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Water activity is the ratio of the water vapour pressure above a food to that of pure water, equal to equilibrium relative humidity divided by 100. Raoult's law estimates it as moles of water divided by total moles of water plus dissociated solute. Below 0.6 no microorganism grows; below 0.85 most pathogenic bacteria cannot.
Raoult's law
aw = n_water / (n_water + n_solute x dissociation factor)
From ERH
aw = equilibrium relative humidity % / 100
Water activity is a critical food safety parameter. Estimates must be confirmed by calibrated measurement for any shelf stability or HACCP decision.
Microorganisms respond to available water, not total water. Two products with the same moisture content but different solute levels have very different water activities and shelf lives.
Staphylococcus aureus grows down to about 0.86, most moulds to 0.80, xerophilic moulds to 0.61 and Zygosaccharomyces rouxii to about 0.62. Below 0.60 nothing grows.