If there is “too much” water in a food product, there is a risk of microbial growth & water migration. This can lead to clumping, changes in consistency & reduced shelf life. Manufactures of food & pharmaceutical products measure water activity to show when the water activity of their product has declined to a point where bacteria can no longer grow. This type of measurement is also a good indicator of when the product is ready for packaging. Water Activity is expressed as “aw”. It is actually a ratio between the vapor pressure of the food itself when in a completely undisturbed balance with the surrounding air media & the vapor pressure of distilled water under identical conditions. A Water Activity of 0.80 means that the vapor pressure of the food is 80% that of pure water.
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