What is the significance of water activity (aw) in controlling microbial growth in food products?

Water activity (aw) is a measure of the amount of unbound, or free, water available in a food product for microbial growth and chemical reactions. It ranges from 0 to 1.0, with pure water having an aw of 1.0. Microorganisms require water to grow and multiply, and each species has a minimum aw level below which it cannot survive. Controlling aw is a crucial strategy for preventing microbial spoilage and ensuring food safety. By reducing the aw of a food product below the minimum level required for the growth of specific spoilage organisms or pathogens, their proliferation can be inhibited, thus extending the shelf life and minimizing the risk of foodborne illness. For example, bacteria generally require a higher aw (above 0.85) than yeasts and molds (which can grow at aw levels as low as 0.60). Methods to reduce aw include drying, adding solutes like salt or sugar, and freezing. Jams and jellies, for example, have a high sugar concentration, which lowers the aw and prevents bacterial growth, even at room temperature. Measuring and controlling aw is therefore essential in food processing to ensure product safety and stability.