The term "water activity" usually applies to the shelf stability of food, but now Colorado marijuana growers are required to test their plants for the same attribute. The requirement, already part of contaminant testing in other states' legal pot industries, was implemented in late July by the Colorado Marijuana Enforcement Division to keep marijuana free of mold, yeast and contaminants that can still pop up after pot leaves a growing facility.
Marijuana plant products such as flower, leaves and kief, a form of extracted trichomes from the plant, contain a certain amount of water after harvest. And two types of water are left over: bound water and free water, according to Jenevieve Klink, analytical scientist for state-certified marijuana testing facility AgriScience Labs.
"Bound water is water chemically bound to components, whereas free water is free for microbial growth," Klink says. "People want to know how shelf-stable their products are, so water activity is essentially the amount of free water in a sample. The higher the water activity, the lower the shelf stability."
While marijuana's shelf stability worsens with higher water activity, the retail and inventory packaging at the store also play a factor. Explains AgriScience lab director James Crisler: "If you don't package it right, there could be a moisture exchange with the outside environment, which could lead to higher microbial activity."
In 2019, the City of Denver conducted a survey of the flower inventories at 25 random dispensaries. Despite the products passing post-harvest microbial testing at the state level, 80 percent of the stores in the survey were carrying marijuana that tested positive for mold when the Denver Department of Public Health and Environment did its tests. Most of the marijuana that failed had been stored "deli-style," or in large amounts that were then weighed out by the order. Since then, more cultivators have transitioned to individual pre-packaging to lessen the moisture content.
Commercial marijuana in Colorado is currently tested for pesticides, mold and yeast, heavy metals, potency and water activity, while concentrates are also tested for residual solvents; vaporizer products have their own standards to meet. The AgriScience team expects that there will be a slight learning curve for growers in order to conform to the new water-activity testing rules, but expects the majority of samples to be fine.
The water-activity tests were one of two new testing requirements for legal marijuana products implemented this year, along with pesticide testing for marijuana concentrates. In 2022, vaporizer products will have to undergo emissions testing to gauge the amount of heavy metals in marijuana concentrate vapor. The new rules largely come out of the MED's science and policy rulemaking groups, which meet throughout the year to discuss developments in the pot industry.
"It's an important balance of risk, public health and cost of business," says Kevin Gallagher, executive director of the Colorado Cannabis Manufacturers Association. "I think what we have right now is adequate as we compile data."
Gallagher would like to see more shelf-stability testing for concentrated products such as hash and vaporizer cartridges, with hopes of seeing expiration dates on all marijuana products one day.
"We want to make sure that these can still pass testing a year after hitting the shelf," he adds.
Update: This story was updated Sunday, September 4.