October 1, 1994
Salt is one of the most ubiquitous food ingredients around. Those who think of salt only as something used to perk up flavor might wonder what could possibly give this ingredient the level of importance it has gained through the ages. In reality, there is much more to this crystalline cube than first meets the eye.
Besides enhancing taste, salt has several other functions in food products. It acts as an antimicrobial or microbiological control agent. It contributes to certain chemical reactions that create a wide variety of food characteristics.
While most people think of salt as a fairly simple compound, there exists a wide range of salt ingredients designed for specific applications. A salt's characteristics vary with the type of process, types of additives and other criteria, giving this ingredient a level of complexity that may come as a surprise to the uninitiated.
Making the grades
Most salt manufactured in this country comes from underground deposits, although some comes from solar evaporation of saline sea water. The underground deposits can be mined conventionally or removed by dissolving the salt with circulating water and collecting the brine. The brine goes through a vacuum pan process that comprises several clarification and purification steps before crystallization.
This process results in a concentric or cubic form of variable size, but which is typically under 20 mesh (0.03 inches) down to approximately 200 mesh. The term "pan run" means unscreened vacuum-granulated salt. A screening process separates the various size grades.
Two other forms of salt - Alberger® and dendritic salt - are commercially manufactured through different processes. The Alberger or grainer method produces a unique, irregular crystal with stair-step sides, also described as a hollow quadrilateral pyramid. The dendritic process uses sodium ferrocyanide, known as yellow prussiate of soda (YPS), to alter the crystallization process. Low levels of YPS added to the brine prevent normal crystallization, and the result is a porous, star-shaped crystal.
"The YPS dopes the crystal so it honeycombs," explains Dave Strietelmeier, technical director of the Morton Salt Div. of Morton International, Inc., Chicago. "This gives the salt a light bulk density-1.7 grams per cubic centimeter versus 2.0 to 2.1 grams per cubic centimeter for common granulated salt."
Besides the various crystal forms, salt can be pressed into flakes through smooth compacting rolls. Flake salt is available in a number of screen sizes.
Also, grinding rolls can break up salt to make a fine particle generally termed flour salt. This was originally developed to mirror the size of wheat flour, about 70 mesh, and it usually contains an anti-caking agent. If the flour salt is formed with grinding rolls, it no longer has a regular cubic form. However, sometimes fine cubic screenings from the Alberger process are used. These may flow better but may not adhere as well in topical applications as the irregularly shaped ground product.
"We have a micro-sized salt that is milled down to 325 microns," says Tom Dommer, Akzo Nobel's director of technical services. "You would use this in an application such as microwave popcorn, where you want the product suspended in the oil as it is added to the package. It helps with the dispersion and contributes to the effectiveness of the microwave popping."
Because salt often reacts with other components of foods, encapsulated salt was developed. Encapsulation allows the salt to be released when it is needed.
"We use a couple of different vegetable oils as encapsulating media for coating the salt," relates Skip Niman, director of quality administration for Cargill's Salt Division, Minneapolis. "We're trying to retard or delay the action of the salt. In meat patties, if you add regular salt it goes to work immediately on the salt-soluble proteins and you wind up with a sticky matrix that won't come off a patty former. Encapsulation doesn't allow the salt to react with the proteins until the meat is cooked. At elevated temperatures, the coating melts and releases the salt."
Another application for encapsulated salt is in frozen doughs. Here, it keeps the salt from reacting with the yeast, so when the dough is thawed the yeast still functions properly. Encapsulation will work in many situations as long as the temperature remains below the melt point of the encapsulating oil, generally around 140°F.
The Food and Chemicals Codex defines food-grade salt on the basis of its purity. This is determined by silver nitrate titration for the chloride ion. Vacuum evaporated salt must be at least 99.0% pure NaCl. Rock solar salt and salt containing various conditioning and flow agents must contain a minimum of 97.5% NaCI. The Codex also limits calcium and magnesium (2% maximum), arsenic (1 ppm maximum) and heavy metals (4 ppm maximum, as lead). A pharmaceutical grade is defined in the U.S. Pharmacopoeia as 99.95% pure NaCl.
"The pharmaceutical grade must undergo processing to remove calcium, magnesium and sulfate," says Akzo's Dommer. "Manufacturers also must provide certificates of analysis for each lot. The only time you'd need to use this grade in food is if there's a necessity to limit the amounts of calcium, magnesium and sulfate."
A major impurity found in salt - calcium sulfate - is precipitated in the vacuum pan process. Other impurities include sodium sulfate, CaCl2, MgSO4, MgC2, KCl, and minute amounts of copper and iron. Calcium and magnesium ions can produce bitter off-flavors, according to Niman, of Cargill Salt Div.
"From a sanitation point the biggest problem in food-grade salt we've had is rust," says Morton's Strietelmeier. "We've started using rare earth magnets. They were developed in the early 1980s and have improved since. A lot of the magnetic scale contamination is only slightly magnetic. You need a very strong magnet, which these are, to remove these trace materials."
Depending on the application, purity is one of the most important features of salt. Because metals can catalyze oxidation reactions such as rancidity and color or vitamin loss, it may be wise to verify the metal content.
"You ordinarily don't need USP standards for food products," advises Niman. "There are certain sensitive foods that a high-purity salt would be appropriate for - salad dressings, mayonnaise and margarine, due to their high fat content, for example. If you have a high complement of calcium or magnesium in the salt, you form soaps with the fats or oils which leads to off-flavors.
"Metals can promote oxidation," continues Niman. "The heavy metal content set by the Food and Chemicals Codex is 4 ppm maximum. Normally, salt will contain less than one-half that amount, which is usually sufficient to prevent problems."
Breaking the cake
Another "impurity" that can affect salt is water. From a shelf life standpoint, sodium chloride is very stable. It is, however, prone to water absorption and subsequent caking. With humid conditions (over 75% RH), water collects on the surface and produces a brine. When the humidity drops below 25%, the moisture disappears. Caking occurs with cyclic wetting and drying as the surfaces between the crystals recrystallize, forming a hard weld. Using anti-caking additives reduces this problem.
"The crystal form can sometimes help to resist caking," remarks Strietelmeier. "Dendritic is very resistant to caking, much more than regular granulated. YPS is an example of a water soluble anti-caking agent added at a low level - 5 ppm. It prevents hard caking by producing flimsy welds between crystals. The welds break easily with the slightest shear. It doesn't keep the salt from actually getting damp."
To prevent the crystals from getting wet, a free-flowing agent is needed. The compound used must possess a small particle size and be water insoluble. It may keep the crystals physically separate, like tricalcium phosphate, or actually absorb the moisture, such as the silicates. These have a higher attraction for water vapor than the salt and can absorb and give off the moisture without forming a brine. Sodium silicoaluminate is widely used as an anti-caking agent in table salt, but other types can be added, including silica dioxides and magnesium carbonate.
Free-flowing agents can create a problem, depending on the application. They form a cloud in water due to their insolubility and small particle size. Because they usually range from 5 to 8 microns in size, they will remain suspended in an aqueous medium and would not be appropriate for use in salt for a pickle brine.
Since such a wide variety of salt products is available, how does a designer determine which works best in a given application? A number of factors besides purity need consideration.
In most food applications - for flavoring, preservation and chemical reactions - salt must be in solution to function. Regardless of form, salt dissolves to a level of approximately 26.4% by weight at room temperature before reaching saturation. However, the physical form, particularly size and structure, affects the rate at which the salt dissolves. The surface area to weight ratio has the greatest effect on this rate.
A porous structure, such as in dendritic salt, solubilizes rapidly. Irregular surfaces found in Alberger and pulverized salt also make these forms go into solution more quickly than a cube. The larger the cube, the longer it takes to dissolve completely. Therefore, when the application requires rapid solubility (adding the salt to a dough at the end of a mix with little or no floor time, for example), a specialty salt may give better results. The solubility is typically expressed as the time for 1,000 grams of salt, mixed in 3,000 ml of 60°F water, to reach 90% saturation.
"With an increased surface area and a low bulk density you're going to have increased solubility," states Dommer, of Akzo. "That's a desirable characteristic in many applications, such as an instant soup mix."
The more irregular the shape and the more porous the structure, the lower the bulk density. Size also plays a role; it becomes an important factor in dry mix applications, for example. If the bulk density of the salt is equivalent to that of the other ingredients it will tend to remain evenly dispersed throughout the mixing, handling and distribution cycles. Bulk density is expressed as weight per volume, usually in grams per 100 cc.
Salt is measured by a standard screen analysis using U.S. standard mesh sizes. It is measured as a percent retained, but an average size or size range often identifies specific salt grades. Another way to indicate size is by crystal count. For each screen size the crystals in a given weight are counted, and a weighed average determines the number of crystals in the sample as a whole. The particle size should be considered in dry blending operations and when appearance is a factor.
The surface area plays a critical role in a number of salt characteristics. It can be measured using an inert gas - usually nitrogen - displacement technique.
Surface area and the geometry of the particle determine how well the salt adheres to a food product. This is important for salt used in topical applications such as snack seasoning. The most widely used technique for testing adherence involves determining the maximum angle a textured plane achieves while retaining one-half ounce per square foot. While this test does not accurately measure how well salt adheres in a given application, it does give a general indication.
Flowability is indicated by timing the rate a given quantity of salt flows from a standardized funnel. This is an important attribute for controlling topical applications.
Shaking up the flavor
Using salt as a flavor enhancer is probably one of its most popular functions. Scientific studies have proved that humans and other mammals exhibit a natural craving for salt.
"Salt has two major roles in flavoring foods; it adds saltiness and enhances flavors," notes Gary Beauchamp, Ph.D., of the Monell Chemical Senses Center, Philadelphia, PA. "Additionally, it suppresses other taste responses such as sweet, sour and bitter."
Besides the salty or saline taste that sodium chloride produces, expert panels have reported that low levels of salt in solution also give a sensation of sweetness. Several theories have been proposed. One speculates that the arrangement of water molecules around a sodium ion triggers the sweetness response in sweet receptor cells.
Another explanation comes from Michael O'Mahoney, Ph.D. University of California-Davis. "Taste may go beyond the basic senses of sweet, salty, etcetera. We must extend our notion of what is meant by these terms. At a certain concentration salt in water tastes sweet. We might perceive a flavor, but don't have the nomenclature for it. So when we describe the sensation, we call it sweet."
Because salt affects the way we perceive other tastes it often can be added to food products to balance the flavor. Some studies report that salt can increase sweetness, and others report a decrease in sweetness. The result seems to be both product- and level-dependent.
"A small amount of salt added to icings keeps the sweet taste from becoming too cloying," observes Cargill's Niman. "Salt can be added to soft drinks in small amounts to potentiate flavors and cut sweetness, especially in artificially sweetened products."
A study conducted in 1984 by Marianne Gillette of McCormick & Company, Inc., Hunt Valley MD, tested the effect of salt in a number of products. She concluded that the addition of salt affects the overall flavor of the products in five areas:
Mouthfeel: Salted products were perceived as thicker or less watery.
Sweetness: Adding salt enhanced the sweetness, in some cases to a higher degree than the increase in saltiness.
Metallic or chemical off-notes: Salt often decreased or masked these flavors.
Flavor balance: Salt rounded out the balance, blended flavors together and increased the perception of flavor intensity.
Saltiness: The increase in the perception of saltiness depended on the level used and the product. Salt added to foods like crackers and pretzels helps cut the pastiness and dryness they generate in the mouth. Some foods are traditionally salty. Many processed meats contain significant levels of salt as a preservative, but since the consumer has grown accustomed to the flavor and expects it, it is not ordinarily deemed excessive.
But you can get too much of a good thing. "In the case of a product like a dressing or butter, you have a high concentration of salt in the aqueous phase," notes Morton's Strietelmeier. "That changes the aromatic intensity of flavors because you have in effect reduced the vapor pressure of that solution. That will be true of other solutes as well.
Along with the effect of the inter-relationship between the four tastes, the ingredients used can play an important role in how the salt is perceived. Fat can increase the duration of the sensation. Also, a recent study by Barbara Klein, Ph.D., of the University of Illinois, indicates that ionic gums, such as xanthan, suppress the flavor of salt.
"We initially thought that the viscosity of the gum solution affected the saltiness of the sample," Klein relates. "But it appears that the sodium binds tonically to charged gum molecules. This flattens the taste curve."
The size and shape of the salt particle also affect the flavor. The faster the salt goes into solution, the quicker the flavor is perceived. The longer it takes to dissolve, the longer the duration of the salty flavor. This is one reason the type of salt can be critical in topical applications.
"The solubility aspect is one consideration for a topical application," points out Akzo's Dommer. "Faster solubility means an upfront hit that brings out other flavors associated with the product. A lingering salt taste tends to be associated with a bitter taste."
"Not only does the large particle of a typical pretzel salt give you a slow dissolving rate, but also a tactile sensation you may not notice in the fine grains," says Strietelmeier. "You also have a very concentrated brine on the surface of that particle. The taste buds are sensing something that's often described as harshness."
From a flavor standpoint, the most suitable salt level depends on a number of factors. Besides the effect of other ingredients and the type of product, the consumers themselves can determine the acceptable level. Different ethnic and regional preferences exist. As people grow older, they lose their taste and flavor acuity and products may require higher salt levels to be appealing. On the other hand, those who have lowered their sodium, and therefore salt, intake often find normal levels of saltiness excessive.
Another of salt's most valuable characteristics involves the control of microbial growth. This results in two very useful functions: preservation of foods, and control of fermentation processes. The preservation aspect is likely the reason that the control of salt sources has been of such immense importance throughout history. The salting of food, particularly meat, was widely practiced in the days before thermal processing.
The two functions, preservation and fermentation control, both rely on the same fact. When dissolved in water, salt slows or stops microbial growth and, at high enough levels, it can kill many microorganisms. As a solute, salt affects water activity and osmotic pressure. With enough salt, the water activity drops too low to support growth or the osmotic pressure causes the cell wall to rupture. Because of its low molecular weight, salt is an extremely effective agent for lowering water activity.
Salt acts as a control agent for fermented foods such as bread and cheese. In bread it slows the gas production by the yeast, promoting a cell structure that creates an acceptable texture. In cheese it helps regulate the amount of acid produced by the culture, which affects the finished product flavor.
A number of foods use salt as a preservative: fermented foods, such as pickles; processed meats; and dairy products. In fermented foods, salt suppresses the growth of spoilage organisms and pathogens while allowing the lactic acid bacteria to produce acid. The increased acidity contributes to flavor and helps limit further microbial growth. Butter and certain cheeses use salt to prevent the growth of unwanted microorganisms.
Processed meat relies on a combination of salt, nitrates and other curing agents to curtail microbial growth. Initially salt was the only means of preservation and required levels as high as 6% when acting as the lone preservative. Those kinds of levels are not typically in use now except with some specialty products such as anchovies.
The functional factor
Salt also acts as a functional additive. Generally, through a complex (and sometimes not completely understood) chemical reaction salt modifies a food, providing some desirable finished product characteristics.
In processed meat, for example, salt solubilizes the salt-soluble proteins in the muscle. This results in the formation of a matrix that binds the meat particles together, retains water and acts as an emulsifier. Insufficient salt reduces yields and often results in fat and jelly pockets on the surface of the product. Adding salt helps provide an acceptable texture. Although levels of 2.0% to 3.0% have been used, due to consumer demands for lower sodium levels some products have come down to 1.5%.
Certain cheeses, such as cheddar, require salt during manufacturing to expel the whey. Without this process the product would have weak body. If too much salt is added, the texture becomes brittle. Salt also can promote the drying of cheese and formation of a rind in those cheeses that require one.
In baked goods salt acts on the gluten in flour providing a tightening effect. This results in an increase in mixing time. Because this is not always desirable, salt is often added later in the mixing procedure, after the gluten has developed. The addition of salt improves the crumb color and grain in bread.
Selecting and using salt is more involved than many product designers would think. Still, its reputation as one of the most common food ingredients is well deserved. Whether it is added for flavor, functionality or a combination of both, salt is definitely an ingredient that's worth its salt.
A sprinkle a day
In the past several decades, salt has acquired a bad reputation because it is the major source of what is believed to be excess sodium in the human diet. It has been estimated that the average American consumes 12,000 mg per day. The NLEA has established 2,400 mg per day as the Daily Value for sodium consumption and allows products labeled with sodium level descriptors such as "low" and "reduced" to link sodium with the risk of hypertension.
"In the past, several studies have linked sodium to hypertension, but more recent research has indicated that there are other minerals - calcium, magnesium, iron - that may have the same effect on hypertension," relates Tom Dommer, Akzo Nobel's director of technical services. "Roughly 20% of the population has the capability of developing hypertension, but the figure that actually needs to limit salt intake is closer to 6%, according to recent research. It also suggests that hypertension is linked genetically rather than induced through your diet. There are now studies that indicate that taking sodium out of the diet would increase hypertension in some cases.
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