Preservation and Physical Property Roles of Sodium in Foods (2024)

Salt’s Role in the Prevention of Microbial Growth

Salt is effective as a preservative because it reduces the water activity of foods. The water activity of a food is the amount of unbound water available for microbial growth and chemical reactions. Salt’s ability to decrease water activity is thought to be due to the ability of sodium and chloride ions to associate with water molecules (Fennema, 1996; Potter and Hotchkiss, 1995).

Adding salt to foods can also cause microbial cells to undergo osmotic shock, resulting in the loss of water from the cell and thereby causing cell death or retarded growth (Davidson, 2001). It has also been suggested that for some microorganisms, salt may limit oxygen solubility, interfere with cellular enzymes, or force cells to expend energy to exclude sodium ions from the cell, all of which can reduce the rate of growth (Shelef and Seiter, 2005).

Today, few foods are preserved solely by the addition of salt. However, salt remains a commonly used component for creating an environment resistant to spoilage and inhospitable for the survival of pathogenic organisms in foods. Products in the modern food supply are often preserved by multiple hurdles that control microbial growth (Leistner, 2000), increase food safety, and extend product shelf life. Salt, high- or low-temperature processing and storage, pH, redox potential, and other additives are examples of hurdles that can be used for preservation. As shown in Figure 4-1, no single preservation method alone would create a stable product; when combined, however, these methods result in a desirable, stable, and safe product. For example, a food might be protected by a combination of salt, refrigeration, pH, and a chemical preservative.

Multiple-hurdle methods offer the additional benefit of improving other qualities of some foods. For example, hurdle methods can be used to reduce the severity of processing needed, allow for environmentally friendly packaging, improve the nutritional quality of foods (by achieving microbiological safety with less salt, sugar, etc.), and reduce the use of preservatives that are undesirable to some consumers (Leistner and Gould, 2005).

Salt’s Role in Fermentation to Preserve Foods

Salt commonly plays a central role in the fermentation of foods. Fermentation is a common process for preserving foods, in which fresh foods are transformed to desirable foods that can be preserved for longer periods of time than their fresh counterparts due to the actions of particular types of microbes (Potter and Hotchkiss, 1995). Products such as pickles, sauerkraut, cheeses, and fermented sausages owe many of their characteristics to the action of lactic acid bacteria. Salt favors the growth of these more salt-tolerant, beneficial organisms while inhibiting the growth of undesirable spoilage bacteria and fungi naturally present in these foods (Doyle et al., 2001). Salt also helps to draw water and sugars out of plant tissues during fermentation of vegetables. This water aids fermentation by filling any air pockets present in fermentation vats, resulting in reduced oxygen conditions that favor growth of lactic acid bacteria. The release of water and sugars also promotes fermentation reactions in the resulting brine, increasing the rate of the fermentation process (Doyle et al., 2001; Potter and Hotchkiss, 1995).

Role of Other Sodium Compounds

A number of other sodium-containing compounds provide preservative effects in foods. There is a wide variety of these preservatives with various product uses. Preservatives can act to reduce microbial activity and also may, like salt, act as a hurdle to microbial growth and survival. Some additives may also play a role in preserving food quality by reducing undesirable chemical reactions, such as lipid oxidation and enzymatic browning. In some cases, the compounds can have more than one function in a food product, with preservative effects being one of several reasons for use.

A brief listing of common sodium-containing compounds used for food preservation and the foods with which they are associated can be found in Table 4-1.

Challenges and Innovations for Lowering Sodium While Maintaining Safety and Shelf Life

For many foods, reducing the sodium content of the product should not create food safety or spoilage concerns. Such foods include frozen products, products that are sufficiently thermally processed to kill pathogenic organisms (e.g., canned foods), acidic foods (pH < 3.8), and foods in which water activity remains low when sodium is removed (e.g., foods with low water activity due to high sugar content) (Reddy and Marth, 1991; Stringer and Pin, 2005). For other foods, reducing sodium content has the potential to increase food spoilage rates and the presence of pathogens. For these foods, product reformulation, changes in processing, and changes in handling may be required to ensure that the product has an adequate shelf life and to prevent pathogen growth. Such efforts do incur additional costs and require careful attention to ensure that new formulations and processes are sufficient to ensure product safety. These issues are discussed further in Chapters 6 and 8.

Foods using sodium as a hurdle to retard microbial growth and survival present a reformulation challenge, since changing the sodium content alters the impact (or height) of the water activity hurdle. Changing this single hurdle may impact the safety and quality of the food because other hurdles that are present (pH, temperature, etc.) may work only in combination with the original sodium level. To maintain a safe, good-quality product, reformulation may have to include the introduction of additional hurdles or an increase in the impact of existing hurdles. If such additional measures are not taken during sodium reduction efforts, the remaining products may not be stable. For example, in cured meats, reducing the sodium content (by removing both salt and sodium nitrite) could allow for rapid growth of lactic acid bacteria and action by proteolytic microorganisms, resulting in a product that spoils more rapidly (Roberts and McClure, 1990; Stringer and Pin, 2005). In some foods, pathogen growth, rather than spoilage, may become a concern.

There is speculation that some past salt reduction efforts may not have adequately accounted for the need to adjust additional hurdles to microbial growth. In the United Kingdom, salt reduction efforts in chilled, ready-to-eat foods were cited as one factor that may have contributed to an increase in the incidence of listeriosis from 2001 to 2005 (Advisory Committee on the Microbiological Safety of Food, 2008). Listeriosis is caused by Listeria monocytogenes, which has a high thermal stability and is able to grow and survive at refrigeration temperatures and elevated salt levels (Zaika and Fanelli, 2003). To decrease the risk of listeriosis, a draft report of the United Kingdom’s Advisory Committee on the Microbiological Safety of Food called on the Food Standards Agency to work closely with food manufacturers to ensure that the microbial safety of food products would not decrease with changes in formulation to reduce salt (Advisory Committee on the Microbiological Safety of Food, 2008).

There is also evidence suggesting that reductions in salt might result in greater risk of toxin formation by Clostridium botulinum (the organism responsible for botulism) in certain foods if additional hurdles are not incorporated. This is particularly the case for foods that have not been heated sufficiently to inactivate C. botulinum spores and have little oxygen present. Processed cheese (Glass and Doyle, 2005; Karahadian et al., 1985), meat products (Barbut et al., 1986), and sous vide products (products that are prepared in vacuum-sealed plastic pouches and heated at low temperatures for long times¹) have been recognized as having potential for C. botulinum control problems when sodium is reduced (Simpson et al., 1995). For example, decreases in salt content from 1.5 to 1.0 percent by weight greatly reduced the time needed for C. botulinum type A and B spores to produce toxins in sous vide spaghetti and meat sauce products when stored at typical refrigeration temperatures. At salt concentrations at or above 1.5 percent, no toxin production was detected from the inoculated products during the 42-day storage period, while at 1.0 percent salt addition, toxins were produced within 21 days (Simpson et al., 1995). Similarly, turkey frankfurters inoculated with C. botulinum and held at 27°C showed more rapid toxin production when salt content was 2.5 percent than when it was 4.0 percent (Barbut et al., 1986).

In addition to C. botulinum and L. monocytogenes, the growth of other foodborne pathogens may be more rapid in foods with reduced contents of salt and other sodium-containing preservatives. These pathogens include Bacillus cereus, Staphylococcus aureus, Yersinia enterocolitica, Aeromonas hydrophila, Clostridium perfringens, and Arcobacter (D’Sa and Harrison, 2005; Reddy and Marth, 1991; Stringer and Pin, 2005).

While the pathogens described above must be taken into account, product developers and researchers have been able to accomplish sodium reductions even in products such as processed cheese and processed meats (Reddy and Marth, 1991). A number of hurdles can be added or increased when sodium is reduced to ensure that a product’s safety is maintained. Examples of additional hurdles are listed in Table 4-2. This list includes a number of emerging technologies (e.g., high-pressure processing, electron beam irradiation) that may have wider applications in the future.

Compounds, such as potassium chloride (Barbut et al., 1986) and mixtures of potassium lactate and sodium diacetate (Devlieghere et al., 2009), that might be used to replace salt and other sodium-containing preservatives have been shown to be somewhat effective at retarding growth and toxin production by pathogens. The effectiveness of alternative salts relative to sodium chloride seems to vary based on the pathogen of interest (Barbut et al., 1986).

Partially replacing salt with other compounds, such as potassium chloride and calcium chloride, may also be possible in fermented products (Bautista-Gallego et al., 2008; Reddy and Marth, 1991; Yumani et al., 1999). However, these alternatives may be less effective than salt so higher concentrations may be needed in formulations to achieve the same functionality (Bautista-Gallego et al., 2008).

Some predictive models have been developed that may be promising methods of screening new product formulations for their potential to grow pathogenic microorganisms. A large study conducted by Kraft foods (Legan et al., 2004) modeled the impact of salt on the growth of L. monocytogenes and used this modeling technique to establish no-growth formulations of cured meat products that contain lactate and diacetate to prevent growth of L. monocytogenes.

Challenges and Innovations for Lowering Sodium While Maintaining Physical Properties

The difficulty of reducing sodium without losing desirable physical properties is dependent on the specific food application and the availability of other ingredients that can fulfill similar functions. In some foods (e.g., certain cheeses and processed meats), the salt used to create special physical properties may be impossible to remove, given current technologies. As previously mentioned in the discussion of challenges to reduce sodium while maintaining food safety, reformulation has a number of costs that are described further in Chapter 6.

Still, for many products, more salt may be added than is truly needed for the desired physical property. In these cases, research to determine critical salt levels may be necessary to quantify the amount of salt that can be removed. For example, attempts to reduce sodium in natural and processed cheese products while maintaining desirable textures and achieving a safe product have been successful using new technologies, such as ultrafiltration (Reddy and Marth, 1991; Van der Veer, 1985). Similarly, in enhanced meat, some brine injection may be desirable to increase the palatability of leaner cuts of meat (Detienne and Wicker, 1999) and help consumers avoid fattier meats that are naturally more tender. However, it is likely that, for many of these products, additional brine is added to further reduce moisture loss (or purge) that normally occurs in the product during its retail shelf life. The benefit that may result from additional brine at that point may be more for economic than sensory reasons, and the brine may not be needed to create acceptable products. In other products, additional salt may be added for enhanced taste and flavor.

Table 4-3 shows the difference in sodium content of similar foods in which sodium plays a role in creating a physical property or in preservation. The varied sodium levels suggest that the sodium levels in some products may be greater than those needed for these functions. Cases such as these may provide opportunities to lower the sodium content of some foods. A similar conclusion was reached by researchers who surveyed the sodium content of processed foods in Australia and found variation in the salt concentration of comparable foods, frequently ≥ 50 percent between the highest- and lowest-salt foods within a category (Webster et al., 2010). Another survey² found differences in the salt content of the same brand name foods, including fast food restaurant items, among different countries. Many branded food manufacturers operate internationally and may participate in sodium reduction programs in other countries.

Alternatives that can replace the texture development functions of sodium are limited. However, advances in ingredient technologies have made it possible to replace some salt. Restructured and emulsified items (e.g., sausages, deli meats), for example, are products for which lower-sodium ingredient options have been identified. In these products, functional proteins (e.g., soy or milk), hydrocolloids (e.g., gums or alginates), and starches have replaced some of the functionality of the salt-soluble proteins that form a gel network and “glue” the meat pieces together in higher-salt products (Desmond, 2006). In addition, sodium tripolyphosphate, potassium phosphates, and transglutaminase have been used to improve the stability of reduced-salt emulsified meats in which there may be less salt-soluble protein available to coat and stabilize fat particles (Ruusunen et al., 2002). In their review on sodium reduction, Reddy and Marth (1991) described several studies successfully demonstrating that sodium reduction in meats could result in products evaluated to have acceptable functionality and flavor. In pork, they described a modified processing procedure referred to as emulsion coating that reduced the salt content by 50 percent in chunked and formed ham products. Successful reductions in sodium were also reported for fresh pork sausage, frankfurters, bologna, and comminuted meat batters.

Another method of reducing sodium in foods is to find alternatives to other (non-salt) sodium-containing additives. A number of alternatives have been developed. Table 4-4 provides examples (although not an exhaustive list) of alternatives to sodium-containing compounds that are often used for leavening, dough conditioning, and emulsifying.

Some industries are conducting their own research or funding universities to research alternative processing methods as another strategy to reduce sodium. For example, these approaches include use of pre-rigor mortis muscle in emulsified and restructured meat products (Desmond, 2006) and the elimination of sodium-containing emulsifying salts in certain processed cheeses (Guinee and O’Kennedy, 2007). These and other changes in processing techniques may have the potential to allow significant sodium reduction, but more research is needed to further develop and implement these technologies.

Grains

Whole grains are naturally low in sodium. Table 4-5 lists the typical sodium content of commonly consumed grains. However, a number of products made from grains have added sodium, and these products are major contributors to sodium intake.

Muscle Foods

Dairy Foods

The sodium content of selected dairy foods is listed in Table 4-8.

Sauces, Gravies, Stocks, Salad Dressings, and Condiments

As shown in Table 4-9, sauces, gravies, stocks, salad dressings, and condiments are often high in sodium. Reasons for sodium use include flavor, preservation, and improving the stability of emulsions (by improving the solubility of emulsifiers). Flavor is a main reason for adding salt to these products, and saltiness is often one of the major characteristics of these items (Hutton, 2002).

In most condiments, salt also plays a role in preservation (Brady, 2002), combined with other hurdles to microbial growth. Sodium-containing additives also may be added to salad dressings, sauces, and condiments to act as emulsifiers or preservatives. For soy sauce, which is very high in sodium, salt is needed to influence the fermentation process in its production (Doyle et al., 2001).

Fruits, Vegetables, Beans, and Legumes

Fresh fruits and vegetables are generally very low in sodium, although salt may be added to fresh produce during home or foodservice preparation. Fruits that are processed further typically remain low in sodium (Van der Veer, 1985). Frozen vegetables generally do not have additional sodium unless components such as breadings or sauces are added to the product (Van der Veer, 1985). Dried pulses (beans, lentils, peas) are naturally low in sodium but they are often salted during home and foodservice cooking.

Canned vegetables are typically much higher in sodium than their fresh counterparts. In canning, a liquid medium is important for heat transfer during processing, and a salt brine is generally used because salt enhances the consistency and flavor of vegetables (Hutton, 2002; Van der Veer, 1985). However, salt is not essential for the canning process and no-salt-added canned vegetables are marketed. One study (Jones and Mount, 2009) that tested multiple brands of five types of popular canned beans showed that draining the beans for 2 minutes reduced sodium by 36 percent, and the draining treatment plus 10 seconds of rinsing followed by an additional 2 minutes of draining reduced sodium by 41 percent. According to one survey, draining and rinsing of canned beans is a relatively common food preparation technique (Bush Brothers and Company, 2009). Other studies have shown that treatment involving draining, rinsing, and/or soaking of various canned and packaged foods results in sodium reduction (Sinar and Mason, 1975; Vermeulen et al., 1983; Weaver et al., 1984). The sodium content of selected fruits, vegetables, beans, and legumes is shown in Table 4-10.

Pickled vegetables such as sauerkraut and cucumbers are also high in sodium because of the salt added to drive the fermentation process and to maintain a crisp texture (Brady, 2002).

Mixed Dishes

Combination foods, such as pizza, soups, stews, casseroles, and ready-to-eat meals, are usually high in sodium, as shown in Table 4-11. Sodium in these foods comes from many sources and has multiple functions; when combined into a single serving, the sodium from these varied sources can easily contribute significant levels to the total diet. Pepperoni pizza is a good example of this because each of the major ingredients contains sodium. The pepperoni has sodium for preservation, meat binding, and flavoring. Sodium in the cheese contributes to texture and preservation as well as taste and flavor. Tomato sauce is seasoned with salt in addition to other herbs and spices. Finally, the crust contains sodium to control the leavening process and dough stickiness. The combination of these ingredients leads to an average sodium content of 668 mg/100 g, according to FDA’s Total Diet Study market basket data (FDA, 2007).

Soups are classic examples of complex, high-sodium foods. Some soups have high-sodium ingredients, such as cheese or sausage. However, even foods made from low-sodium ingredients, such as vegetables, are high in sodium due to the use of salt for flavoring. In soups, salt contributes not only to salt taste, but also to overall flavor, as discussed in Chapter 3 (Gillette, 1985; Rosett et al., 1997).

In chilled foods, sodium-containing compounds can play a role in preventing the growth of pathogens. Vacuum and modified-atmosphere packaging can create oxygen-free environments that favor the growth of Clostridium botulinum. Salt, in addition to other hurdles, can help prevent the growth of this organism. If oxygen is present, Listeria monocytogenes is often a concern because it can grow even at low temperatures. Salt addition can serve as one hurdle to the viability of this organism (Hutton, 2002).

Refrigerated or frozen meals often contain sauces or gravies. Besides contributing flavor, these sauces have a secondary role of preventing or masking warmed-over flavors. The fats in precooked meats have a tendency to experience lipid oxidation upon storage, resulting in rancid and “painty” odors and flavors (Hedrick et al., 1994). Using strongly flavored sauces can help to mask these flavors, and coating meats in sauces before storing can help to exclude the oxygen needed for these reactions to take place (Kuntz, 2000). Unfortunately, the sauces are often high in sodium.

Savory Snacks

Most savory snacks, including chips, nuts, pretzels, popcorn, French fries, and extruded snacks (cheese balls, shaped potato snacks, etc.), have added sodium in the form of salt. The function of salt in these foods is to contribute to salt taste and overall flavor. For many flavored snack products, salt is used to distribute minor ingredients, such as flavors and colors. Mixing minor ingredients with salt before application can help to ensure even distribution of these components over the surface of the snack (Matz, 1993). In fried products, antioxidants may also be incorporated in these mixtures to prevent the development of rancidity (Ainsworth and Plunkett, 2007). The sodium content of selected savory snacks is shown in Table 4-12.

Secondary functions of sodium in some extruded products are to modify texture and color. Extruded products have a puffy texture and the degree of expansion and airiness has been found to change with the salt concentration of the extrudate and is thought to be due to interactions between salt and starch (a main component of these snacks). Color has also been found to change with salt content, and this relationship has been proposed to be due to the ability of salt to change the water activity of the extrudate and thus change the rate of browning reactions (Ainsworth and Plunkett, 2007).

Confections

As shown in Table 4-13, hard candies are generally low in sodium, and other confections may have low levels of sodium-containing leavening or texture-modifying agents (Saulo, 2002). Dairy-based confections will contribute to sodium intake due to the sodium naturally present in milk. Chocolates may also contain small amounts of sodium to contribute to flavor and texture. Some confections are likely to contain added salt for flavoring purposes, particularly those with fillings, such as crèmes or jams (Van der Veer, 1985). Other confections that may include salt for flavoring purposes are caramels, taffy, and nut-containing candy.

Beverages

Water is relatively low in sodium, but sodium levels vary by water source and with the use of water-softening systems (Bradshaw and Powell, 2002; Pehrsson et al., 2008). Tea and coffee are also very low in sodium, although the level may increase slightly with the addition of milk and cream.

Sodium-containing preservatives are sometimes added to carbonated beverages and fruit drinks (Doyle et al., 2001). Even though these beverages contain sodium, the levels are generally low compared to those of many other solid food items.

The vegetable juice category of beverages is one in which sodium levels are traditionally quite high. Taste and flavor improvements are the reasons for addition of salt to tomato, carrot, and vegetable blend drinks. The sodium content of selected beverages is shown in Table 4-14.

Salt is often present in sports drinks for the stated purpose of rehydrating the body during or after physical activity, although the medical justification for the sodium contained in these drinks under the conditions consumed (e.g., high school sports activities) is not clearly demonstrated (Jeukendrup et al., 2009; Shirreffs et al., 2007). While no data on the sodium content of sports drinks was available from the Total Diet Study, data from USDA’s National Nutrient Database³ suggest that such drinks contain 100 mg or less per 8 oz. (240 mL) serving. It is reported that the sodium in these products is not added for taste or preservative effects (Man, 2007).