Water activity is a crucial factor in determining shelf life. The measurement of water activity makes it possible to predict which microorganisms will and will not be sources of potential spoilage.

Temperature, pH, and other factors have an influence on if and how fast organisms will grow in a product. However, water activity (a_w) may be the most important in controlling spoilage. For example, most bacteria will not grow at water activities below 0.91and most moulds cease to grow at water activities below 0.80.

Water activity, not water content, determines the lowest limit of available water for microbial growth. As well as influencing microbial spoilage, a_w can also determine the activity of enzymes and vitamins in foods. This can have a major impact on colour, taste and aroma.