Drying coffee in a Guardiola is similar to drum roasting coffee in that it involves pushing moving air through the drum, which leaves the air temperature generally hotter than the bean temperature. The maximum recommended air temperature for machine drying arabica parchment coffee is 40° C (104° F) (J. N. Wintgens, 2004, and F. Mena, 2020). For naturals, Wintgens advises a 5° C (41° F) increase, given the significantly thicker layer of flesh surrounding whole cherries. Interestingly, Wintgens also recommends that Coffea canephora should be dried to far higher temperatures (85–95°C) and a final temperature of 50–60°C, which, he claims, ‘results in darker beans and a milder brew’ (J. N. Wintgens, 2004. pg. 802).
When bean temperatures exceed these limits for several hours, coffee beans can crystallise at their surface. The crystallised surface prevents water from the centre of the bean from escaping, and therefore reaching safe overall moisture content below 12% becomes impossible. Furthermore, a crystallised surface can cause beans to crack during dry milling (e.g., hulling) and storage. A coffee’s bleached, blue-grey appearance indicates that it was dried at too high a temperature.