Understanding Water Stability
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 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.
FOR EXAMPLE, let’s look at cannabis-infused cookies.
Let's say a dozen cookies, when measured, had a water activity value of 0.95. 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.