Drying Cannabis: Is There a Science Behind the Art?

It’s Fall and farmers around the country are harvesting their crops and preparing them to be consumed, stored, or processed. For outdoor cannabis farmers, it’s the season of drying and curing the flower they spent the past 6 months growing. For indoor and greenhouse cannabis growers, drying cannabis happens year-round. Drying cannabis is the first step in post-processing the crop after it’s harvested. Drying the flower is critical to preventing spoilage and helps prepare it for curing, extraction, lab testing, storage, and eventually consuming.

Water Activity

When drying food and agricultural products such as cannabis, the most important factor to consider is water activity. Water activity (aw) is defined as the ratio between the vapor pressure of water in a material to the vapor pressure of pure water at the same temperature and pressure. Unlike moisture content, which is the amount of water contained in a material, water activity is the free moisture and equal to the equilibrium relative humidity(1). Water activity allows chemical, physical, microbial, and enzymatic activity to occur. Therefore, it is one of the most important factors for food safety, quality, and maintaining shelf-life.

The water activity stability diagram is widely used to understand the water activity in which degradation occurs (Figure 2). Most bacteria cannot grow below a water activity of 0.9, spoilage molds below 0.7, and all microorganisms cannot grow below 0.6. Keeping the water activity below 0.6 will also inhibit lipid oxidation, which changes the chemical profile of oils, such as terpenes, and can cause rancidity in foods.

The ASTM International Cannabis Committee recommends during Drying and Curing to keep water activity levels between 0.55 and 0.65, or an equilibrium relative humidity (RH) of 55-65%. Therefore, by controlling the Drying Room’s relative humidity, you are ultimately controlling the water activity! Allowing a Dry or Cure room to climb above 65% RH puts the flowering product at risk for microbial growth. However, if the Drying Room is kept too dry (less than 50%) the flower terpenes can dry out, reducing the intensity of the smell and taste.

Moisture Content

But what about moisture content? Most growers we talk to are targeting 11-14% at the end of the drying cycle. These values pass the grower’s “snap test” and have been demonstrated through many years of trial and error to produce a quality dried flower product that meets the standards of both consumer and lab. Health Canada requires 9% moisture content at the end of the drying cycle, a controversially low MC that many claim reduces shelf-life and boosts %THC and %CBD content reported by lab tests. This moisture content may also allow for a little rehydration of the flower before it becomes susceptible to spoilage, an important factor to consider if the 60% RH cannot be maintained during cure, transport, or packaging. On the other hand, if you cultivate cannabis in a dry climate like Arizona or Colorado, you may want a higher moisture content to prevent the flower from desiccating below 9% during those post-harvest activities. Perhaps local climate and the controllability (or lack thereof) of the post-processing environment is why we see such a wide range of targets?

Unfortunately, there is no published scientific research (yet) that correlate the moisture content of dried cannabis flower to water activity, curability, shelf-life, or the quality of smoke to establish clear targets and test these claims. However, there have been studies done on hops, a dried flower product that has a similar morphology to its cannabis cousin. Those studies(2) have demonstrated that a final moisture content of about 9% is equivalent to a water activity of 60%. Perhaps that’s where Health Canada got it’s standard? But hops is not cannabis and research is needed to help growers and regulators alike find the moisture content best-suited for its final use and post-harvest handling.

Managing Water Activity

Measuring water activity can be challenging as there aren’t many reliable sensors available. Luckily, because water activity and relative humidity of the air are closely correlated, we can be confident that if the RH is around 60%, that microbial activity will be inhibited. Therefore, when we design HVAC systems for Drying Rooms, we target a maximum 60% relative humidity throughout the drying period. In the first day or two, we need a lot more dehumidification to achieve this target, as the plants are still filled with water after they’re harvested. But after a couple of days, it’s a slow burn to the end of the drying cycle as plants release less and less water and the 60% RH is easier to maintain with less dehumidification.

Water activity is an important concept used not only in the drying of cannabis, but also food processing and storage, pharmaceuticals, and electronics. By controlling water activity to around 0.6, we can minimize the growth of microbes and mold spores while optimizing the physical properties, chemical content, and quality of the product we’re drying. A relative humidity of 60% is equivalent to a water activity of 0.6; therefore, targeting 60% RH in the Drying Room is critical to preventing spoilage. If it’s too high, mold and bacteria could grow. It it’s too low, the final product quality may be compromised. At the end of the drying cycle, the dried cannabis flower should have a moisture content ranging between 9-14%, depending on local climate, post-drying activities, and local regulations.

Tell us about your experience with drying cannabis. What final moisture content and relative humidity are you targeting? Have you ever measured water activity?

1. Relative humidity is defined as the ratio of vapor pressure of water in air and the saturation vapor pressure of air at the same temperature and pressure.

2. Henderson, S.M. and G.E. Miller Jr. 1974. Hop Drying Details and In-Bale Quality. University of California Cooperative Extension. Davis, CA.

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