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Preventing moisture mayhem

December 3, 2019  By Gagandeep Singh Bhatoa and Mohyuddin Mirza

Cannabis growers know very well that molds can develop at different stages of the plant’s life cycle.

Different molds and mildews can spoil crops at any stage of growth. They include powdery mildew, botrytis and others. The most serious ones are the molds which can attack buds and render them unsellable. Molds can cause product recalls and result in economic losses. It is therefore vital to understand water activity inside the cells.

Growers often do physical examination of harvested flower buds. However, this may not show any sign of microbial infestation. Outdoor grown plants are likely to get mold infection if there is ample exposure to moisture. However, this can be kept under control by natural checks.

When indoor plants are exposed to inappropriate watering, humidity, fertilizers and ventilation, it results in high risk of mold threat. Inadequate curing and leftover water content can also encourage microbial growth later on. When moisture levels are too high after harvesting, mold growth is likely to occur on cannabis flower either during the drying and curing process or during packaging for distribution. Buds are most likely to get attacked a week before or a week after the harvest.


Handle with care
Handling during harvesting and trimming creates significant opportunity for pathogens, which can be easily transferred to flowering stage. However, depending on climatic conditions, harvested cannabis can carry a microbial load with diverse, potential spoilage-causing organisms, which include bacteria, yeasts, and filamentous fungi belonging to different genera.

Growers must sufficiently dry the harvested flowers in a cool environment (temperature 180 – 220C with RH 35 per cent) to prepare them for storage and transport.

Storing in a wet and humid environment encourages the growth of microbes and increases volatilization and chemical reactions, including degradation of THC and CBD.

Microbial contamination due to filamentous fungi and bacteria is one of the possible reasons for post-harvest losses in cannabis, with the end-product mycotoxins potentially inducing subsequent consumer health issues.

Quality assurance
Moisture content is the total amount of water contained in the harvested flower or expressed as percentage of the total weight. It can be measured by using moisture analyzer or by weight of water loss before or after drying the cannabis sample.

Measuring moisture content in cannabis determines the effectiveness of drying and curing techniques, which also affect the terpene concentration as well as chemical profile of cannabis. The moisture content of adequately cured cannabis flower should be less than 13 per cent.

Nevertheless, measuring water content in the product does not offer enough information to determine safety and product shelf life. Water activity is the key determinant for microbial spoilage.

Water activity (aW) is the measurement of available free moisture in cannabis or those not chemically bound to the cannabis product. It’s the ratio of vapour pressure (p) in cannabis bud to the vapour pressure of pure water (P0) at the same temperature. Water activity value ranges from 0.0 aW (absolute dried sample) to 1.0 aW (pure water).

aW= P/P0

If we multiply this ratio by 100, we obtain the Equilibrium relative humdity (ERH) that cannabis would produce if enclosed with air in a sealed container at constant temperature.


Thus, a stored cannabis sample that has 0.65 aW would have 65 per cent relative humidity. It has been recommended that dried cannabis should be stored in sealed container with water activity below 0.65aW.

Growers often mistakenly assume that a higher moisture content in the product means a higher water activity – but it’s not always correct. It is possible to have low moisture content in the product and have water activity that is more than 0.65 aW.

Previous research have proven that higher water activity in the plant product means availability of unbound water molecules that act as potential source of food for microbial growth, resulting in spoilage.

Measuring water activity in cannabis is key in determining how vulnerable the cannabis product is to microbial contamination. Mitigating water activity reduces the chance of microbial growth and allows the product to be stored for extended periods.

Gagandeep Singh Bhatoa is a plant health care specialist in Winnipeg. Email him at Moyhuddin Mirza, PhD, is chief scientist with the Cannabis Nature Company in Edmonton, and a consultant with the cannabis industry. Email him at

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