Understanding Water Activity
Measuring the water activity in a cannabis product is an excellent way to test how susceptible the product is to microbial contamination. The higher the measured water activity is in a product, the more freely water can be used by microbes as a food source or to support chemical and enzymatic reactions leading to spoilage.
In other words, the higher the water activity value, the more vulnerable cannabis products are to microorganism growth.
Water activity is a thermodynamic measurement that describes how tightly bound the water’s available energy is. The results can range from 0.000, which would be the measurement of a dry sample devoid of water content, to 1.000, which would be the measurement of pure, liquid water.
Water activity (Aw) is defined as the partial vapor pressure of water in a substance (P) divided by the saturation vapor pressure (P0), also known as the standard state partial vapor pressure of water:
Aw = P/P0
Water activity can range from 0.000, which would be the measurement of a dry sample devoid of water content, to 1.000, which would be the measurement of pure aqueous water free of any bonding forces.
Water activity is measured by obtaining the saturation vapor pressure and the vapor pressure of water in the sample at a specific temperature. Common instruments take these measurements from a small sample of the product that is inserted into a water activity meter (the instrument labs have to measure water activity).
First, the meter measures the temperature of the sample, which directly correlates to the saturation vapor pressure (P0). Once vapor equilibrium is established, an infrared beam focuses on a small mirror in the instrument’s chamber to measure the vapor pressure of the water in the sealed headspace above the sample tray. This equilibrium vapor pressure equals the vapor pressure of water in the sample (P).
These two measurements are then used to calculate the water activity (Aw). Measuring water activity in this manner can usually be done in just a few minutes. Accuracy can be determined by measuring calibration standards (salt solutions of varying concentrations) with known water activity values.
— Dane Oberhill and Stephen Goldman
To control spoilage, it is recommended to have a water activity value of less than 0.600.
Most enzymatic activity is inactive at values below 0.850, and values below 0.750 prevent the growth of most bacteria. But a water activity value below 0.600 greatly inhibits all growth and cellular activity, including yeast, molds, fungi, bacteria, enzymes and other chemical moieties that could lead to spoilage.
Some state regulators are currently establishing water activity limits for the cannabis industry. For example, California and Oregon limit flower products to a value of less than 0.650 water activity, while California has an additional cutoff of 0.850 for cannabis-infused edibles.
It’s important to note that water activity is not to be confused with total water content (percent moisture), which is a measurement of the total amount of water present in a material. While a sample can have very low moisture content, the water activity can still be greater than 0.600 if the water is not energetically bound, making the sample susceptible to spoilage. This makes water activity more important and relevant to cannabis and food spoilage than total water content.
As an example, consider a sponge dipped into a bowl of water. Without the sponge, the water in the bowl would have a water activity close to or at 1.000. It would be freely available for movement and binding. The water’s availability could be demonstrated by dipping the sponge into the bowl so that the water actively moves from the bowl into the sponge.
Once the water is in the sponge, its water activity would be much lower than 1.000, since it is bound up by the sponge, making it not as readily available. To remove the water from the sponge, you would need to actively squeeze it, utilizing energy to make the water once again accessible. While in the sponge, the water has a higher boiling point, lower freezing point and lower vapor pressure.
Again, it is water activity, not total water content, which is more relevant to food or product shelf life. Further understanding of this concept can be applied to product formulations to extend their longevity. By utilizing certain ingredients and methods that can bind up the water, it is possible to reduce the water activity and decrease the chance of microbial growth.
With lower water activity, products can be stored for extended periods at room temperature without needing to worry about such things as microbial growth, browning and lipid oxidation reactions, chemical stability or physical product characteristics.
Different methods may be implemented to reduce the water activity of a product. One of the most effective methods is to remove water content through drying. Other methods include adding ingredients such as sugars or salts to cannabis products, which can bind up the water and reduce its availability to be used by microorganisms.
For example, let’s look at cannabis-infused cookies. Let’s say these cookies, when measured, had a water activity value of 0.905. These cookies would be susceptible to microbial growth due to the amount of freely available water and would not pass some states’ standards, such as those in California. But if more sugar were mixed into the cookie dough recipe, some of that freely available water would be bound to the sugar molecules and effectively reduce the water activity. Other methods can also be tried, such as adding more salt or baking the cookies longer to dry out more water.
Building tables of different ingredient effects on water activity during product development is a good industry practice. That way, all failing products can be quickly mitigated with predictable results leading to passing products and full dispensaries.
Dane Oberhill is an analytical chemist for PhytaTech in Colorado. He has a bachelor’s degree in chemistry from Michigan State University and was scheduled to earn his master’s degree in environmental health and safety from the University of Denver in March 2018.
Stephen Goldman is the lab director of PhytaTech in Colorado. He is an analytical chemist with extensive industrial and academic laboratory experience. Prior to joining PhytaTech, he served as an analytical chemist at Forensic Laboratories, overseeing toxicology testing.