Moisture Content: How Much Water?
Moisture content describes the amount of water in a sample, commonly reported as a percentage. Check whether a result uses a wet-weight or dry-weight basis before comparing it with another report.
For example, 10% moisture on a wet-weight basis means water accounts for 10% of the sample's total mass.
Water Activity: A Different Property
Water activity, written as aw, describes water's energy state rather than its total quantity. It is a dimensionless value generally expressed between 0 and 1.
Two samples can contain similar amounts of water yet have different water activity because water interacts differently with each material.
How Relative Humidity Fits In
At equilibrium in a sealed measurement chamber, water activity equals the relative humidity above the sample divided by 100, at the same temperature. An equilibrium reading of 60% relative humidity therefore corresponds to a water activity of 0.60.
This example explains the relationship; it is not a recommended target. A room humidity reading alone does not establish the water activity of flower.
Why Both Measurements Matter
Moisture content helps describe drying and material composition. Water activity helps assess moisture movement and conditions relevant to microbial growth. Neither number alone establishes that a batch is safe, contaminant-free, or compliant.
Packaging and storage conditions can change moisture over time. A measurement describes the sample tested under the conditions recorded; it does not guarantee future shelf life.
Reading a Result Carefully
- Confirm the sample or batch identification and test date.
- Distinguish a moisture percentage from a water activity value.
- Check the method, reporting basis, and measurement temperature.
- Consider the result alongside other batch documentation, rather than as a substitute for it.
Further Reading
Background measurement science: FDA: Water Activity in Foods and AQUALAB: Water Activity and Moisture Glossary. Food-science examples explain the measurements; they do not establish cannabis testing requirements or imply FDA approval.
Learn about batch testing and COA documentation.
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