one of the best methods for inhibiting the growth and survival of undesirable microorganisms. Although modern-day advances in food storage and packaging techniques and the speed of transportation have largely diminished this role, salt does remain in widespread use for preventing rapid spoilage (and thus extending product shelf life), creating an inhospitable environment for pathogens, and promoting the growth of desirable microorganisms in various fermented foods and other products. Other sodium-containing compounds with preservative effects are also used in the food supply.
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).