The importance of regulating moisture in green coffee is well known. Improper drying causes multiple defects, from mould found in coffee that isn’t dried enough, to poor storage qualities for coffees that are dried unevenly. However, to measure and monitor these effects, green coffee professionals are moving away from just measuring moisture content as a percentage, towards a measure called ‘water activity’ (also written as Aw).
Water activity is a relatively new measurement in speciality coffee, though it is well established in other parts of the food industry. Rather than measuring the total amount of moisture in a sample, it measures how tightly bound the water is in the sample. In a sample with a high water activity, the water is less tightly bound and can therefore participate in chemical reactions or other processes more easily. It is water activity rather than moisture content that has been shown to affect, for example, how quickly food goes mouldy or stale.