sodium reduction in this recipe.

Salt

Background

Salt is the common name for the substance sodium chloride (NaCI), which occurs in the form of transparent cubic crystals. Although salt is most familiar as a food supplement, less than 5% of the salt produced in the United States is used for that purpose. About 70% is used in the chemical industry, mostly as a source of chlorine. Salt is also used for countless other purposes, such as removing snow and ice from roads, softening water, preserving food, and stabilizing soils for construction.

The earliest humans obtained their salt from natural salt concentrations, called licks, and from meat. Those people who lived near the ocean may have also obtained it by chewing seaweed or from the natural evaporation of small pools of seawater. Meat became a more important source of salt as hunting was developed, as did milk when sheep, goats, horses, camels, reindeer, and cattle were domesticated. Even today, certain peoples—such as the Inuit of the far north, the Bedouin of the Middle Eastern deserts, and the Masai of east Africa—use no other form of salt.

As agriculture developed, leading to an increased population and a diet consisting mostly of plants, it became necessary to devise ways of obtaining salt in greater amounts. The earliest method of salt production was the evaporation of seawater by the heat of the sun. This method was particularly suited to hot, arid regions near the ocean or near salty lakes and is still used in those areas. Solar evaporation was soon followed by the quarrying of exposed masses of rock salt, which quickly developed into the mining of underground deposits of salt. Two thousand years ago the Chinese began using wells to reach underground pools of salt water, some of which were more than 0.6 miles (1.0 km) deep.

In areas where the climate did not allow solar evaporation, salt water was poured on burning wood or heated rocks to boil it. The salt left behind was then scraped off. During the time of the Roman empire, shallow lead pans were used to boil salt water over open fires. In the Middle Ages these were replaced with iron pans which were heated with coal. In the 1860s a procedure known as the Michigan process or the grainer process was invented, in which salt water was heated by steam running through pipes immersed in the water. This process is still used to produce certain types of salt. By the late 1880s open pans were replaced by a series of closed pans, in a device known as a multiple-effect vacuum evaporator, which had been used in the sugar industry for about 50 years.

Today the United States is the world's largest producer of salt, followed by China, Russia, Germany, the United Kingdom, India, and France.

Raw Materials

Salt is obtained from two sources: rock salt and brine. Rock salt is simply crystallized salt, also known as halite. It is the result of the evaporation of ancient oceans millions of years ago. Large deposits of rock salt are found in the United States, Canada, Germany, eastern Europe, and China. Sometimes pressure from deep inside the Earth forces up large masses of rock salt to form salt domes. In the United States, salt domes are found along the Gulf Coast of Texas and Louisiana.

Brine is water containing a high concentration of salt. The most obvious source of brine is the ocean, but it can also be obtained from salty lakes such as the Dead Sea and from underground pools of salt water. Large deposits of brine are found in Austria, France, Germany, India, the United States, and the United Kingdom. Brine may also be artificially produced by dissolving mined rock salt or by pumping water into wells drilled into rock salt.

Natural brines always contain other substances dissolved along with salt. The most' common of these are magnesium chloride, magnesium sulfate, calcium sulfate, potassium chloride, magnesium bromide, and calcium carbonate. These substances may be as commercially valuable as the salt itself. Rock salt may be quite pure, or it may contain various amounts of these substances along with rocky impurities such as shale and quartz.

For table salt, however, additives are usually mixed in. Most table salt is iodized in order to provide the trace element iodine to the diet. This helps to prevent goiter, a disease of the thyroid gland. To supply iodine, a small amount of potassium iodide is added. Table salt also contains a small amount of various chemicals used to keep the salt from absorbing water and caking. These chemicals include magnesium carbonate, calcium silicate, calcium phosphate, magnesium silicate, and calcium carbonate.

The Manufacturing
Process

Processing rock salt

• 1 Underground salt deposits are usually discovered by prospectors searching for water or oil. When salt is detected, a diamond-tipped, hollow drill is used to take several regularly spaced core samples throughout the area. These samples are analyzed to determine if salt mining would be profitable.
• 2 When a site is selected for mining, shafts are sunk into the center of the salt deposit. Then a machine that looks like a gigantic chain saw is used to cut a slot about 6.0 inches (15 cm) high, about 66 feet (20 m) wide, and about 10 feet (3 m) deep into the salt at floor level. This process is known as undercutting. A series of holes are drilled into the undercut salt with an electric drill containing a tungsten carbide bit. These holes are filled with an explosive such as dynamite or ammonium nitrate. Electric blasting caps connected to long wires are attached, and the explosive is detonated from a safe distance. Cutting and blasting are repeated in a pattern that leaves pillars of salt standing to support the roof of the mining area. This is known as the room-and-pillar method and is also used in coal mines.
• 3 Chunks of blasted rock salt are transported to an underground crushing area. Here they are passed over a grating known as a grizzly which collects pieces smaller than about 9 inches (23 cm). Larger pieces are crushed in a rotating cylinder between metal jaws with spiked teeth. The salt is then transported outside the mine to a secondary crushing area where a smaller grizzly and a smaller crusher reduce the particle size to about 3.2 inches (8 cm). At this point foreign matter is removed from the salt, a process known as picking. Metal is removed by magnets and other material by hand. Rocky material may also be removed in a Bradford breaker, a rotating metal drum with small holes in the bottom. Salt is dumped into the drum, breaks when it hits the bottom, and passes through the holes. Rocky matter is generally harder than salt, so it does not break and does not go through. The picked salt then goes to a tertiary crushing area, where an even smaller grizzly and crusher produce particles about 1.0 inch (2.5 cm) in size. If smaller particles are needed, the salt is passed through a grinder consisting of two metal cylinders rolling against each other. If purer salt is needed, rock salt is dissolved in water to form brine for further processing. Otherwise the crushed or ground salt is passed through screens to sort it by size, poured into bags, and shipped to the consumer.

Processing brine

• 4 The simplest method of evaporating brine is solar evaporation, but it can only be used in hot, dry, sunny places. The brine is collected into shallow ponds and allowed to evaporate in the sun. Insoluble impurities such as sand and clay and slightly soluble impurities such as calcium carbonate settle to the bottom as evaporation begins. The brine is pumped or moved by gravity flow to another pond where calcium sulfate settles out as evaporation continues. The remaining brine is moved to yet another pond where the salt settles out as evaporation proceeds. The brine is moved one more time before evaporation is complete to prevent highly soluble impurities such as magnesium chloride, magnesium sulfate, potassium chloride, and magnesium bromide from settling out with the salt. These substances may be collected separately for commercial use.
• 5 The salt is scooped up by machines running on temporary railroad tracks laid on top of the layer of salt. It is then washed with highly concentrated salt water. This water contains so much salt that it cannot hold any more, so the salt is washed free of any trace impurities without dissolving. The washed salt is removed from the salt water, rinsed with a small amount of fresh water, and piled into huge stacks to drain for two or three months. At this point the salt is about 99.4% pure and can be used for many industrial purposes. If purer salt is needed, it is rewashed in salt water and fresh water, allowed to drain for one or two days, then dried in a hot air oven at about 365°F (185°C). This salt is about 99.8% pure and can be used for food processing.
• 6 Most brine is processed by a multiple-effect vacuum evaporator. This device consists of three or more closed metal cylinders with conical bottoms. Brine is first treated chemically to remove calcium and magnesium compounds. It then fills the bottom of the cylinders. The brine in the first cylinder passes through tubes heated by steam. The brine boils and its steam enters the next cylinder, where it heats the brine there. The steam from this brine heats the brine in the next cylinder, and so on. In each cylinder the condensation of steam causes the pressure inside to drop, allowing the brine to boil at a lower temperature. Salt is removed from the bottom of the cylinders as a thick slurry. It is filtered to remove excess brine, dried, and passed through screens to sort the particles by size. Salt made this way is known as vacuum pan salt and consists of small cubic crystals.
• 7 Brine may also be processed in a grainer. The brine is chemically purified and pumped into a long open pan heated by steam running through pipes immersed in the brine. The brine is heated to a temperature slightly below the boiling point and flakes of salt form on its surface as it evaporates. Usually a temperature of about 194°F (90°C) is used. Lower temperatures produce larger flakes and higher temperatures produce smaller flakes. The flakes grow until they sink to the bottom of the pan, where they are collected and dried. Grainer salt consists of small flakes rather than cubes and is preferred for certain uses in food processing. Sometimes the Alberger process is used, in which the brine is first partially evaporated in a vacuum evaporator then moved to a grainer. This process produces a mixture of flakes and cubes.
• 8 At this point salt used for most purposes is ready to be packaged in bags or boxes and shipped to consumers. To make iodized table salt, however, potassium iodide is added, then magnesium carbonate, calcium silicate, calcium phosphate, magnesium silicate, or calcium carbonate is added to make it free-flowing. The salt is then packaged and shipped to restaurants and grocery stores.

Quality Control

Specifications for salt vary widely according to the intended use. Salt intended for human consumption must be much purer than salt used for melting snow and ice, but salt used for certain scientific purposes may need to be even purer.

For most purposes, rock salt is allowed to have a gray, pink, or brown tinge rather than being pure white. The impurities that cause these colors may make up as much as 4% of a test sample. To test solubility, a 0.7-ounce (20 g) sample is placed in 6.8 fluid ounces (200 ml) of water. It should completely dissolve in no more than 20 minutes.

Evaporated salt intended for food processing is very pure, containing as much as 99.99% sodium chloride before additives are mixed in. This is important not only for safety and good taste, but because certain impurities can cause problems with certain foods. For example, small amounts of calcium tend to toughen vegetables. Traces of copper or iron tend to destroy vitamin C and to increase the rate at which fatty foods become rancid. In addition, calcium and magnesium both tend to make salt absorb more water, causing it to cake.

Health Aspects

Salt intake—or more precisely, sodium intake—is a controversial topic in health care today. Healthy adults can safely consume 0.2-0.4 ounces (6-11 g) of salt daily, which is equivalent to 0.08-0.14 ounces (2400-4400 mg) of sodium. For some people with high blood pressure, salt intake should be reduced. About one-third to one-half of all hypertensive people are salt-sensitive and will benefit from a low-sodium diet. Since there is no way to tell who these people are, most hypertensives under medical care will be placed on such a diet to see if it helps. A low-sodium diet usually aims to reduce sodium intake to less than 0.08 ounces (2400 mg) per day. While some have suggested that everyone should reduce salt intake, others point out that there is no evidence that salt restriction is of any benefit to otherwise healthy individuals.

Where To Learn More

Books

Adshead, Samuel A.M. Salt and Civilization. MacMillan, 1992.

Multhauf, Robert P. Neptune's Gift. Johns Hopkins, 1978.

Periodicals

Dornberg, John. "A 700-Year-Old Mine in Poland Is a Shrine to Human Ingenuity." Smithsonian, March 1994, pp. 96-106.

Vogel, Hans Ulrich. "The Great Well of China." Scientific American, June 1993, pp. 116-21.

Young, Gordon. "The Essence of Life: Salt." National Geographic, September 1977, pp. 380-401.

—Rose Secrest

SALT

SALT. As a commodity of near universal demand, common salt, or sodium chloride, has been produced and traded on a large scale in most countries throughout history. As a national industry in the United States, salt production can be studied in three distinct phases. First, salt served as a vital commodity drawn from oceans and surface waters by boiling. Second, producers discovered rock salt deep beneath the earth's surface and began large scale drilling operations. Finally, the salt industry moved into the realm of high technology as producers scientifically derived compounds from raw materials, reducing salt to—and marketing it as—its component chemicals.

Salt production in America dates from before colonial settlements, and it was vital to those settlements as a preservative and curing agent for perishable meats and other goods. The British colonies were well situated to produce sea salt; however, although there were saltworks at the Jamestown and Plymouth colonies, they were costly operations, and colonists therefore largely tended to import salt. During the American Revolution there was a frantic, and largely successful, attempt to produce salt on the American coast, either by the solar evaporation of sea-water in lagoons laid out along the shore or, more commonly, by boiling it down in cast-iron pots. With the end of the war, these establishments became unable to compete economically with salt imported from England or the West Indies, and the United States again became a salt importer.

As settlement moved west and away from the coastline, inland sources of salt became more cost effective. Interior America possessed many brine springs, known as "licks" because wild animals, especially buffalo, congregated around them to lick the salt deposits. Buffalo trails to these licks became some of the first roads beyond the Appalachians. Many licks were known to the French, who largely controlled that region. Among the first salt-lick regions the British settlers appear to have paid attention to was the Onondaga country of central New York. French travelers reported that Indians were making a little salt there in the mid-eighteenth century, and, in 1788, the Anglo-Americans began to manufacture salt near present-day Syracuse, New York. A little later, buffalo licks gave rise to salt production from brine at two localities now known as Saltville, Virginia, and Charleston, West Virginia. Saltworks began as early as the late 1770s in the Kentucky settlements and quickly became a cornerstone of the frontier economy. Salt making employed scores of landless workers, and the salt produced became a vital currency in the cash-poor region. As late as the 1870s, salt was produced from buffalo licks in Kansas.

As in Europe, salt was regarded as important enough in the United States to justify government intervention, and most salt-producing lands were reserved to the states, which leased them to private producers. The state of New York provided brine to producers and applied a tax, which became a major source of funds for construction of the Erie Canal. Salt production from brine began on the Scioto River in Jackson County, Ohio, before 1800, and when the state was organized in 1803, the area was set aside as a state reservation. On the Wabash, near Shawnee-town, Illinois, the federal government actually took on the operation of a saline works in the early nineteenth century. As salt proved plentiful, however, interest of governments waned. Salt exploration in Michigan began in 1838 under state auspices, but the actual production that began in 1860 was entirely private.

Salt became plentiful as a result of the discovery of rich in-ground sources at great depths. Salt production by well drilling appeared in the United States in the early nineteenth century in Kanawha country near present-day Charleston, West Virginia, through the effort of brothers David and Joseph Ruffner. From 1806–1808, their success in finding strong brine ninety-eight feet below the earth's surface made Kanawha a leading salt-producing region. Many other wells followed. By 1835, forty furnaces in the region boiled down brine, and, by 1845, one well reached 1,500 feet deep.

After reaching production of 2 million bushels (1 bushel equals 56 pounds) a year by 1850, Kanawha's output declined. Onondaga's output similarly declined after reaching 9 million bushels in 1862 and again dropped drastically after 1883. At that time, salt production began in Wyoming County, New York, from a deep well drilled originally in search of oil. Rock salt was found, however, and was produced at various places in New York from 1886.

Rock salt was not always deep, and it is now known that Indians mined salt at several shallow deposits in the Far West. During the emergency conditions of the Civil War, the Confederate government began to work a salt spring in Louisiana, and, in 1862, rock salt was found at a depth of only sixteen feet. Large-scale mining began, only to be terminated by the destruction of the works by Union troops in April 1863. Mining has been continuous at Avery Island since 1883.

Deep salt strata can either be mined or, often more economically, turned into brine by adding water. Michigan's salt production began in 1860 with a 636-foot well at East Saginaw. Near Hutchinson, Kansas, rock salt was found in 1887 by drilling to 800 feet. Drilling has also uncovered salt deposits in many other states—so many, in fact, that salt has lost its status as a precious commodity.

Since the 1850s, one of the most important sources of salt in the United States has been the tideland of San Francisco Bay. Here, solar salt production is successfully accomplished by a method practiced in France since the Middle Ages. Seawater is admitted to enclosed, rectangular basins and transferred to smaller and smaller enclosures as the sun reduces its volume. Ultimately, the water evaporates leaving the salt deposits.

Up to the mid-nineteenth century, nearly all salt was produced for human and animal consumption, although about half was used in meatpacking. In England, large quantities were used in making artificial soda, or sodium carbonate. This industry came to the United States in 1882 and, by 1900, consumed about half of the salt used in the country. By 1957, nearly 80 percent of salt consumed in the United States went to the chemical industry, and the artificial soda industry became the primary user in industries based on sodium and chloride, the elemental constituents of salt. Uses of sodium include the manufacture of caustic soda (sodium hydroxide), which is, in turn, used to make the artificial fiber rayon, to produce aluminum, and to manufacture plastics and detergents. The chlorine-consuming industries are even newer, although they depend on the mid-nineteenth-century discoveries of chlorinated hydrocarbons, organic compounds in which one or more carbon atoms have been replaced by chlorine. By the 1970s, more than half the salt used in the United States was broken down into chlorine and sodium. Chlorine is ultimately converted into the chlorinated hydrocarbons used in plastics, such as vinyl chloride; solvents for dry cleaning; automotive fluids such as anti-freeze; and pesticides such as DDT.

Most of these uses date from about 1940. However, despite the growing chemical industry, the share of American salt used by the industry dropped to 63 percent by 1974 because of an even newer application for the product. Beginning in the 1950s, the salting of highways to remove snow and ice increased continuously until 1974 when 17 percent of all salt consumed was for this purpose. Since the automobile also accounts for the salt used in making automotive fluids and uses much of the plastics, it has clearly become the largest consumer of salt.

American salt production in 1974 was more than 46 million tons, by far the world's largest. Even so, to meet demand, 3 million tons were imported, an amount equal to the entire consumption of the country in 1900.

BIBLIOGRAPHY

Bathe, G. "The Onondaga Salt Works of New York State." Transactions of the Newcomen Society 25 (1945–1947): 17.

Chatard, Thomas Marean. Salt-Making Processes in the United States. Washington, D.C.: U.S. Government Printing Office, 1888.

Lonn, Ella. Salt as a Factor in the Confederacy. New York: Neale, 1933; Tuscaloosa: University of Alabama Press, 1965.

McMurtrie, D. C. "Negotiations for the Illinois Salt Springs, 1802–03." Bulletin of the Chicago Historical Society. (March 1937).

Quinn, William P. The Saltworks of Historic Cape Cod: A Record of the Economic Boom in Nineteenth Century Barnstable County. Orleans, Mass.: Parnassus Imprints, 1993.

Stealy, John E. The Antebellum Kanawha Salt Business and Western Markets. Lexington: University Press of Kentucky, 1993.

Robert P.Multhauf/h. s.;a. e.

See alsoAutomobile Industry ; Chemical Industry ; Food Preservation ; Industries, Colonial ; Meatpacking .

salt has always been an important commodity, especially in hot climates, where salt loss through the sweat can be considerable. Soldiers of the Roman legions were often paid part of their stipend in salt — the so-called salerium argentinium, giving us the word salary.

Salts in general are formed, together with water, when acids react with bases, but the common meaning of ‘salt’ refers in particular to sodium chloride, the same material that is found in salt cellars. Normal saline is a solution of salt in pure water containing 0.9 g of sodium chloride per 100 ml. This solution is often used for bathing and cleaning wounds, or given by intravenous infusion after excessive blood loss — but why is it called ‘normal’? It is because the solution has the same tonicity as blood. Tonicity is a term related to osmotic strength, a property determined by the total number of molecules or ions in a given volume of solution. If living cells are bathed in a hypertonic saline (with a greater tonicity than normal saline) then the cells shrink and cease to function properly, as water passes outwards from the cells into the concentrated solution. Conversely, if living cells are bathed in hypotonic saline (lower tonicity than normal saline) they swell, water passing from the dilute solution into the cells, and eventually the cells may burst. Thus fluids that exist in different body compartments must be of the same tonicity, to avoid any such shrinkage or swelling. Salt — that is, sodium chloride — is one of the most important salts used by the body to keep fluids at their correct osmotic strength.

In a 70 kg person, the extracellular fluid contains an amount of sodium ions equivalent to 125 g of salt, while in the intracellular fluid — the sum total inside all body cells — there is the equivalent of 25 g of salt. With an average urine output of 1 litre per day there is a loss of about 9 g of salt per day. The kidneys filter off from the blood the equivalent of around 1500 g/day of salt, of which 99.5% is reabsorbed as the filtrate passes down the kidney tubules, so that only 0.5% ends up in the urine. There are also small losses of salt in the saliva and faeces, and during strenuous exertion — particularly in a hot environment — there is significant salt loss through the sweat. Clearly the salt loss must be made up by dietary intake, but this alone is not sufficiently precise to keep the tonicity of body fluids constant. Therefore the body has control mechanisms to regulate salt levels, by either increasing or reducing its excretion. If the intake of salt is insufficient, keeping the concentration correct causes the extracellular fluid volume to decrease, with consequent dehydration.

As with most bodily control mechanisms, there is a system for dealing with deficiency as well as one for dealing with excess. They are, respectively, the renin-angiotensin system and atrial natriuretic peptide.

When the body is short of salt the extracellular fluid volume, including the circulating blood volume, decreases, and the blood pressure may fall; the sodium concentration falls and the potassium ion concentration may rise, especially in those eating a low sodium diet. All these changes act directly or indirectly as stimuli for the release of the enzyme renin in the kidneys, triggering a sequence of chemical events in the blood of which the end product is angiotensin II. This is a powerful constrictor of blood vessels and therefore counteracts any fall in blood pressure. Many people with hypertension are treated with drugs which block the enzyme required for angiotensin II formation. (This same converting enzyme also breaks down kinins, which are powerful vasodilators, and therefore tend to lower the blood pressure. When kinins are preserved by inhibiting the converting enzyme, the decrease in blood pressure is probably due to both a lack of angiotensin II and also an excess of kinins.)

Angiotensin II also acts on the adrenal cortex to liberate aldosterone, which in turn causes the kidneys to increase the reabsorption of sodium ions from the filtered fluid.

When salt intake is excessive, the extracellular fluid volume and the blood pressure rise; stretching of the atria of the heart causes the release of stored granules that contain atrial natriuretic peptide. As the name implies (natrium; sodium: ouron; urine) this peptide causes natriuresis; that is, it acts on the kidneys to increase salt loss in the urine by reducing its reabsorption. More water is lost along with the salt, so the excess fluid volume is corrected.

In man excess salt intake has been considered to cause hypertension, but the supporting evidence is equivocal. Certainly there are salt-sensitive strains of laboratory animals that become hypertensive when fed salt, but other strains do not. In the animal kingdom low salt content of the diet is a problem. In seed-eating birds, like parrots, the seeds contain very little salt and an avid salt retaining mechanism has developed in the terminal part of the gut, the coprodaeum, so that little or no salt is lost in the faeces. Similarly, in frogs and toads, salt is avidly reabsorbed from the bladder, so that urine is free of salt. Darwin described how some primitive peoples would pick up a large toad (Bufo marinus), gently squeeze it, and be rewarded with several fluid ounces of almost pure water.

The old medical name, from the time when prescriptions were written in Latin to prevent patients knowing what they were getting, is nat. mur., standing for natrium of muriate. Muriatic acid is hydrochloric acid, therefore nat. mur. is the chloride salt of natrium — that is, sodium. Many popular homeopathic remedies of today contain nat. mur. in infinitesimally low amounts. The reader may ponder how adding such miniscule amounts of salt to the very large quantities already present in the body can have any effect whatsoever.

Alan W. Cuthbert

Salt

The word "salt" is a general chemical term that refers to ionic compounds formed when an acid reacts with a base. They may be simple salts such as

NaCl, KCl, and Na₂SO₄; acid salts like NaHCO₃ and NaH₂PO₄; or double salts like KAl(SO₄)₂. Table salt is sodium chloride, a chemical compound with the formula NaCl. Sodium chloride has been used to flavor and preserve food for thousands of years. As a result, salt became an essential part of commercial transactions and was often used as money or barter. Exchange of salt for slaves in ancient Greece gave rise to the expression "not worth his salt." The Romans gave a salarium (salary) to those who were "worth their salt," and Roman soldiers were given salt rations known as salaria argenta. Today, salt continues to be of major economic importance, with thousands of uses in addition to flavoring and preserving food.

Sodium chloride forms colorless, cubic crystals that are made up of large numbers of NaCl formula units, to give a three-dimensional crystalline lattice in which each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions. The strong electrostatic attractions between the positive and negative ions, known as ionic bonds, hold the solid sodium chloride together.

Sodium chloride occurs naturally as the mineral halite, commonly called rock salt, in large underground deposits on every continent. Seawater contains about 3.5 percent dissolved minerals, of which 2.8 percent is sodium chloride and the other 0.7 percent is primarily calcium, magnesium, and sulfate ions. Natural brines, or salty waters other than seawater, are found in wells and lakes, such as the Great Salt Lake of Utah and the Dead Sea. Salt is also found in surface deposits in regions subject to arid climates.

The manufacture and use of salt is one of the oldest chemical industries. The three main methods for recovering salt are: (1) underground salt mining, which uses techniques similar to those for mining coal; (2) solar evaporation of seawater or natural brine in lakes or large lagoons; and (3) evaporation of brines obtained by pumping water into a rock salt deposit, dissolving the salt, and bringing the brine to the surface. If the salt is going to be used for food flavoring or preservation, additional purification is required, usually by methods that use multi-stage evaporation of brine solutions. The annual world salt production is about 200 million tons (181,436,948 metrictons). The top producers are the United States and China with 45 million and 30 million tons (40,823,313 and 27,215,542 metric tons), respectively.

Uses

Table salt is pure salt that has been ground into fine particles. Because salt tends to cake in humid climates, an anti-caking agent such as magnesium carbonate or calcium silicate is often added. Table salt is also available as iodized salt with 0.1 percent potassium iodide by weight. Because iodide ion is essential to thyroid gland function, the routine use of iodized salt ensures adequate iodine in the diet.

Salt is used to cure meat and fish by soaking them in brine, rubbing salt onto them, or injecting them with a salt solution. Bacon and cured ham are examples of meats preserved by the use of salt. Salt is also used to make pickles by soaking cucumbers in brine.

Rock salt is sprinkled on highways to melt ice. The lowest temperature at which salt can melt ice is −21°C (−6°F), at a concentration of 23 percent salt, and it works best when temperatures are at or slightly below freezing, 0°C (32°F).

Nearly half of the 45 million tons (40,823,313 metric tons) of salt produced each year in the United States is used in the chemical manufacture of chlorine and sodium hydroxide (caustic soda) by electrolysis of brine solution

2 NaCl (aq) + 2 H₂O (l) → 2 NaOH (aq) + H₂ (g) + Cl₂ (g)

Electrolysis of molten sodium chloride produces sodium metal and chlorine gas

2 NaCl (l) → 2Na (l) + Cl₂ (g)

Salt is also one of the raw materials used in the Solvay process for manufacturing sodium carbonate (soda ash).

Both sodium chloride and potassium chloride are essential to the electrolyte balance in body fluids. Good health depends on the proper ratio of potassium ions to sodium ions. Typical values are greater than one. Natural, unprocessed foods have high K⁺/Na⁺ weight ratios. Fresh, leafy vegetables average a K⁺/Na⁺ ratio of 35, and fresh, non-leafy vegetables and fruits average a ratio of 360, with extreme values of 3 for beets and 840 for bananas. After such foods are eaten, the body achieves K⁺/Na⁺ ratios of greater than 12. However, during processing (and cooking, if foods are boiled), both potassium and sodium compounds are dissolved by the water used, which is then discarded. The sodium ions are replenished by the salting of the food, which gives it a lower K⁺/Na⁺ weight ratio. One solution is to eat unprocessed, natural foods and to salt foods with a commercial product that contains both potassium and sodium chlorides called "Lite Salt."

see also Alkali Metals; Chlorine; Halogens; Sodium.

Melvin D. Joesten

Bibliography

Joesten, Melvin D., and Wood, James L. (1996). The World of Chemistry, 2nd edition. Fort Worth, TX: Saunders College.

Kurlansky, Mark (2002). Salt: A World History. New York: Walker and Co.

Internet Resources

Salt Institute. Information available from <http://www.saltinstitute.org>.