Quality Control: Water Activity Considerations for Beyond-use Dates.

There are minimum levels of water activity that are required for growth of bacteria and molds. Water activity is the ratio of vapor pressure of water in a product to pure water at the same temperature. Pure distilled water has a water activity of 1. Water activity is also a measure of water in a material that is available to react with or attach itself to other material; it is also called "free" water while the unavailable water is called "bound" water. Higher water activity substances tend to support more microorganisms; bacteria usually require water activity values of at least 0.91 and fungi at least 0.6. Every microorganism has a limit of water activity below which it will not grow. To lower water activity, one can add or change the concentrations of ingredients such as sodium chloride, sucrose, alcohol, propylene glycol, or glycerin so the preparation becomes self-preserving. Also, if the preparation has too high a water activity and the addition of other substances to lower the water activity, then a preservative may be required for the preparation. Nonaqueous liquids or dry solids will not support spore germination or microbial growth due to their low water activity.