How does water activity affect cannabis and hemp?

Water activity is a description of how ‘bound’ water molecules are, chemically and structurally, in a system. The less tightly bound a water molecule the more likely for microbial growth, i.e. growth of mold. For example, curing meat by salting it. When meat is salted, the water molecules and salt molecules bind together meaning there is less ‘free’ water molecules to mold the food. Salt cured meat would therefore have a low water activity and a longer shelf life.

Water Activity relationship with humidity

Relative humidity is the ratio of the vapor pressure of water in the air compared to its saturated vapor pressure. This assesses the amount of water the air contains compared to how much it theoretically could contain. If the temperature of the sample and the air are in equilibrium the relative humidity of the system is equal to its water activity multiplied by 100%. So a sample that has a aw=.78 would have a relative humidity of 78%.

Temperature dependence of Water Activity

Water activity is the comparison of two vapor pressures at equal temperature; that of the liquid water in the material and pure water vapor in the chamber. If these two vapor pressures are measured at a temperature difference of one degree Celsius an error of up to 0.06 could result in the water activity. To minimize this error, the sample and the chamber must be as near temperature equilibrium as possible. Water activity meters contain a fan to circulate air and promote this equilibration.

What is Moisture Content?

Moisture content is the physical water weight in a sample. This is commonly determined by a loss on drying method. A representative amount of sample is weighed, then the sample is dried. After drying, the sample is measured by dry weight. The difference between the two weights is the moisture content by weight.

How do Moisture Content and Water Activity work together?

The higher the moisture content of a sample, the higher the water activity as there are more water molecules present and available. The inverse is also the same, the higher the water activity the more percentage by weight the water occupies in the sample.



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