Introduction to Wet-Hulled Coffee Processing Method_Characteristics of Coffee Bean Wet-Hulling Method_Is Wet-Hulled Coffee Delicious

Wet Hulling (Giling Basah)

Today we would like to introduce a coffee processing method you may not have come across often. Wet hulling is a processing method exclusive to Indonesia and a few nearby producing countries. Giling Basah is Indonesian and literally means "scraping off the hard shell under wet conditions." It also goes by the English name Wet Hulling.

The earliest Chinese rendering of the term appeared in Teacher Han Huaizong's book "Specialty Coffee Studies," and his translation remains the best way to understand the process. Other incorrect names, such as the common "semi-washed," confuse several distinct processing methods and have left many coffee lovers with almost no real understanding of wet hulling, which is a shame.

Wet hulling matters to the global coffee industry no less than honey processing does. Coffee lovers must have tasted the essential "Mandheling," which is made this way. It could even be said that most of the Arabica coffee from Sumatra and Sulawesi in Indonesia is wet-hulled. According to 2014 International Coffee Organization (ICO) statistics, Indonesia is the world's fourth-largest coffee-producing country. A processing method that contributes so much to world coffee production is certainly worth studying and analyzing.

Why Use Wet Hulling

Because of Indonesia's unique tropical rainforest climate, drying conditions for coffee beans are relatively harsh. The washed method dries too slowly and can lead to processing failures. So once the parchment shell has dried to a certain point, it is ground off and the raw beans are exposed directly to the environment to dry, which effectively shortens the drying time. Because the parchment is still moist when removed, this distinctive method is called wet hulling. The short fermentation and drying time also explain why Indonesian coffee has low acidity and its own herbal flavor and character. Now that we understand why wet hulling is used, let's look at the processing steps.

Processing Steps

Floating the Coffee Cherries

Whether on small farms or somewhat larger estates, all coffee in Indonesia is hand-picked. After picking, the routine is to soak the cherries in water so they sink or float for rough screening: the sinking cherries are kept, while debris and green fruit floating on the surface are screened out.

Farmers Using Machines to Remove the Fruit Skin

The ripe red fruit is then put into a pulping machine to remove the skin. Unlike the washed method, wet hulling uses smaller pulping machines. They are small enough that almost every farmer can afford one, so the pulping stage is almost entirely completed at farm level.

Next comes fermentation. In the washed method, fermentation means putting the de-pulped beans with their mucilage into tanks to soak, using bacteria to remove the mucilage. This lasts about 24-48 hours, with the water quality in the soaking tank roughly equal to the bean quality. Wet hulling fermentation is slightly different: only 12 hours, with very little water, and often carried out in smaller containers such as plastic basins or even plastic bags. After this brief fermentation most of the mucilage clinging to the bean shell rinses off easily, leaving a small amount behind. This step too is completed at farm level.

Farmers load the wet, sticky parchment coffee into bags and carry it to weekly agricultural markets to sell, often to buyers from processing stations. The purchasing unit calculates by volume using locally pre-measured buckets, one bucket marked as one liter or two liters, and pays the farmer a price per liter of coffee beans. Buyers often set the payment level by watching how much residual mucilage clings to the bean shell. More residue means a lower price; less residue means a higher price.

After collecting many parchment beans, the processing station sends the still-undried parchment beans (moisture content 35%-45%) straight into the hulling machine, which scrapes off the bean shell together with the mucilage clinging to it. This is the so-called wet hulling. This step differs from every other processing method. Whether washed, natural, or honey processed, all of them dry the beans with parchment (moisture content 11%) before "dry hulling" to scrape off the bean shell.

The hulling in wet hulling is also particular. The machine shown above is the dedicated huller for Indonesian wet hulling. In size alone it is three times larger than the huller used for washed processing. More important than size is the nature of the grinding core.

From the cross-section of undried, high-moisture parchment beans in wet hulling (shown below), we can see that the wet-hulled bean shell is still soft, whereas the shell of the washed method has become brittle and dry after exposure to the scorching sun. During the drying process of the washed method, the inner raw bean loses water and shrinks relatively, leaving an obvious gap layer between the raw bean and the shell. Parchment beans going through wet hulling do not have such an obvious gap layer and remain very swollen. Both points make the shell-removal step in wet hulling increasingly difficult, and the defective, broken beans caused by intense friction in wet hulling are also quite numerous.

At this point the exposed raw beans, still high in moisture, are spread out on cement floors covered with straw mats or plastic sheets for drying. Although there are no perishable organic materials such as mucilage on the surface, workers still need to turn the beans frequently. After all, with such high humidity and the directly exposed raw beans being so fragile, they can rot if left covered.

Without the "sealing" of the bean shell, wet hulling should in theory dry faster. It does not. People in the coffee industry describe drying in Sumatra as a "nightmare." Sumatra, Indonesia, is one of the few coffee-producing regions with a very humid climate. According to one coffee professional who has visited Indonesian growing areas, "the maximum drying time within a day can be 4 hours: taken out in the morning, brought back when it rains heavily in the afternoon." Some processing workshops with a little money build greenhouses; drying in the shed may be more cost-effective than carrying beans in and out.

The humid weather also brings great trouble to picking. In producing countries with clear and stable weather patterns, such as Central America, pickers can gather all the fruit by going to the fields an average of 3 times. As for Sumatra, which knows nothing of stable weather patterns, pickers need to visit the fields countless times without stopping throughout the entire 6-month period when the fruit ripens, because coffee trees flower and bear fruit according to the weather.

Under such harsh conditions, drying the raw beans takes about half a month, with the exact number of days depending on luck. Without the bean shell, with the freshly hulled raw beans moist and warm, and with Indonesians not using raised beds for drying, the raw beans are affected by many external enzymes and microorganisms, especially those on the ground. Perhaps it is due to these factors during drying that Sumatran coffee generally carries an "earthy" flavor. The drying of exposed raw beans gives wet-hulled beans a common characteristic: a forest-green color.

Flavor of Wet-Hulled Coffee Beans

Having followed the whole process, we can now discuss flavor. Wet-hulled beans are generally thought to have earthy, smoky, and chocolate flavors, with acidity described as "low" and rather dull. The low acidity comes from the shorter, less effective fermentation and the longer drying time. As for the "earthy" notes in wet-hulled beans, opinions vary. Some say it relates to the varieties chosen in wet-hulling regions (Timor hybrid & Catimor: with Robusta lineage), some say it is the result of organic interaction between the raw beans and the external environment (Peter Giuliano), and some even say it is because there are simply too many defective beans.

First, natural defects from the coffee fruit stage are inevitable. Another contributor to defects is the hulling itself, along with corrosion during the transport of wet parchment beans, damage to the exposed raw beans from sunlight and moisture, and the irregular drying speed brought by unpredictable weather, all of which detract from the flavor.

Although Indonesian wet-hulled coffee beans have a high defect rate, we should still have confidence in them. In recent years the Indonesian Specialty Coffee Association (SCAI) has been established, strengthening quality supervision throughout the coffee production chain. Some growing areas and estates have even introduced partial-washing mechanisms, and some regions have begun to follow the path of high-quality, closely supervised characteristic wet hulling. Either way, I remain optimistic about coffee development in Sumatra, Indonesia. I do not want such a large and important producing country to keep carrying an unclear burden. I believe it still has plenty of room to improve.

FrontStreet Coffee Recommendation

To taste the finest expression of wet hulling, the highest grade is FrontStreet Coffee PWN Golden Mandheling. The PWN company buys high-quality Mandheling green beans and, after machine selection and hand selection, picks out plump, defect-free beans so the output is specialty grade; Golden Mandheling is its flagship product. PWN applies very strict standards: beans must be 18 screen or larger, and each 300g green bean sample must contain fewer than 3 defective beans to meet the top G1 grade. On top of that, three rounds of hand sorting fully remove defects, raising the cup's cleanliness and the intensity of its sweetness. Because of these standards, PWN Golden Mandheling is produced in limited quantities and is the leading expression of Mandheling: even, plump, translucent beans with a low defect rate, and after a medium-dark roast, rich roasting aromas and a clean, full-bodied black coffee that is ideal for anyone who dislikes acidic coffee. The beans are small-batch roasted to order, within 5 days.

FrontStreet's Golden Mandheling shows best as pour-over. Use 87-88°C water, 15g of coffee, a 1:15 ratio, and a coarse grind (like coarse granulated sugar) in three pours. The first pour of 30ml fully wets the grounds and lets them degas; the second pour of 100ml flushes out the golden foam and lifts the bed; the final gentle, centered pour of 95ml lifts the sweetness. The result is a thick, full body with caramel sweetness and chocolate and nut flavors.

For a machine-friendly alternative, FrontStreet's classic house blend adds medium-dark-roasted Mandheling to a creamy, nutty Brazilian base, with butter, roasted chestnut, and caramel standing out. Mandheling not only raises the body of the whole cup but lends it distinct sandalwood, herbal, and spice notes; a small amount of Robusta is added to boost the richness of the crema, which makes latte art or a layered Dirty effortless.