What is the significance of water activity (aw) in food preservation, and how is it measured and controlled?

Water activity (aw) is a measure of the free moisture content available for microbial growth and chemical reactions in a food product. Controlling water activity is crucial for preventing microbial spoilage and maintaining food quality. It is measured using a water activity meter and controlled through techniques such as dehydration, adding salt or sugar, or modifying packaging materials.