All microorganisms must have an abundant supply of water to grow. Perishability of a food is related to the moisture content, and the water activity level.
Moisture content is the amount of water in food expressed as a percentage. Water activity (aw) is the amount of water available for use and is measured on a scale of 0 to 1.0.
Bacteria, yeast, and molds multiply rapidly with a high water activity level, above 0.86. Meat, produce and soft cheeses are examples of foods with aw in this range (between 0.86 and 1.0).
Foods preserved with salt or sugar, such as beef jerky or jams and jellies have a lower aw because salt and sugar deprive microorganisms of water and inhibit their reproduction. These products are shelf-stable (i.e. they do not need refrigeration, unless opened).
Pathogenic bacteria have difficulty growing in foods such as dry noodles, flours, candies and crackers, where aw is below 0.85.
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