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Effective vs. Ineffective Preservation Using Water Activity*
By: David Steinberg, Steinberg & Associates
Posted: January 4, 2011
page 3 of 7
However, it is practically impossible to calculate water activity, therefore it is far better to measure it by instrumentation. The foods industry has used water activity for many years to determine the need for preservatives.
There are several ways to measure water activity including: vapor pressure manometry, electric hygrometry, hair hygrometry and dew point. The Association of Official Analytical Chemists (AOAC) has officially recognized the dew-point/chilled mirror method.
The approximate aw needed for growth of various microorganisms was published around 1960. While bacteria requires 0.94-0.99 aw for growth, yeast needs >0.7 aw and mold needs >0.6 aw for growth.
The specific aw needed for growth of the five common test organisms are: 0.77 aw for Aspergillus niger, 0.86 for Staphyloccous aures, 0.95 aw for Escherichia coli, 0.97 for Pseudomonas aeruginosa and 0.87 for Candida albicans.
It should be noted that some molds can grow with water activities as low as 0.7, so a good rule is to keep the water activity below 0.7 in order for the product to not need the addition of chemical preservatives.