What scientific measure, distinct from moisture content, indicates a food's preservation potential by measuring available water for microbes?

Answer

Water activity ($a_w$)

Water activity ($a_w$) is the scientific measure related to how much water is available in a food item for microorganisms to use for growth; preservation methods aim to lower this value.

What scientific measure, distinct from moisture content, indicates a food's preservation potential by measuring available water for microbes?
foodstoragepreservation