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United States Department of Agriculture

Agricultural Research Service

PMP FAQs - Water Activity
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1.  What are the consequences of calculating water activity based on a known level of NaCl that is added to the product?

Water activity of food is a measure of the level of water that is not tightly bound to the food matrix, and that is available for bacterial growth. This value varies from 0 to 1, with most hazardous foods being in the range of 0.85 to 0.99. This value is based on all of the salts, not just NaCl, that may be in the food matrix. Consequently, if you add say 1% NaCl to a food, you can enter this value in the PMP input box. However, the calculated water activity level is a reflection of this level of NaCl and not that of the total level of salts that are in a food. For this reason, we advise that you measure the water activity of your product and then change the value in the input box to match this value.

Water Activity (aW) may be calculated from the NaCl concentration. It is presented for convenience and is not used in any calculations based on NaCl level. At high water activities (> 0.974 = 4.5% NaCl), the effect of different humectants on microorganisms is approximately equal. If the water activity of a food is known, the equivalent NaCl level can be entered into the model.


Last Modified: 1/20/2006
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