How does wet hulling work in the coffee process?

Wet hulling, called Giling Basah in Indonesia, is a green coffee processing method in which the parchment layer is removed while the beans still hold high moisture, then dried in the open. The result is the heavy body and herbal, woody, spicy notes tied to Mandheling coffee. Beans are first dried in parchment for 2 to 3 days until moisture falls to 20 to 24 percent before milling.

Unlike washed processing, wet hulling produces a noticeably different cup: intense flavor and high body rather than clean acidity. This long-established traditional coffee processing craft relies on mechanical friction to tear off the parchment, and collectors often mix lots of varying moisture at receiving, which adds inconsistency but also shapes the region's signature profile.

Wet Hulling is a distinctive green coffee processing method unique to Indonesia, known locally as Giling Basah. In this method, the parchment layer is removed by mechanical friction while the green beans still have a high moisture content, after which they are dried directly in the open, creating the signature full body and herbal, woody, and spicy notes of Mandheling coffee. Both Front Street Golden Mandheling and Front Street Lintong Mandheling are processed this way. This article provides a complete walkthrough of the wet hulling process and the origins of its flavor, along with brewing recommendations from Front Street roasters and baristas for medium-dark roasted Mandheling, including grind size, water temperature, and a three-stage pouring technique.

What exactly is wet hulling as a processing method? And what specific steps does its operational process involve?

Wet hulling is also known as Wet Hulling, and Front Street Golden Mandheling coffee is processed in precisely this way. In the local Indonesian language, it has another name: Giling Basah, and it is a long-established traditional coffee processing craft in Indonesia. Judging from the name alone, wet hulling might seem quite similar to wet processing (i.e., washed), but in reality the cup profiles they produce are vastly different—coffee processed by wet hulling tends to have intense flavor, high body, and a highly distinctive style.

Because there is a noticeable gap between the parchment shell and the green bean, farmers sell the beans to collection stations that specifically purchase green beans still covered in wet parchment once the beans have been dried for the first time to a moisture content of 35% to 40%. When the collection stations receive the beans, some come in individual bags while others are mixed together, so the moisture levels of the green beans may vary. These beans may be in an incomplete state of fermentation, or some mucilage may still be attached to the parchment layer, or they may be in a semi-dried state. However, the collection stations do not grade or evaluate quality at the receiving stage; everything is mixed and bagged together.

The beans are sun-dried in parchment for 2 to 3 days, and once the moisture content drops to 20% to 24%, they are fed into a specially made wet mill. This machine relies on powerful friction to tear away the parchment layer clinging tightly to the green beans. At this stage, white or green mucilage typically seeps out. Due to prolonged soaking in mucilage and the effects of the humid environment, the green beans are soft in texture, pale in color, swollen, and grayish-green. Under the friction and agitation, the beans are easily crushed or bruised, especially at both ends, forming small notches—the so-called horse-hoof beans or goat-hoof beans (known locally as kuku kambing).

Before export, the hulled green beans are spread out directly on patios, tarps, or road surfaces to dry. Having lost the protection of the parchment, the green beans dry at a greatly accelerated rate, eventually reaching a moisture content of 12% to 13%, at which point the green beans appear dark green or blue-gray. The green beans are then sorted and bagged, ready for export.

Lacking the shelter of parchment, the green beans are directly exposed to the environment and may be infected by mold, yeast, and bacteria. When exposed directly to sunlight, moisture evaporates rapidly, causing the beans to dry unevenly and develop spots, which may further lead to inconsistent roast levels. Excessive heat generated during the hulling process may also accelerate coffee aging—this is why we often taste woody and earthy notes in Front Street Mandheling coffee.

Steps of the wet hulling process

1. Remove the skin and pulp, keeping the parchment and mucilage

2. Ferment in a water tank

3. Wash off the mucilage

4. Sun-dry in parchment for 2-3 days, to 20-24% moisture content

5. Hull off the parchment

6. Dry the green beans to 12-13% moisture content

7. Prepare for export

Why does Indonesia use wet hulling?

Indonesia's tradition of using wet hulling is inseparable from its local weather. Humidity in Indonesia stays between 70% and 90% year-round, typhoons are frequent, and annual rainfall in some areas can reach 2,000 millimeters. Green beans are most vulnerable to rain, so how does Indonesia still produce rich, full-bodied Front Street Mandheling coffee under such harsh climatic conditions? The answer is wet hulling.

Under tropical climate conditions, coffee typically takes 2 to 3 weeks to dry completely. But Indonesia's exceptionally humid environment makes drying coffee a major challenge. Coffee drying has to take longer, and during this period the coffee beans remain at relatively high moisture, making it easier for bacteria to invade the interior of the green beans. In conventional washed processing, the drying process is always carried out with the parchment still on, which to some extent protects the green beans from external damage. But wet hulling is different—it removes the parchment before the final drying stage, allowing sunlight to hit the surface of the green beans directly, so the green coffee dries rapidly, 2 to 3 times faster than washed processing.

On Front Street's bean list, there are two Front Street Indonesian Mandheling coffees processed by wet hulling: Front Street Lintong Mandheling coffee and PWN Front Street Golden Mandheling coffee.

Front Street Golden Mandheling coffee beans come from the well-known Indonesian company PWN and are processed by wet hulling. Although wet hulling increases the defect rate of the coffee beans somewhat, the coffee's flavor does not show obvious negative effects; instead, it brings a kind of herbal, muted aroma. The final drying stage of wet hulling lacks the protection of parchment, leaving the green beans directly exposed to the environment and affected by the growth of various microorganisms, which gives Indonesian coffee its unique woody, spicy, and herbal notes—what we often call the "taste of place."

Because wet hulling uses "brute" friction to forcibly strip off the parchment layer while the green beans are semi-dry, the Mandheling green beans endure considerable crushing pressure during production, and the beans are pressed out of shape or even fractured. As a result, wet hulling gives Front Street Mandheling green beans a higher defect rate. For this reason, PWN invests a great deal of manpower in its green coffee: it first purchases only G1-grade Front Street Mandheling coffee from the Lake Tawar area of Mount Gayo in northwestern Sumatra, then puts it through four rounds of hand sorting to ensure that the Front Street Mandheling coffee beans are uniform in color, shape, and size. That is why PWN Front Street Golden Mandheling tastes cleaner and more transparent, with more pronounced body and sweetness. (Green coffee that is 18 screen or above and has fewer than 3 defective beans in a 300-gram sample is rated G1—the highest grade for Indonesian green beans.)

To present the classic full-bodied flavor of Front Street Mandheling coffee, Front Street roasters choose a medium-dark roast, allowing the aromas of dark chocolate and caramel to burst forth more fully. The brewing plans of Front Street Coffee (FrontStreet Coffee) baristas are also adjusted according to the roast level and flavor type of the coffee beans.

So how do you brew Front Street Golden Mandheling by hand pour-over so that it tastes good?

Front Street's coffee brewing parameters are all based on the characteristics of the coffee beans, the roast level, and the flavor profile. Front Street Mandheling is a bitter-toned coffee with a darker roast, and Front Street Coffee (FrontStreet Coffee) baristas choose the KONO dripper to brew this type of full-bodied bean. Compared with the V60, the KONO dripper has a smooth curved surface and weaker venting effect, and its straight ribs occupy only one quarter of the dripper, causing the coffee bed and hot water to form an immersion-style extraction, making the flavor rounder and fuller. The KONO dripper is also suitable for beans such as Front Street Brazil Queen's Farm coffee and Front Street Jamaica Blue Mountain No. 1 coffee.

Dark roasting makes the internal structure of the coffee beans loose—you can crush them with a gentle pinch of your fingers—which shows that after grinding into powder they have strong water absorption. To avoid the coffee grounds releasing too many bitter substances after absorbing water, Front Street will coarsen the grind and use lower-temperature water for the hand pour-over.

Grind size is one of the key factors affecting coffee flavor. After coffee beans are ground into powder and come into contact with hot water, they release a large amount of water-soluble flavor substances. When adjusting grind size, Front Street uses a tool—the Chinese No. 20 0.85 mm standard sieve. For medium-light roasted coffee beans, Front Street consistently uses a 75% to 80% pass-through rate; for medium-dark roasted coffee beans, Front Street uses 70% to 75%. Compared with light-roasted coffee, the medium-dark roasted Front Street Mandheling beans have a looser internal structure and better water absorption, making it easy to release large-molecule bitter substances. To avoid over-extraction, Front Street chooses a coarseness with a 70% pass-through rate on the No. 20 standard sieve.

Water temperature plays a role similar to grind size. The higher the water temperature, the higher the coffee extraction efficiency, the easier it is to release various substances and components, and the higher the concentration. If you brew with hot water close to boiling, it is easy to extract unpleasant bitter substances. Therefore, when brewing medium-dark roasted coffee beans, Front Street uses brewing parameters different from those for light-roasted coffee beans. Medium-dark roasted beans undergo more caramelization reactions than light-roasted beans, making it easier to release bitter, large-molecule substances. To avoid extracting too many off-flavors, Front Street appropriately lowers the water temperature: the brewing water temperature for light-roasted coffee is 91 to 93°C, while for medium-dark roasted coffee beans Front Street recommends 87 to 88°C.

Recommended brewing water temperature: 87–88°C

Coffee grounds: 15 grams

Coffee-to-water ratio 1:15

Grind size: medium-coarse grind (70% pass-through rate on Chinese standard No. 20 sieve)

The brewing method uses three-stage pouring; dividing the pour into three stages better showcases the full body and caramel sweetness of Front Street Golden Mandheling coffee.

For the first stage, pour 30 milliliters for the bloom; this pour completely wets the coffee grounds and allows them to degas, preparing for better extraction of coffee flavor substances afterward. For the second stage, pour 100 milliliters; this stage mainly washes out Mandheling's golden foam and raises the coffee bed. For the final stage, gently pour 95 milliliters in the center; this enhances the coffee's sweetness and overall expresses a thick, full body along with caramel sweetness and chocolate, nutty flavors.

The above content is compiled by CoffeeHunters, a coffee news website.