The water activity is the energy of the water present in an organism. This sounds complex, but to understand it in relation to our green grading report we only need to know it is measured on a scale of 0 to 1 - with 1 being pure water, and our acceptable range being from 0.5-0.65.Anything lower can be a sign of an old crop, or too long a drying period. Anything higher can be caused by bad or uneven drying, or moisture absorption after drying (e.g. in the warehouse).
This reading should also never change. A change in water activity usually precedes a change in moisture content, and can serve as an early warning sign of bad storage conditions or ageing. We know from our own observations that, before a coffee dies, the water activity suddenly goes up for a short period. Which is a last warning sign - but it should never come to that in our case.
A water activity meter (the coffee beans sit under the black part on the right)