Freeze-dried to fresh-brewed, coffee and tea are manufactured in many different ways. Moisture has always been a consideration in determining how long to dry or roast these products. However, measuring only moisture in tea and coffee leaves a lot to be desired. If a buyer of Ethopian 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?
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. If you are used to referencing moisture numbers, no problem. The AquaLab Duo was designed to give you both moisture and water activity from one sample in about 5 minutes.
Although most tea and coffee products can't support microbial growth, that's not always true. For example, in 2005, the EU adopted regulations placing limits on ochratoxin A (OTA), a harmful mytoxin 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.