Water activity: a better moisture metric for coffee and tea

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.  Water activity provides an alternative moisture measurement that is much more precise and is tied to standards that guarantee both parties are looking at the same thing.

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.

Learn more about water activity and coffee

Wendy Ortman, Aqualab research scientist, and Ian Fretheim, Café Imports, discuss the role of water activity in coffee quality.

Topics covered include:

• Water activity vs. moisture content
• Basics of water activity
• How to measure water activity
• Water activity in roasted coffee
• Water activity in green coffee
• Coffee beans and respiration
• How water activity influences coffee quality
• Fats absorbing tastes/odors
• Chemical changes
• Loss of organic material
• Water activity, moisture content, and scores
• How water activity extends shelf life
• How water activity affects quality during drying and storage