Drying coffee

Three phases on the drying yard

Sourced

The FAO guidance divides a drying run into three stretches, by how fast the coffee is losing water and by which microbes can live in it.

The figures in the guidance

Sourced

All moisture figures are on a wet basis. These are limits for safety in storage, set with a margin: the guidance reports that the water activity at which the moulds can start to grow corresponded, on average across more than two thousand samples, to about 16 per cent moisture in arabica parchment and 18 in robusta cherry. They are not a statement about flavour.

Why two measures, moisture and water activity

CoffeeHQ explanation

Moisture content says how much of the coffee's weight is water. Water activity says how available that water is to a microbe, on a scale from nought to one. Mould responds to the second, which is why the guidance states its thresholds in water activity and then translates them into moisture, the thing a producer can measure with a meter or judge by biting a bean.

The translation is not exact. The guidance notes that its relationship between the two was measured only as coffee dried, and that coffee taking water back up might behave slightly differently. That is one reason the recommended moisture is set well below the point at which growth becomes possible.

How water leaves a seed

Sourced

Drying is two movements, one after the other. Water evaporates from the surface of the seed into the air, and water from inside the seed travels outward to replace it. A 2026 review of coffee drying puts it in those terms: moisture is removed by internal diffusion and surface evaporation, with heat carried in by moving air and by contact between seeds. Whichever of the two is slower sets the pace. While the surface is wet, the air decides. Once the surface has dried, the seed does, and no amount of sun or wind makes water cross the inside of a seed much faster.

What drives evaporation is a difference between the seed and the air: water leaves when the air next to the seed holds less water vapour than the seed's surface would be in balance with. Warm air and dry air both widen that gap. A laboratory study in Thailand dried thin layers of arabica parchment at 50, 60 and 70 degrees Celsius and at relative humidities between 10 and 30 per cent, with the air speed held constant at one metre a second. Coffee dried fastest in the hottest and driest air, and the authors give the reason for the humidity's part in those terms: the vapour pressure of the drying air was lowest when its humidity was held at 10 per cent. Those temperatures are the conditions of an experiment designed to fit an equation, not settings anyone is advised to use.

The same study estimated how readily water moves through the parchment seed, a quantity called effective diffusivity, at about one billionth of a square metre per second, rising with temperature, and notes that this is within the range reported for other crops. The number matters less than what it implies. Diffusion over the few millimetres from the centre of a seed to its skin takes hours, and it is this, not evaporation, that makes the last stretch of drying slow.

Where drying stops: equilibrium with the air

Sourced

Coffee does not dry to nothing. It dries until the seed and the air around it are in balance, and then it stops. In the Thai study every run ended at a moisture that no longer changed, and that end point differed with the temperature and humidity of the run: between 12.5 and 4.5 per cent on a dry basis across the conditions tested. The passage read does not say which condition gave which figure. This end point is the equilibrium moisture content, and it belongs to a particular temperature and humidity, not to the coffee alone.

Two things follow. The first is that the air at the place of drying sets a floor. Coffee cannot be sun-dried below what the local air will hold it at, and coffee left out in damp air will not reach a storage moisture however long it stays there. The second is that the balance works in both directions. Dry coffee placed in air more humid than the air it was dried in takes water back up until it reaches the new balance. That is rewetting without rain, and it is why the guidance treats storage and transport as part of the same problem as drying.

A note on units, because the two conventions give very different numbers for the same coffee. Dry basis divides the water by the weight of the dry matter; wet basis, which the trade and the guidance use, divides it by the whole weight. The study's starting moisture of 122 per cent dry basis is 55 per cent wet basis, and its end points of 12.5 and 4.5 per cent dry basis are about 11.1 and 4.3 per cent wet basis. A moisture figure with no basis given cannot be compared with another.

Moisture movement during drying: a model

CoffeeHQ explanation

A diagram drawn by CoffeeHQ from the physics above. It describes one seed in moving air. A bed of coffee adds a second problem, that seeds at different depths are at different stages at once.

What sets the rate, and how each is known

Sourced

Six things a producer can influence. The evidence for them is of different kinds, and each entry says which: a controlled laboratory run, a field experiment, or the guidance's report of practice.

Air temperature

Controlled experiment. In the Thai laboratory, higher air temperature shortened drying, and the diffusivity the authors fitted rose with temperature at each of the three humidities. The study measured drying behaviour and did not taste the coffee, so it says nothing on what temperature does to the cup.

Humidity of the air

Controlled experiment. The shortest drying in the laboratory was in the driest air at the highest temperature, and the authors explain it by the low vapour pressure of that air. Humidity is the variable somebody drying in the sun has least control over.

Air movement

Named, not tested, in what was read. The laboratory study lists air velocity among the important factors and then held it constant. The review describes uneven airflow as a cause of uneven drying in dryers of several designs.

Depth of the layer

Field experiment. A study at Jimma, in Ethiopia, dried washed parchment at two, four and six centimetres deep in a solar tunnel and in the open sun. The thinnest layer at the hot end of the tunnel reached a steady moisture in 14 hours; the deepest layer in open sun took 47. The authors attribute the slowness of deep layers to water having to travel from the inside of the layer to its surface. The share of beans carrying mould at the end ranged from 4 to 93 per cent across the treatments.

Turning

Report of practice. Turning brings wet seeds to the surface and breaks up the gradient through a layer. The FAO guidance admits it is hard to show that raking more than once a day shortens drying, and records that coffee left in a static bed has been seen to become covered in mould. The case for turning is evenness and safety more than speed.

Where in the dryer

Field experiment. In the same tunnel the air was about 45 degrees Celsius and 30 per cent humidity near the inlet and about 65 degrees and 16 per cent near the exit, and coffee at the same depth dried at different speeds along its length. One dryer is several drying conditions.

Case hardening: a borrowed word

Accounts differ

Case hardening is a term from the drying of other foods and of timber. It describes a surface dried so fast that it stiffens and slows the escape of the water still inside. The 2026 review uses it of coffee several times, listing it with overheating and uneven airflow among the faults of badly ventilated solar houses and badly designed dryers, and crediting tumbling dryers with reducing it.

The physics above makes it plausible: a steep gradient between a dry skin and a wet centre is what fast surface drying produces. But in the passages read the review asserts case hardening and does not show it. CoffeeHQ has read no study that measured a hardened outer layer in a coffee seed, or the moisture trapped behind one, or its effect on storage or taste. It is reported here as a hypothesis with a mechanism, used by people who dry coffee for a living, and not as an established fault with a known threshold.

Where coffee is dried

Sourced

Patios and terraces

Cement or brick, with the coffee spread thin and raked. The review describes patios as needing a lot of land and being the most exposed to the weather. The guidance lists compacted earth, mats and plastic sheet among the surfaces in common use, and notes that plastic has been seen to sweat under the coffee and encourage mould.

Raised beds

Tables covered in mesh or matting, which let air move under the coffee. The review says they cost more to build and work than patios and are still vulnerable to rain and to night-time damp unless covered.

Parabolic houses

Arched frames covered in clear plastic, also called solar tents. They keep rain off and run warmer than the air outside. The review says field evaluations found they cut drying time by roughly a fifth to two fifths against open patios, citing one source that was not opened for this page, and lists overheating and condensation among the faults of a badly ventilated one. The guidance found their value highly dependent on the weather.

Mechanical dryers

Heated by burning a fuel, and controlled by inlet temperature and time. Most often used to finish coffee that has been part-dried in the sun. The guidance names two concerns: an inlet temperature high enough to turn immature beans black, and drying too far, which costs the producer weight.

The surface matters less than how it is used

Sourced

The guidance reports extensive side-by-side tests of drying surfaces, and its conclusion is unexpected. Different surfaces sometimes dried coffee at different rates, but the differences were generally small and not consistent, and the tests failed to condemn any particular surface. Drying rates varied enormously with how the equipment was used and with the weather during the run.

That finding is about how fast coffee dries, not how it tastes. The 2026 review credits raised beds with better preservation of quality, citing studies that were not opened for this page. The two statements are not in conflict, but the stronger claim is the one with the weaker support here.

What the guidance advises on the yard

Sourced

These are the FAO guidance's recommendations for sun-drying, summarised. It offers them as a basis for national codes, not as rules for every farm, and its numbers are guides.

1. Spread thin. About 25 to 35 kilograms of fresh coffee to the square metre, which is a layer of roughly three centimetres for parchment and five for cherry; thinner in cloudy, damp or still weather.
2. Turn. Four times a day if possible. The guidance admits it is hard to show that raking more than once a day shortens drying, but coffee left in a static bed has been seen to become covered in mould.
3. Heap and cover at night, once it has begun to dry. After about a day for parchment and three for cherry. Covering coffee that is still fully wet produces condensation.
4. Keep it from getting wet again. Parchment should always be protected from rain. Cherry in its first three days on the yard is little affected by a shower.
5. Check dryness before you expect it. Start two or three days ahead and check daily, sampling from several places. Biting or shaking a bean can work, the guidance says, but should be verified against a calibrated meter.
6. Keep lots apart. Different days' harvests and different grades are dried separately and labelled.

None of this is addressed to somebody drinking coffee. It is here because it is what the word drying stands for when a bag mentions it.

Getting wet again, and staying wet

Sourced

The guidance singles out rewetting: it may be more serious than drying slowly. Mould that established itself while the coffee was drying has more of itself in place, and is poised to grow quickly if water returns. The code of practice makes the same point about storage and transport, where the moisture should not exceed its limit anywhere along the way.

The method that deliberately keeps coffee wet for longest is wet hulling, in northern Sumatra, where parchment is traded wet and the hulled bean is commonly held at about 17 per cent moisture for weeks before it is finished. The study that documented it did so as part of the same project on mould.

Is slow drying better?

Accounts differ

It is widely said that coffee dried slowly and gently tastes better and keeps longer as green coffee. The guidance records the idea and its limits in one sentence: some workers recommend that parchment be protected from rapid drying in the midday sun in the early stages, but many origins have no such tradition. It also records that sun-drying, once considered essential for good coffee, is no longer agreed to be.

Against the idea stands the guidance's main concern, which is time spent in the middle stretch where mould can grow. Slower is not safer. CoffeeHQ has read no study that tested drying speed against cup quality with a tasting panel, and reports the belief as a belief.

What this page gives no figure for

CoffeeHQ explanation

How many days coffee takes to dry. The guidance gives none, because the answer depends on the weather.

A drying temperature. The guidance warns against excessive inlet temperatures in dryers without giving a figure in the passages read, and the temperatures in the laboratory and tunnel studies are the conditions those studies happened to use.

A drying rate to aim for, or a point at which fast becomes too fast. Case hardening is asserted in the review read and measured in nothing read.

That raised beds, shade or slow drying improve the cup. Each is believed; none was tested in a document read for this page.

How dry is too dry. The guidance says over-drying costs the producer weight and is undesirable, and a figure of about eight per cent is often quoted in the trade as a lower limit; no document read for this page gives it.

What to do next

• Natural process — The method with the most water to remove, and the one most dependent on the yard.
• Wet-hulled — The method that reverses the usual order and dries the bean without its parchment.
• Coffee fermentation — The stage before drying, and the other way a wet crop is kept from spoiling.
• Coffee Expert: green coffee — What happens to dried coffee next: moisture, storage and grading.