Product safety is emerging as a key purchasing factor for cannabis consumers. More than ever they’re seeking products and brands they can trust. That’s why it’s surprising that water activity as a moisture measurement method isn’t well known in the industry, despite the fact that many are familiar with moisture content as an important quality concept.
Being able to measure and communicate the water activity of your products throughout the supply chain can help to earn trust and distinction in the market. Recently, the ASTM Cannabis Committee set out to establish water activity as the primary moisture measurement method that producers need to be concerned with.
Water activity (Aw) tells you how much water “is free” or unbound. “Free” water is H20 that’s available for reactions, such as for fueling growth of microorganisms. Historically experts and connoisseurs alike typically tout 9% to 12% moisture content as the acceptable range in dried flower, but now there is a more specific measure that’s been established by international industry scientists. The best, science-backed level is set forth in ASTM D8196 – 18, the Standard Practice for Determination of Water Activity in Cannabis Flower.
The ASTM standard has an acceptable water activity range of 0.55 to 0.65. Microorganisms (including mold) have difficulty thriving below 0.65 Aw. Cannabis above 
this level is at risk for microorganism growth. Water activity below 0.55 raises quality concerns. For example, below 0.55 Aw key compounds in the flower, such as terpenes, can dry out, compromising the flavor and aroma. Water activity is used in many cases as a critical safety control point for Hazard Analysis and Critical Control Points (HACCP) programs. Controlling moisture should not be seen as a kill step, but rather as a control focused on preventing the growth and proliferation of microorganisms.
Do you have a means to measure water activity? Here at Cann Drying Systems, we use a tool that one of our medical marijuana customers recommended. The Rotronic Hygrow Palm AW is easy to use, and costs around $3,300. Our Rotronic meter comes with a convenient mobile carrying case. The Rotronic meter also has the ability to store data sets into bins to be downloaded and accessed later. With this meter, our engineers can get accurate sample results in less than 10 minutes. We use the hand-held meter to measure whether a cannabis drying cycle in one of our low temperature dehumidification systems is complete and meets the industry standards and specs.
Not only is moisture an important metric for quality and safety related to mold prevention, it can also have an effect on the end value of the product. Pricing in the cannabis industry is primarily based on weight. The more moisture a product has the more value it will retain once it goes to market. A goal with drying is to get the product as close to an Aw of .65 without going much below to maximize the profit of each grow. Water activity is also important because it can affect the laboratory analysis of cannabis, as well as other concepts related to THC levels and regulations.