What does water activity (Aw) measure?

Water activity (Aw) specifically measures the amount of water that is available for use by an organism. It is a critical factor in microbiology and food science, as it influences the growth of microorganisms and the stability of food products. Aw is defined as the ratio of the partial vapor pressure of water in a sample to the vapor pressure of pure water at the same temperature. A higher water activity means that more water is available for biochemical reactions and cellular processes, which is essential for the survival of organisms.

In contrast, while temperature, oxygen levels, and concentration of solutes can influence water availability and are related to water properties, they do not directly measure the availability of water to organisms. Each of these factors has its own significance in different contexts, but Aw specifically focuses on the potential for water to facilitate biological activity. This is why the measurement of water activity is particularly important in fields that deal with food preservation, spoilage, and microbial growth.