Water activity is widely recognized for the safety it provides to food because of its control of microbial growth and it is considered a critical control point for preventing microbial contamination. Water activity provides this same determination of safety in cannabis and consequently is already a part of cannabis regulations in many states. However, at this point, it remains primarily associated with edibles, linking it back to food safety. While water activity is important for the safety of cannabis edibles, it is also important for storage stability of harvested buds and extracts. If cannabis buds are not dried to water activity levels below 0.70 aw, they can support mold growth during storage and transport, leading to the subsequent potential for inhalation of mold spores or mycotoxins. In addition, water activity controls the rate of decarboxylation that can lead to the breakdown of tetrahydrocannabinol (THC). Finally, water activity is correlated to additional critical product quality attributes that are associated with shelf life. In fact, it is possible to predict shelf life based on water activity and to establish an ideal water activity specification for any cannabis or hemp-based products. The objective of this manuscript is to explain the theory and measurement of water activity, explain its mode of action for microbial control, discuss existing water activity regulations for cannabis, and describe additional uses for water activity in the cannabis industry.
With the legalization of cannabis-based products for both medicinal and, in some locations, recreational use, the need to implement safety initiatives has also arrived. Microbial contamination in either dried buds, extracted oils, or processed edibles can result in allergic reactions, respiratory complications, or foodborne illnesses. In addition, breakdown because of chemical reactions can result in changes in efficacy and potency. Water activity is an effective tool used in the food and pharmaceutical industries to maximize microbial, chemical, and physical stability. Water activity provides this same safety and control to the cannabis market; therefore, it is important that cultivators and processors understand water activity and how to maximize its usefulness. Safety regulations for the cultivation and processing of cannabis-based products are currently handled at the state level, resulting in inconsistent recommendations. As a result, not all states currently require water activity testing of cannabis. However, based on its established relationship with common safety and quality modes of failure, it should be the most important analytical test run by anyone in the cannabis market. The objective of this manuscript is to describe the theory of water activity, outline its current inclusion in state regulations, and describe its impact on microbial and chemical stability.
Theory of Water Activity
Water activity is defined as the energy status of water in a system and is rooted in the fundamental laws of thermodynamics through Gibb’s free energy equation (1). It represents the relative chemical potential energy of water as dictated by the surface, colligative, and capillary interactions in a matrix. Practically, it is measured as the partial vapor pressure of water in a headspace that is at equilibrium with the sample, divided by the saturated vapor pressure of water at the same temperature. The water activity covers a range of 0 for bone dry conditions up to a water activity of 1.00 for pure water, resulting from the partial pressure and the saturated pressure being equal. Water activity is often referred to as the “free water” and while useful when referring to higher energy, it is incorrect since “free” is not scientifically defined and is interpreted differently depending on the context. As a result, the concept of free water can cause confusion between the physical binding of water, a quantitative measurement, and the chemical binding of water to lower energy, a qualitative measurement. Rather than a water activity of 0.50 indicating 50% free water, it more correctly indicates that the water in the product has 50% of the energy that pure water would have in the same situation. The lower the water activity, the less the water in the system behaves like pure water.
Water activity is measured by equilibrating the liquid phase water in the sample with the vapor phase water in the headspace of a closed chamber and measuring the equilibrium relative humidity (ERH) in the headspace using a sensor (2). The relative humidity can be determined using a resistive electrolytic sensor, a chilled mirror sensor, or a capacitive hygroscopic polymer sensor. Instruments from companies such as Novasina utilize an electrolytic sensor to determine the ERH inside a sealed chamber containing the sample. Changes in ERH are tracked by changes in the electrical resistance of the electrolyte sensor. The advantage of this approach is that it is very stable and resistant to inaccurate readings because of contamination, a particular weakness of the chilled mirror sensor. The resistive electrolytic sensor can achieve the highest level of accuracy and precision with no maintenance and infrequent calibration.
While water activity is an intensive property that provides the energy of the water in a system, moisture content is an extensive property that determines the amount of moisture in a product. Water activity and moisture content, while related, are not the same measurement. Moisture content is typically determined through loss-on-drying as the difference in weight between a wet and dried sample. While useful as a measurement of purity and a standard of identity, as this paper will describe, moisture content does not correlate as well as water activity with microbial growth, chemical stability, or physical stability. Water activity and moisture content are related through the moisture sorption isotherm.
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Brady P. Carter is a Senior Application Scientist with Neutec Group Inc., in Farmingdale, New York. Direct correspondence to: [email protected]
How to Cite This Article
B.P. Carter, Cannabis Science and Technology 2(4), 30–35 (2019).