Water activity
also: aw, a_w
A 0 to 1 measure of how available the water in green coffee is, affecting shelf life and stability.
Water activity (written aw) measures how freely available the water inside green coffee is, on a scale from 0 to 1, rather than how much water there is in total. That distinction matters: two lots can share the same moisture content yet behave differently in storage because microbes and chemical reactions only respond to water that is actually available to them.
Why it matters: water activity is the better predictor of microbial growth and staling. Lower aw means water is more bound up and less available, so the coffee resists mold and degrades more slowly. Most quality green coffee falls roughly in the 0.50 to 0.60 range; readings creeping toward 0.65 and above signal a higher spoilage risk and a shorter shelf life.
You will not taste water activity by name, but its effects show up as how long a green lot holds quality before turning flat, baked, or musty (see how-long-do-beans-stay-fresh). Many specialty buyers now measure aw alongside moisture as a routine quality check to catch defect risk early.