Water Activity Deep-Dive — The Isotherm, the Meter & the Release Gate
Why aw (not moisture %) governs mold, how the sorption isotherm links the two, and a bench protocol for reading aw correctly enough to defend a batch release.
Moisture is inventory; water activity is availability
Moisture content answers how much water is in the flower (9–13% wet basis at spec). Water activity (aw) answers the only question mold cares about: how much of that water is chemically free to fuel microbial metabolism, versus bound to cell walls, sugars, and resins. aw is a unitless ratio from 0 to 1 — the equilibrium relative humidity of the air in a sealed headspace around the sample, divided by 100. Flower at 0.60 aw sitting in a closed jar will drive the headspace to 60% RH. That equivalence is why your in-jar hygrometer and a lab aw meter are measuring the same physics from two ends.
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