Drying, a floor and a ceiling
What water remains available to mold?
Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.
Brazilian trials compare sun drying, eight to ten days to reach around 11% moisture, and mechanical drying that does the same job in twenty-four hours, provided the mass of coffee is kept under 40 °C.
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- Sun or machineBrazilian trials compare sun drying, eight to ten days to reach around 11% moisture, and mechanical drying that does the same job in twenty-four hours, provided the mass of coffee is kept under 40 °C.
- Get the air everywhereThe dryer is designed so that air reaches all the coffee, not only the top of the pile: seven-centimeter columns alternating with voids, airflow reversed every ten minutes, a static phase down to about 30% moisture and then a dynamic phase.
- Read the water activityThe quantity that matters is water activity, the share of water actually available to a microorganism, expressed from 0 to 1. All the treatments tested drop below 0.6, and below 0.5 microbial growth is prevented. Mechanical drying reduces filamentous fungi by 38 to 44%.
- A floor and a ceilingThe ICO gives 8 to 12.5% moisture, the trade aims for 10 to 12%, storage requires a water activity below 0.65, and the parchment must not exceed 40 °C. The lower limit is forgotten: below 10%, around 0.4, the coffee is no better protected, it is flat and stale.
To remember: Too dry is not safer, it is only worse: a successful drying is first of all sanitary, and the cup score follows.
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Brazilian trials compare sun drying, eight to ten days to reach around 11% moisture, and mechanical drying that does the same job in 24 hours. The condition is to keep the temperature of the mass under 40 °C.
The whole setup is designed so that air reaches all the coffee, and not only the top of the pile: seven-centimeter columns alternating with voids, airflow reversed every ten minutes, a static phase down to about 30% moisture and then a dynamic phase.
The quantity that matters is not the moisture content but the water activity. It is the share of the water that remains actually available to a microorganism, expressed from 0 to 1. A bean can contain water without that water being usable, and it is this water that molds go looking for. All the treatments tested drop below 0.6, and below 0.5 microbial growth is prevented.
The rest of the measurements follow from there. Mechanical drying reduces filamentous fungi by 38 to 44%. Ochratoxin A was not detected in any sample: 2.5 µg/kg is the detection threshold of the kit, not a measured value. The mechanical lots score 80 to 81 points against under 80 for the solar ones, with total phenols of 3.24 against 2.20 g of gallic acid equivalent per 100 g. The first function of a successful drying is sanitary, and the cup score follows.
Abreu, Lorenço, Machado, Silva, de Azevedo and Carvalho, Foods 14(9):1463 (2025), solid levelTargets: the ICO gives 8 to 12.5% moisture, commercial practice aims for 10 to 12%, and storage requires a water activity below 0.65. On parchment, the maximum temperature is 40 °C, 42 tolerated by some. Beyond that, the embryo is damaged and the parchment split.
The lower limit, for its part, is always forgotten. Below 10% moisture, at a water activity close to 0.4, the coffee is no better protected: it is flat and stale. Too dry is therefore not safer, it is only worse.
Two symmetrical accidents frame the window. Drying too fast produces case hardening: the surface closes up, the core stays moist, the water no longer escapes. The bill arrives later, in splits at hulling or at roasting. Drying too slowly in high humidity produces the opposite accident, molds, acid pockets and stinkers.
ICO; Yang et al. 2026, p. 15-16; Abreu et al. 2025; LC ch. 17b