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Water Activity Testing

Water activity testing measures the amount of freely available moisture within a cannabis product — the water that microorganisms such as mold and bacteria can access and use for growth. Unlike moisture content (total water by weight), water activity directly predicts microbial stability and product shelf life.

Maintaining appropriate water activity levels is critical for dried cannabis flower, edibles, and certain infused products. It is a required testing parameter for some product categories under Minnesota's regulatory framework.

What Is Water Activity?

Water activity (aw) is a dimensionless scale from 0 to 1 that represents the ratio of the vapor pressure of water in a substance to the vapor pressure of pure water at the same temperature. Pure water has a water activity of 1.0; completely dry products approach 0.

Microbial growth requires available water. Most mold species cannot grow below an aw of approximately 0.70, and most bacterial pathogens require aw values above 0.85–0.90. Cannabis products with water activity above regulatory thresholds are considered at risk for microbial proliferation during storage or distribution.

For dried cannabis flower, regulatory frameworks typically establish a maximum water activity threshold. Products exceeding this threshold may fail testing regardless of microbial test results, because the conditions for microbial growth are present even if organisms have not yet colonized the product.

How Water Activity Is Measured

Water activity is measured using a water activity meter, which places a sample in a sealed chamber and equilibrates the headspace to the sample’s vapor pressure. The instrument measures the relative humidity of the equilibrated headspace using a chilled mirror dew point sensor or capacitance sensor, and reports the result as an aw value.

Measurement is non-destructive and relatively rapid compared to incubation-based microbial methods. Results are reported as a numeric aw value and compared against applicable regulatory thresholds.