What was for decades a relatively obscure scientific offshoot borne of the 1940’s processed-food revolution, the measurement of water activity (Aw) is now known in the specialty coffee industry as being an important tool in assessing the potential shelf-life of green coffee. However, while its efficacy is appreciated in origin and roaster labs alike, it can be complicated to understand. Perhaps you know that your coffee should measure between the unitless digits of 0.5-0.6 to be deemed of sufficient quality, but what does that really mean, and why is Aw considered so important?

At Caravela, we measure Aw along with the total moisture content of beans at various stages while we process the coffee we buy. When coffee is delivered to our purchasing warehouses, Aw is among the first properties we check before continuing with any sensory analysis. If the measured Aw and total water content do not sit within our stable ranges, the coffee is cupped but not bought. Aw and moisture content are checked again in our main labs at each origin and also before the coffee is exported, as part of the quality assurance process.

Caravela started measuring Aw at the end of 2012, initially with water activity readers in Colombia, El Salvador and the US, with the goal of collecting data from the coffee we were buying to see how this related to the way the coffees behaved over time. By the second harvest in 2014, all of our buying stations in Colombia had Aw readers, as we realised that the technology was the most accurate predictor of coffee longevity, resulting in Caravela adopting Aw evaluation as one of our green coffee purchasing criteria. In 2015, all origins that we were working in had embraced Aw measurement, including Nicaragua, El Salvador, Colombia, and Ecuador. Since then it has become an integral part of our QC evaluation protocol at origin as well as at our importing offices, where offer samples are checked before being forwarded to customers, and arrival samples are measured upon availability.

So, what really is Aw and how does it differ from coffee moisture content? Aw can be described as the state of energy of water in the coffee, which is to say the amount of water inside the bean that is readily available to react with other elements. It is measured on a scale from 0.00 to 1.00, with 1 being the absolute ‘wetness’ of distilled water, and zero being devoid of any moisture (think chalk). A water activity reading of 0.50 represents an equilibrium between the two ends of the scale, and Caravela’s acceptable range for green coffee lies between 0.50 – 0.60; the closer to 0.50, the better. An excess of loosely-bound water – high Aw – means that the water molecules want to leave the bean, coming and going as they please. This not only makes the coffee very inconsistent flavour-wise but can also damage the coffee to the extent that it is rendered unusable if not roasted rapidly.

As a jute bag contains over 35,000 beans, blending and packing coffee all within stable ranges is essential as well, as it minimizes the possibility of great coffee being compromised by coffee beans with high Aw. Once mixed and packed, coffee with higher Aw will raise the Aw of coffee possessing a lower measurement. Through data collection we have seen that when the difference is small and within the stable range this potential issue is dramatically reduced.

Not all water inside the bean is like this, however; at a given temperature some water is bound securely, meaning it cannot react with other elements. Moisture content is a measure of this bound water as well as the active water, the total percentage of water inside the coffee. For us the stable range for moisture content is between 10-11%. Although not identical, Aw and moisture content are linked, and measuring both gives us greater opportunity to identify any irregularities. We have found that our respective ranges of acceptability serve as a target equilibrium of sorts, and that coffee which sits outside of one of these ranges is normally the result of bad drying practices.

If coffee is not dried evenly it can arrive at a point where even though total water content is between 10-11%, the water activity can exceed the desired range, pushing towards 0.70 or higher. This is frequently the result of coffee that has been dried too fast, for example coffee that is fully sun-dried at high temperatures, usually in concrete patios or parabolic patios with very little airflow. Parts of the bean remain wetter than others, which also means that although the total water content of the bean might be between 10-11%, much of this total water content will exist in a loosely-bound state. For us, this was a key discovery, as it yielded new guidelines for producers to dry at lower temperatures, for longer amounts of time (i.e., 15-20 days versus 3-7 days), using raised beds to ensure more even airflow, shade nets to control and maintain more stable temperatures throughout the day, plus a consistent movement of the beans. When done properly, this type of drying enables producers to more easily achieve results in water activity and total moisture content within the stable ranges, providing better shelf life for roasters but also means that coffee farmers end up with a better-tasting, more marketable product.

The effects of temperature change can also be somewhat mitigated through having Aw standards. As temperature increases so does the Aw in green coffee, which can create difficulties when transporting coffee, as temperature is guaranteed to fluctuate on the journey from farm to warehouse to port, and on to destination, and this often cannot be controlled. However, the effect of temperature change on Aw is reduced when the coffee in question possesses water activity and total moisture content within the stable range.

There is much about coffee purchasing that is beyond the buyer’s control; we do our best to mitigate as many of these factors at origin as possible. This is why we only accept coffee within 0.50 to 0.60 for Aw and 10-11% for total moisture content, all the way from parchment purchase to exportation as green coffee. Collecting data on Aw and total moisture content is what initially enabled us to make recommendations to farmers regarding the best way to dry their coffee, and it is now an integral component of PECA team’s agenda. Our agronomists work diligently to educate producers on how to perfect their drying practices. A better drying protocol is a small adjustment that can have a huge impact for a farmer in terms of the quality of what they sell. For producers, it offers a more sustainable livelihood, as well-dried coffee that resides within the required ranges, is much more likely to be bought at a higher price. For roasters, coffee will be more consistent, from pre-shipment sample through the exporting process, until it is finally enjoyed by the consumer.