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Testing the Water: The Top Techniques for Moisture Content Analysis In Cannabis

By Alexander Beadle

Published: Jul 25, 2019    Last Updated: Jan 13, 2023
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When it comes to testing cannabis, one of the most important variables worth knowing is the water content of the flower in question.

For example, take one of the most common types of cannabis tests: cannabinoid content analysis. Usually, cannabinoid levels are expressed as a percent by weight; so if a 10-gram sample of cannabis is tested and it contains 2 grams of tetrahydrocannabinol (THC), that will be then taken as a THC content of 20 percent.

While this looks like a fairly straightforward calculation, changes in water content between batches of flower can make these results inaccurate, and its presence (or absence) can have a significant effect on the overall weight of the sample which is used in calculating the THC content.

Imagine two cannabis flower samples, both containing exactly the same amount of THC, but one sample has been completely dried out. The dried sample will be lighter overall on account of having less water weight, and so will be calculated as having a higher THC percentage than the second sample, despite the numerical amount of THC being equal in both samples.

To account for this, companies submitting cannabis samples for testing are increasingly being encouraged to submit samples that have been cured and dried to a point that is equivalent to the water content, or moisture content, the flower is expected to have at the point of sale. In order to do this, producers need to be able to determine the moisture content of their product at every stage of the process, from the harvest, until it sits on a dispensary shelf.

As well as moisture content testing being generally good practice for businesses to ensure that their advertised cannabinoid levels are accurate in their products, knowing the moisture content of a specific sample can also be helpful to processors who want to examine their curing process, or extractors evaluating the extraction efficiency of their operations.

In addition to having industrial relevance, moisture content testing is now starting to become a mandatory part of the routine testing process in some areas of the country.

California requires all cannabis to undergo moisture content testing and for the result to be printed on the sample’s official certificate of analysis. In Nevada, moisture testing for all processed cannabis and cannabis resin is required, and the products must return a moisture content reading of less than 15 percent in order to “pass” the test.

A cannabis moisture meter: moisture content testing

The most common method used to determine the moisture content of a cannabis sample is also the simplest: weighing the flower then completely drying it out and measuring the difference.

For a relatively low-tech version of this method, cannabis processors can use a low-temperature oven. Using an accurate balance, a small amount of cannabis would have its weight recorded. Then, the sample would be placed in the low-temperature oven to slowly dry out, which is normally taken to mean dried until the flower visually crisps up, and is reweighed. The oven-dried weight divided by the weight of the original sample would then give the moisture content of the plant as a percentage.

Bench-top moisture analyzers are also commonly used as an easily repeatable and reproducible way of measuring moisture content. They operate on the same principle of using heat to dry out the cannabis sample and normally feature an in-built heat source and balance that can take weight and temperature measurements throughout the drying process, and are able to automatically calculate the moisture content of a given sample, thus removing any human error that basic oven drying might introduce. These sorts of moisture analyzers are already used routinely in the food and drink industry for determining the moisture content of long-shelf-life products, and are known to be fast and reliable pieces of equipment.

A less commonly used, but reportedly effective technique, is the use of a wood moisture meter. The method, as created and reported by Ed Rosenthal of Cannabis Now uses a wood moisture meter with a touch sensor and two small detector probes. By setting the meter to the “softwood” setting and pressing a piece of cannabis material firmly against the touchpad, Rosenthal says that he is able to get a near-immediate readout of the sample’s moisture content.

“And even if the actual number isn’t strictly accurate, as long as it’s consistent, it can be used as a standard,” writes Rosenthal in his piece. “If you have tested the meter against two subjective but accurate tests and the meter reads [the same], all is well.”

This is clearly not a commercial method but could be used by a home cultivator (in regions where home cultivation is legal) who wants to dry and cure their own cannabis. Rosenthal also provides a short list of moisture content percentages associated with different qualities of smoke, which would also seem to imply that this guide is for somebody curing their own cannabis for personal taste, not for a commercial operation.

What’s the difference between water content and water activity?

Water content is an important metric to know when calculating cannabinoid content and evaluating your processing techniques, but it also used to be considered paramount to product safety. The thinking was, the higher the moisture content, the greater the risk of microbial infection and mold and fungal growth.

But recently, there has been a shift away from associating microbial growth risk with moisture content, and towards an association with water activity.

Water activity (Aw) is a thermodynamic concept that describes how “tightly bound” the water’s available energy is, and is measured on a scale from 0 to 1, where zero would indicate a completely dry sample devoid of all water, and one would be equivalent to pure liquid water.

There are several handy ways to visualize the difference between moisture content and water activity. For example, a water-soaked sponge would have a moisture content of almost 100 percent, given the weight of the water versus the light sponge, but its water activity would be significantly below 1.0, as much of the water would be “bound” to the structure of the sponge. The sponge would have to be squeezed to return that bound water back into its free state.

“Moisture content refers to the total amount of water that something has in it, while water activity refers to how easy it is to actually access or use the water that is present,” explains Dane Oberhill, an analytical chemist with the Colorado-based cannabis testing facility PhytaTech. “A low water activity value means that the water present is bound on the molecular level and you must input energy to break those bonds and free the water.”

In 2017, ASTM International, a leading global standards organization, formed a committee on cannabis to evaluate and develop standards relating to cannabis and its safe processing. At the time, water activity was not a very widely known or used moisture measurement within the cannabis industry, but with the release of its first cannabis standards, specifically, a standard detailing standard practice for determining water activity, the organization labeled the concept as “the only moisture measurement method that producers need to be concerned with.”

This is because, as Oberhill explains, “a high water activity value means that moisture content is unbound and can be more easily accessed, making the water a potential food source for things such as microbes, which could lead to spoilage.”

Related to this, the committee has published a standard for acceptable water activity ranges for cannabis flower, in which it lists a water activity of 0.65 as a maximum limit before mold will start to grow and contaminate the cannabis. It also notes that a water activity of below 0.50 can cause some of the terpenes in cannabis to dry out, which can result in the flower having little smell or flavor.

Generally, the committee recommends maintaining the water activity level in cannabis somewhere between 0.55 to 0.65 Aw to minimize the risk of mold or fungal infection while preserving the quality of the flower.

This recommendation is already being reflected in regional legislation, for example, California’s cannabis regulations require all dried cannabis to test out at under 0.65 Aw and all solid edibles under 0.85 Aw, to “pass” testing and be allowed to be sold on the state’s legal market.

Measuring water activity in cannabis

In practice, water activity can also be related to relative humidity when the cannabis is held within a sealed container. This is because an equilibrium can be established between the liquid phase water in the sample and the vapor phase water in the air (or “headspace”) above the sample.

This relationship is exploited by water activity measurement methods, which use the properties of this headspace to characterize a sample. Common secondary methods (requiring calibration) include the use of electric hygrometers, which determine water activity using capacitance or resistive electrolytic methods.

1. Capacitance hygrometers are made up of two charged plates acting as electrodes, separated by a polymer membrane dielectric material. As the dielectric material absorbs moisture from the air, there will be a corresponding increase in the sensor’s ability to store an electric charge – its capacitance. By calibrating the sensor against known salt standards, it is possible to relate these changes in capacitance to the relative humidity at equilibrium, and thus to the water activity of the sample.

2. Resistive electrolytic hygrometers are similar to capacitance hygrometers in that they both interpret changes in electric signals with air humidity to calculate water activity, but here the main component of the sensor is a liquid electrolyte. As the electrolyte gains or loses water vapor as humidity fluctuates, its resistance will fluctuate in turn. By calibrating or verifying the electrolyte resistance read-out against known salt solutions at equilibrium, it is possible to determine the water activity of the relevant sample.

A primary method of measuring water activity

An alternative to these methods is the chilled mirror dew point hygrometer, which relies solely on fundamental thermodynamic principles to calculate the water activity of a sample.

The dew point is defined as the temperature at which air (at a constant pressure) reaches saturation with water vapor. At temperatures cooler than the dew point, the vapor will begin to condense onto nearby surfaces. Chilled mirror dew point hygrometers use a temperature-controlled mirror, a light beam, and a photodetector to create a feedback loop that keeps the mirror at a temperature such that the rate of dew condensation exactly equals the rate of the dew evaporation, thus maintaining a dynamic equilibrium with the water vapor pressure of the air in the sample. This allows for an extremely accurate determination of the dew point temperature of the gas.

Using this, the partial vapor pressure of the water in the headspace above the sample (denoted as P) can then be easily calculated from first principles. By measuring the temperature of the sample, it is also possible to calculate the saturation vapor pressure (P0) of the sample.

As water activity can be defined mathematically using the equation Aw=P/P0, it is possible to accurately calculate the water activity of a sample using this method.

The chilled mirror dew point method is accurate to around ±0.003 Aw and can provide a water activity measurement in as little as five minutes, making it an extremely popular analysis method in the industry. The analyzers themselves are also usually small bench-top devices that are simple to use, and capable of storing information and automatically making calculations.

This method, as well as the secondary capacitance and resistive methods, are already routinely used outside of the cannabis industry; for example, the food and drink industry uses water activity measurements in a similar way to the cannabis industry in order to reduce risk of mold growth on food and to forecast the shelf-life of products.

Having trusted and well-established methods in place that allow cannabis producers and processors to accurately measure the moisture content and water activity of their products is a strong line of defense against poor labeling and microbiological contamination in cannabis products. 

Alexander Beadle

Science Writer

Alexander Beadle has been working as a freelance science writer since 2017 and has covered the cannabis industry for Analytical Cannabis since 2018. He has also written for our sister publication, Technology Networks, and the cannabis industry consultant firm Prohibition Partners, among others. Alexander holds a Master's in Materials Chemistry from the University of St. Andrews, where he won a Chemistry Purdie scholarship, and conducted research into zeolite crystal growth mechanisms and the action of single-molecule transistors.


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