Dr Brady Carter on maintaining green coffee quality through water activity control.
Coffee beans are harvested as a coffee cherry that is then processed to remove the fleshy cherry covering to produce green coffee beans. These green coffee beans are the form in which the coffee is stored and transported. When it is time to produce coffee, the green beans are roasted to create the familiar roasted coffee beans that are purchased at a shop or utilised at a coffee house. Coffee quality is tracked through cupping score, with speciality coffees expected to score 80 or higher.(1) Unfortunately, finding that a sourced coffee bean does not cup well may be too late as the beans have already been purchased. It would be preferrable to be able to conduct tests on the green beans themselves at the time of harvest to predict the cupping quality. Water activity has been investigated as a potential tool to predict the cupping quality of sourced coffee beans.
WATER ACTIVITY AND CUPPING QUALITY
The value of tracking water activity of green coffee beans is not in a direct correlation to cupping score. Instead, the value of water activity is in providing an optimal range for storage to avoid microbial contamination, control both degradative and beneficial chemical reactions, and maintain metabolic activity. The recommended optimal water activity range for stored green coffee beans is 0.45-0.55 aw.(2) Drying and storing green coffee beans at this water activity range will not improve the cupping quality of the green coffee beans, but instead will maintain the quality for 6-8 months.
WHY LESS THAN 0.55 aw
The main reason for using 0.55 aw as the upper limit for stored green coffee beans is to prevent microbial growth. The growth of all microorganisms stops at water activities less than 0.60 and would therefore not grow on green coffee beans that are stored below 0.55 aw. An additional reason for maintaining green coffee beans at water activities less than 0.55 is to limit the rates of potentially harmful chemical reactions. In general, as water activity increases so do reaction rates and keeping water activity below 0.55 aw will slow reaction rates enough to minimise their effects during storage.
WHY GREATER THAN 0.45 aw?
The main reason for not allowing the water activity of stored green beans to drop below 0.45 aw is to maintain the viability of the green seeds including the enzymatic activity that is critical to maintaining the expected flavour and aroma profile for a coffee. Green coffee beans at water activities less than 0.45 aw will potentially lose their viability, resulting in aged flavours when the coffee is roasted. In addition, at water activities less than 0.45, the surface of green coffee beans can potentially become more rigid and brittle, resulting in poor grinding performance in preparation for roasting.
Like most products, green coffee is sold on a weight basis, so maximising the amount of water, the cheapest ingredient, that can be in a product while remaining safe and stable will maximise profitability. The ideal water activity range for storing green coffee beans has been identified and moving above or below this range will render the product undesirable.
Dr Brady Carter is an application scientist with Novasina.
(1) Galova, V. 2020. https://www.roestcoffee.com/roestblog/how-to-evaluate-the-quality-of-your-coffee-part-3-cupping
(2) Agudelo, Y.M. 2021. https://dailycoffeenews.com/2021/06/23/an-introduction-to-water-activity-in-green-coffee/