Why Does Salt Preserve Food?

When salt is applied to food (or dissolved in a brine), it creates a hypertonic environment — one with higher solute concentration outside microbial cells than inside. Through osmosis, water flows out of the microbial cells to equalise the concentration, dehydrating and killing them (plasmolysis). Salt also reduces the water activity (aw) of food — a measure of how much moisture is available for microbial growth. Most spoilage bacteria need water activity above 0.91, and most pathogenic bacteria above 0.85. Heavily salted foods like salt cod or cured ham have water activity below 0.75, making them inhospitable to nearly all dangerous microorganisms. Salt curing has been used for at least 5,000 years — salt was so valuable for food preservation that Roman soldiers were partly paid in salt ("salarium," the origin of "salary"). Combined with smoking, nitrates, and cold storage, salt remains essential in modern food preservation.