The ERH of a food products is defined as the relative humidity of the air surrounding the food at which the product neither gains nor loses its natural moisture and is in equilibrium with the environment.
Microorganisms vs. water activity value
The definition of moisture conditions on which pathogenic or spoilage microorganisms cannot grow is of paramount importance to food preservation. It is well known that each microorganism has a critical water activity beloe which growth cannot occur. For instance, pathogenic microorganisms cannot grow at water activity < 0.68; yeast and moulds are more tolerant and usually no growth occurs at water activity < 0.62. The so-called intermediate moisture foods (IMF) have water activity in the range of 0.65-0.90 ( Figure 3.1 )