Ask the Experts; May 2006; Scientific American Magazine; by Mickey Parish, Rick Watling; 1 Page(s)
Salt (usually sodium chloride) and sugar (generally sucrose) interfere with microbial growth in several ways to block decay in food.
The most notable way is through simple osmosis, resulting in dehydration. The salt or sugar, whether in solid or dissolved form, attempts to reach equilibrium with the salt or sugar content of the food product with which it is in contact. This action has the effect of drawing available water from within the food to the outside and inserting salt or sugar molecules into the food interior. The result is a reduction of the so-called water activity (aw), a measure of unbound, free water molecules in the food that are necessary for microbes to survive and reproduce. The aw of most fresh foods is 0.99, whereas the aw required to halt growth of most bacteria is roughly 0.91. Yeasts and molds usually survive with even lower amounts of water.