What is Wet-Processed Coffee?

November 20, 2014

Wet-Processed Coffee

You might know it for its clean, bright taste, but wet-processing is a means to get the seed separated from the fruit, get it dried and prepare it for roasting. Wet-processing means the skin (and pulp) is peeled from the fruit before it is dried.

This is not an exhaustive treatise on wet-processing of coffee. It’s a quick overview based on the printed card I made that (perhaps) you received in a Sweet Maria’s order. This online version has more information than the card since there is less space constraint on the interweb. Still, it is not exhaustive, and since the technique is implemented differently, it’s difficult to cover all the variations.

While all processing has some effect on the coffee flavor, one can argue that wet-process does not overlay the same intensity of fruit taste as dry-processing does. Even though it involves more steps and in some ways, more resources, wet-processing is perhaps more “transparent” to the final coffee flavor, allowing other differences in the coffee (variety, farm environment and altitude) to be discernable in the cup.

Yet as farms begin to experiment with the variables, I have found dry-process coffees that do not overlay fruit notes in such a heavy-handed way, and wet-process techniques (such as multi-day, multi-picking fermentation, or infusing pectinase or yeast into the fermentation) that absolutely does color the final cup with flavors from the process. So perhaps the old ideas of difference between wet- and dry-process are changing. Let’s leave that as an open debate and focus on the card image.

I added a couple of steps not pictured on the card, ones I like because they can result in greater uniformity of quality (namely, floating coffee cherry, sorting wet parchment, and skin drying). Anyway, here goes …

Coffee comes from a shrubby tree. Someone has to pick it. What is this fruit from a flowering tree? I took this macro image of the coffee fruit to show the difference between the outer skin, inner pulp, and the thin mucilage layer. Pulp and mucilage are two distinct parts of the fruit.

The coffee is taken from the farm to a wet mill, AKA  washing station, or beneficio humedo. In Kenya, its called a factory. That might be a donkey ride, a truck trip, or a walk from the trees to the side of the house on a small-holder farm. Whether coffee is pulped and fermented by the farmer, or in a central wet mill is not a factor; it’s whether it is done well no matter the scale of the production.

(The coffee is selectively picked for ripeness at the tree. In some places, it is sorted again before pulping, such as Kenya. While the machines can remove the under-ripe cherries it is best they never enter the system. In some places, like Burundi, they float the coffee cherry in water to remove lightweight floaters. This can also happen a fancy pulper or in the washing channel, but redundant ways to remove defects is a good thing).


Small-holder farmers use a simple hand-crank or motor operated pulper that uses pressure and a textured metal drum to scrape the skin off the fruit. Larger mills have pulpers with additional features, that can remove unripe green coffee cherries, and remove defect floater beans too. The skin and pulp are separated more effectively in these larger pulpers as well.

Pulpers strip the outer skin (epicarp) and the pulp layer underneath it (mesocarp) comes off as well. This leaves the seed with the mucilage layer strongly attached. Technically this mucilage is also considered the mesocarp, the pulp of the fruit. But it does not separate from the parchment layer (endocarp) that shields the plant seed. It might not be the best comparison, but think of mango: When you get down to the seed there is a layer off fruit that clings tenaciously to the seed. Some types of plum have this same characteristic and perhaps are a better example.

To remove the mucilage, the coffee is fermented in a tank, basin or bucket as micro-organisms act on the fruit. As sugars in the mucilage are consumed the acidity of the coffee mass drops, and some say it is this that allows the mucilage to break down and “slip” off the parchment layer. In any case, the seed is not directly acted on as the parchment layer forms a barrier. And yet the fermentation process in not simply the mechanism to remove mucilage; there is an impact on flavor, positive or negative.

After the fermentation period  (which varies based on ambient temperature and other factors) the coffee is washed in a channel to remove remaining mucilage. Freshly pulped coffee is slimy and slippery. After fermentation, the slime slips off and you can feel a rough texture like sandpaper from the parchment layer. In the washing channel, the coffee is pushed around with a wood paddle to help work off the broken-down mucilage. The washing channel also separates good dense seeds from light bad ones: floating lightweight coffee is separated, heavy coffee (good coffee) sinks.

(New “eco-pulpers” such as the Penagos pictured here can strip off mucilage with pressure, and the farmer can skip the entire fermentation and washing process. See the red star in the image? That vertical cylinder is the demucilage burr. However some mucilage always remains, and many wet mills that use the de-mucilage machines soak the coffee in a water bath afterward. Whether this is a supplemental fermentation or not is an open question. This method is called machine-washed, forced demucilage or “cereja descascada” in Brazil. The taste of the coffee approximates the flavors from a traditional wet-process. We prefer the traditional method, but accept this type when the cup is great, and water resources are limited).

The coffee is ready to dry after washing, ideally in the sun on a patio or raised bed of stretched fabric. Places like Rwanda hand-sort the coffee when it is wet, and dry in shade for the first few days. Some defective beans can be spotted easier when the parchment is slightly transparent with wetness. Also, the slow introduction of air first, then sun later, helps keep the parchment intact and un-cracked. If you think of parchment as a tiny protective shell buffering the seed from the shock of too much heat, then initial “skin drying” on a raised bed makes the changes more gentle for the bean.

Many origins sort out bad or broken beans during drying, which can last 10-20 days. In this time the seed slowly shrinks away from the outer shell, called parchment (Sp. pergamino).  Good parchment coffee is yellow-white in color and isn’t too cracked or split. Parchment that is yellow-brown might have stayed in the cherry too long before pulping and fermenting, or fermented too long. Coffee should be pulped the day it is picked. Cracked parchment can still be good coffee, but it means there has been too much heat exposure, too much direct sun. In some places of intense equatorial sun, the coffee is covered in the middle of the day to prevent too-rapid drying.

Mechanical drying, done right, can be okay for quality too. Ideally, the coffee is sun-dried for several days, then exposed to gentle heat (less than 50c input air temperature) in a drum dryer. When there are intermittent rains, access to a dryer is important because coffee that is re-wet after the drying phase begins can lose quality quickly.

 After sun-drying, the coffee is bulked up in bins or sacks and is left in a clean dry warehouse to rest (Sp. reposo).  This allows for moisture to stabilize within the seed. Water literally come to rest in this dried seed, and with less “water activity” it means that the quality of the positive taste attributes will be preserved until roasting.


A few days before export, the parchment coffee is prepared at the dry mill. The parchment layer is stripped off by the huller, and the bean graded for density and size. Dense coffee is often better coffee, so the vibrating air bed of the density table is critical for quality. Interestingly, the dense coffee travels to the high side of the tilted table because it responds strongly to the vibrating movement of the table, and light-weight coffee gravitates to the low side.

(Why is dense coffee generally good coffee? Usually, it is because it comes from a healthy fruit that was on the tree for a longer time and matured slower. Therefore the cellulose pockets that form the green beans structural matrix are tighter-packed and smaller. High altitude coffee and or well-shaded coffee trees tend to slow fruit maturation because of limited light and colder climate. The opposite of dense coffee is called “soft coffee”, grown in a lower hotter climate, and reaching maturity faster).

There are other machines to remove leaves or strands of jute, and the screener to separate various size grades and peaberry, as well as broken bits of coffee bean (called triage). In the past coffee was polished to remove remaining silverskin (called the integument technically, which becomes chaff when roasting), but this exposes coffee to heat and serves no quality purpose. We ask to skip this step when our coffee is milled.

As a last step, the coffee might be checked by a color sorter for off colors (the notorious “black bean”). Some defect beans have the same form and weight of a good bean, and only a color difference distinguishes them. Some mills use different light spectrums, such as ultraviolet wavelengths, in addition to white light. It’s interesting to note that if a coffee is really well-picked, processed and the density table is set right, optical color sorters will do little to improve the cup taste of the best coffees. They work well to take a commercial coffee from 74 points to 78 points in cup score: they will not transform a specialty coffee from 84 to 88 points though).

Hand-picking the final green bean occurs in many places and is an important step. But with the competition for labor, it is becoming rarer. Static hand-picking at a table with a good white light is ideal. In some countries, they use conveyors for handpicking that stop and start on a timed interval. This works too. It is difficult, eye-fatiguing work and is done by women. ( I was going to write “most often” or “usually”, but I have only seen men in the role of hand-picking coffee once that I can recall from 12 years of traveling! The good news is that it offers a decent local job to unskilled workers, and increasingly I see a friendly, safe and respectful environment for hand-picking coffee, and solid wages paid for this work.)

If everything is done to high standards, there’s a chance the coffee is a winner! It does not make the coffee good … it just means it has not been ruined along the way. To appreciate all that goes into making a great cup, it’s important to appreciate all the human labor behind coffee. There is much focus on the farm, the altitude, varieties of coffee trees, soil. But it is the post-harvest steps that are truly critical to great quality.   — T. Owen