We recently purchased a moisture meter and have been monitoring the moisture content of our coffees. We record the moisture content of samples of the coffee at reasonably regular intervals. At the moment we are at the data collection stage and are not drawing too many conclusions. We are interested to see what happens to our green beans over a period of time, and to what extent the moisture content stays within the parameters outlined above. At the very least we hope to identify any downward trends and to take action before the quality of the beans deteriorates. In the short time we’ve been taking readings, it has made us reflect on how we view the notion of seasonality.
One aspect of seasonality relates to the moisture content of the green beans. Following processing, green coffee beans are dried to a specific moisture content. One of our current coffees, Gatuyaini for example, is dried to a moisture content of between 10 and 12% before it is shipped. This is pretty typical of a speciality coffee: the Speciality Coffee Association of America defines a standard for speciality coffee as having a moisture content of 10–12%, with 11.5% the optimum.
Over time the coffee is likely to lose its moisture content and as it dips below 10%, aroma, acidity and freshness begin to fade, pretty much disappearing by the time the coffee reaches a moisture content of 8%. In practice, the longer we hold on to the coffee, the more likely it is to lose moisture, and this will have a negative impact on the roasted bean. In addition, as the coffee dries, it will roast differently: the heat will transfer quicker through the bean resulting in quicker roasts. It should also be noted that green coffee beans with a moisture content above 12% may be susceptible to mould development, and at higher moisture contents still, the beans become unstable – they readily lose moisture.
Storage conditions can also have an impact on the moisture content in the coffee. If the coffee is stored in a location where the humidity in the air is too low, some of the moisture will leave the bean. The water molecules in the bean will have bonded to other particles, such as some of the desirable volatiles, and as the moisture leaves the coffee, so do these desirable substances. Similarly if the humidity in the air is too high, the beans are likely to absorb moisture and this may bring with it undesirable substances which taint the bean.
For the majority of coffees we’ve used over the past months, the above makes sense. Their moisture content remains stable and comfortably within the 10–12% range. However, there have been a couple of notable exceptions. Earlier in the year we used a coffee from Bolivia, Tres Estrellas. The sample tasted good and the purchase price reflected our high expectation of the coffee. We did not have access to the moisture meter when the coffee first arrived so were only able to start monitoring about a month in. We were surprised to have readings in the low 9% range, which was also reflected in the cup quality. I never felt this coffee lived up to expectations and it didn’t have the brightness we’d come to associate with other coffees from Bolivia.
Contrast that with another coffee we were using at the time, Regalo de Dios. We picked this coffee up very late in the season based upon the sample we tasted. In addition we probably over committed to this coffee which meant we were still using it much later than we would have liked. The moisture content of this coffee remained consistently high, around the 11% mark, despite being around 12 months after harvest. The coffee also cupped very well and proved to be a popular addition to our range. One explanation for this is the increased use of GrainPro bags to protect the coffee within the outer hessian sack, though the Bolivian example above was also shipped in GrainPro.
These experiences also introduced us to the notion of water ativity, which is the energy status of water in a system. Whereas moisture content is concerned with the total water in a coffee bean, water activity is concerned with the degree to which that water is bound at a cellular level. Of the two coffees highlighted above it may be reasonable to hypothesise that they have different water activity levels: the water activity in the Tres Estrellas being higher, thus making it succeptible to greater overall moisture loss. We have yet to invest in the technology to measure water activity though I have a feeling it will only be a matter of time before we do. In the meantime we will continue to monitor the moisture content of our coffee, adding a quantitative indicator to accompany that tried and trusted subjective measure, taste.