Water Activity

The water activity (aw) of a food describes the energy

status of the water in the food (1). Water activity, not

moisture content, determines the lower limit of avail-

able water for microbial growth. Controlling water

activity is important to maintaining the chemical

stability of foods (2) due to its availability to act as a

solvent, medium, or reactant in chemical or biochem-

ical reactions. Nonenzymatic browning reactions and

spontaneous autocatalytic lipid oxidation reactions are

strongly influenced by water activity, as well as playing

a significant role in determining the activity of enzymes

and vitamins in food. Finally, aw plays an important

role in physical properties such as texture and in the

shelf life of foods. This chapter defines water activity, describes its

effect on the microbial, chemical, biochemical, and

physical properties of foods, and discusses measure-

ment methods. The authors discuss the advantages and

disadvantages, accuracy, repeatability, speed of mea-

surement, stability of calibration, linearity, and con-

venience of a variety of measurement methods (3).