ABSTRACT
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).