Moisture has always been a consideration in determining how long to dry or roast coffee or tea, but measuring only moisture in tea and coffee leaves a lot to be desired. If a buyer of Ethiopian coffee specs green beans at 12% moisture, someone will probably check her shipment before it’s sent. But how does she know that her 12% and her supplier’s 12% are equivalent?
Water activity is a better moisture metric
Measuring the water activity of coffee and tea gives more reliable results for two reasons. First, water activity readings are based on known standards, allowing two instruments on different continents to give equivalent readings. Second, products like tea (1-2% moisture) have so little water in them that loss-on-drying methods aren’t very precise. Manufacturers used to referencing moisture numbers can use the Series 4TE Duo water activity meter. It was designed to provide both moisture and water activity from one sample in about 5 minutes.
Fight fungi with water activity
Although most tea and coffee products can’t support microbial growth, that’s not always the case. For example, in 2005, the EU adopted regulations placing limits on ochratoxin A (OTA), a harmful mycotoxin produced by fungi that grow on green coffee beans. Water activity can predict whether ochratoxin A or other mycotoxins can be produced by microorganisms growing on coffee. There is no correlation between moisture content and microbial growth–only water activity can be used for this application.
Predict how storage will affect quality
Increasingly, importers and roasters are using water activity to predict how well green coffee will store. A certain water activity can indicate that the coffee will deteriorate quickly and should be sold and roasted sooner for best results.