The first large-scale drying machines, called Guardiolas, were developed over 120 years ago by J. Guardiola, of Chocola, Guatemala (W.H. Ukers, 1935). This name is still used in Central America to refer to modern rotary dryers. These machines spin beans around in a drum, in the manner of an enormous clothes dryers. They are usually fuelled by a heat exchange system, with air travelling through an external furnace, though some modern machines are gas powered. Husks from a previous season’s harvest can be used as a fuel source in the burners. Others are sometimes powered by burning timber. Just as with wood-fired coffee roasters, it is paramount that furnaces don’t exhaust into the drying area, lest they taint the coffee with a smoky flavour.
As discussed in the context of fermentation in Lesson 5.04, weight losses can lead to a reduction in profit for producers. Particularly with the washed process, parchment coffees continue to metabolise during drying, right up until their moisture levels drop to below 12%. When coffee is mechanically dried over a period of 40 hours, the process of weight loss is interrupted much earlier. This is an argument in favour of mechanical drying: by reducing the time available for metabolic processes to occur in the beans as they dry, producers can minimise the coffee’s weight loss.