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Preservatives
Effective vs. Ineffective Preservation Using Water Activity*
By: David Steinberg, Steinberg & Associates
Posted: January 4, 2011
page 2 of 7
aw = P/Po = (n2/(n1 + n2))
where P is the vapor pressure of the product, Po is the vapor pressure of pure water, n1 is the number of moles of solute and n2 is the number of moles of water.
Pure water has an activity of 1.00 while something considered “bone dry” is 0.00, which is numerically equivalent to 1/100 of the relative humidity (RH) generated by the product in a closed system. RH can be calculated from the direct measurements of partial vapor pressure or dew point or indirect measurements by sensors that are altered by being exposed to RH.
Therefore, the relationship between water activity and the equilibrium relative humidity (ERH) is expressed by the following formula.
ERH (100%) = aw X 100
