Salt as microbial inhibitor

Salt is an excellent microbial inhibitor, mainly due to its suppression of the water activity of the material to which it is added. Its effectiveness is enhanced when the food is also dried or smoked or both. Smoking also imparts a partial preservative effect.

The early food uses of salt were for the purpose of preserving meats. This use is based on the fact that at high concentrations, salt exerts a drying effect on both food and microorganisms.

Salt can act in several ways to inhibit microbial growth and thus preserve the food. Perhaps most important, it reduces the amount of water available to micor0rganism for growth processes. Water activity (aw) is term used to express numerically degree to which water in foods is available to microorganisms.

The use of salt as a preservative serves to inhibit microbial growth by reducing the water activity and increasing the osmotic pressure of the food.

Salt (saline) in water at concentrations of 0.85-0.90% produces an isotonic condition for non-marine microorganisms. Because the amounts of NaCl and water are equal on both sides of the cell membrane, water moves across the cell membranes equally in both directions.

To damage microbial enzymes, NaCl must gain access to the intracellular pool. Enzymes can be damages in a variety of ways by high ionic environment and perhaps by NaCl specifically. Many of those involved in the preservation effect of NaCl are involved with cellular recovery from stress.
Salt as microbial inhibitor

Sodium chloride as food additive

Common salt or sodium chloride, is not only the most frequently used salt, but is also the most common food additive in food processing. When sufficient salt is added to food, it makes water unavailable to microorganisms.

Sodium chloride decreases the water activity of foods, thus helping to control the growth of pathogenic bacteria. Sodium chloride this used as a preservative in meats and is necessary to make fermented products.

Since microorganisms require water to survive, they cannot exist when their water requirement is diminished by the addition of salt. Amount of water can be reduced availability to microorganisms by lowering the water activity (aw).

Water activity is an excellent stability factor for microbial growth, because the creation of osmotic pressure difference between the hydrated microbial cell and the surrounding food leads to cessation of microbial growth.

Microorganisms require high levels aw.  Most bacteria require a minimum aw level of 0.96, although halophilic bacteria can grow at a aw of 0.75.

Most yeasts grow at aw levels of 0.90 and above, although a few can grow at a aw level of 0.81.  Mold grow at lower aw levels with some able to grow at a aw level of 0.62.  While salt preserves foods mainly by lowering the aw, the chloride ion is believed to inhibit bacterial growth independently.
Sodium chloride as food additive