Short AnswerBasically, salt works by drying food. Salt absorbs water from foods, making the environment too dry to support harmful mold or bacteria.
Long AnswerSalt draws water out of cells via the process of osmosis. Essentially, water moves across a cell membrane to try to equalize the salinity or concentration of salt on both sides of the membrane. If you add enough salt, too much water will be removed from a cell for it to stay alive or reproduce. Organisms that decay food and cause disease are killed by a high concentration of salt. A concentration of 20% salt will kill bacteria. Lower concentrations inhibit microbial growth, until you get down to the salinity of the cells, which may have the opposite and undesirable effect of providing ideal growing conditions!
What About Other Chemicals?Table salt or sodium chloride is a common preservative because it is non-toxic, inexpensive, and tastes good! However, other types of salt also work to preserve food, including other chlorides, nitrates, and phosphates. Another common preservative that works by affecting osmotic pressure is sugar.
Salt and FermentationSome products are preserved using fermentation. Salt may be used to regulate and aid this process. Here, salt dehydrates the growing medium and acts to maintain fluids in the yeast or mold growing environment. Un-iodized salt, free from anti-caking agents, is used for this type of preservation.
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