Free water versus bound water. Water activity describes the continuum of energy states of the water in a system. These terms are easier to conceptualize, although they fail to adequately define all aspects of the concept of water activity. A portion of the total water content present in a product is strongly bound to specific sites on the chemicals that comprise the product. These sites may include the hydroxyl groups of polysaccharides, the carbonyl and amino groups of proteins, and other polar sites.
Water is held by hydrogen bonds, ion-dipole bonds, and other strong chemical bonds.
Many preservation processes attempt to eliminate spoilage by lowering the availability of water to microorganisms. The processes used to reduce the amount of free water in consumer products include techniques like concentration, dehydration and freeze drying. Freezing is another common approach to controlling spoilage. Water in frozen foods is in the form of ice crystals and therefore unavailable to microorganisms for reactions with food components. Because water is present in varying degrees of free and bound states, analytical methods that attempt to measure total moisture in a sample don't always agree.