Water activity is measured using an Aqualab DUO 4TE, the industry standard for dew point moisture analysis. WA state compliance samples will no longer require weight % for cannabis flower to be recorded. USP <1112> outlines water activities required to support microbial growth.
Q: What’s the difference between % moisture and water activity?
A: % moisture is the weight percent of water in the sample and water activity is how much is on the surface. The analogy I like to use is your body is ~70% water (% moisture) but the water activity is similar to how much sweat is on your skin at any given time. This measurement (aw units) is used to predict the likelihood of bacteria growing on the surface and is commonly used in the food industry. Do you ever wonder why cookies and crackers don’t grow mold/bacteria colonies immediately in your cupboard? They have very little water activity on the surface, so bacteria and fungi are less likely to grow. The same goes for any surface, including cannabis bud; the more moisture on the surface, the more likely bacteria and fungus can survive there and the sample might fail those tests.