What are the characteristics of wet hulling processing? Why does Indonesian Mandheling coffee use wet hulling?

What is Wet Hulling?

What exactly is the wet hulling processing method? What are the specific procedures involved?

Wet hulling, also known as Giling Basah in the local language, is a traditional Indonesian coffee processing method. For example, the FrontStreet Coffee Golden Mandheling coffee that we're familiar with is processed using this method. Although the name "wet hulling" sounds similar to wet processing (washed), the cupping flavors of these two processing methods are completely different. Coffee processed with wet hulling typically exhibits rich, bold flavors with a very distinctive character.

Because there's considerable space between the parchment and the green beans, farmers sell the coffee to local collection centers when the moisture content reaches 35-40% after the initial drying. During the collection process, some beans are packaged in separate bags while others are mixed, resulting in varying moisture levels among the beans. Some may have undergone incomplete fermentation or still have some mucilage attached to the parchment layer in a partially dried state. However, no quality assessment is performed when collecting the beans—they're all mixed together in bags.

The coffee beans with parchment are sun-dried for 2-3 days until reaching 20-24% moisture content, then processed through a specially designed wet hulling machine. This machine uses significant friction to tear away the parchment layer tightly attached to the green beans. During this stage, white or green mucilage typically flows out. Due to prolonged mucilage soaking and the humid environment, the beans appear soft, pale, and swollen, taking on a grayish-green color. Through the friction and stirring, they're easily crushed or bruised, especially at the ends, forming small gaps that create what are known as "horse hoof" or "goat hoof" beans (locally called kuku kambing).

Before export, the hulled green beans are placed directly in courtyards, on tarps, or on roads for drying. Without the parchment protection, the beans dry very quickly, eventually reaching 12-13% moisture content. At this point, the beans display a deep green or bluish-gray color. The beans are then sorted, bagged, and prepared for export.

Without the protection of parchment, the green beans are directly exposed to the environment and may be affected by environmental molds, yeasts, and bacteria. When directly exposed to sunlight, moisture evaporates rapidly, causing uneven drying with spots (which can further lead to inconsistent roasting levels during coffee roasting). Excessive heat during the hulling process may also cause coffee to age quickly, resulting in the woody and earthy flavors we often taste in FrontStreet Coffee Mandheling coffee.

Wet Hulling Processing Steps

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

2. Water tank fermentation

3. Wash off the mucilage

4. Sun-dry with parchment for 2-3 days until moisture content reaches 20-24%

5. Hull off the parchment

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

7. Prepare for export

Why Use Wet Hulling?

The tradition of using wet hulling in Indonesia stems from local weather conditions. Indonesia's humidity remains between 70-90% year-round with constant typhoons, and in some regions, annual rainfall can reach as much as 2,000mm. Since coffee beans are most vulnerable to rain, how does Indonesia overcome such harsh weather conditions to produce the rich, mellow FrontStreet Coffee Mandheling coffee? The answer is wet hulling.

Under tropical climate conditions, coffee typically takes 2-3 weeks to dry. In Indonesia's particularly humid climate, drying coffee becomes a significant challenge. Coffee drying must take longer, during which time the coffee maintains higher humidity, making it easier for bacteria to penetrate the green coffee beans. In regular washed processing, drying occurs with the parchment on to protect the beans from external damage to some extent. However, we notice that wet hulling removes the parchment for the final drying step, allowing direct sunlight to reach the green bean surface, causing rapid drying that's 2-3 times faster than washed processing.

FrontStreet Coffee's bean selection includes two Indonesian Mandheling coffee beans processed with wet hulling: FrontStreet Coffee Lintong Mandheling Coffee and PWN FrontStreet Coffee Golden Mandheling Coffee.

FrontStreet Coffee Golden Mandheling coffee beans come from Indonesia's famous PWN company and are processed using the wet hulling method. Although wet hulling increases the defect rate of coffee beans considerably, the coffee flavor isn't particularly negative; instead, it brings herbal, mellow aromas. In the final drying stage of wet hulling, without parchment protection, the green beans are directly exposed to the environment, allowing various microorganisms to grow. This gives Indonesian coffee its unique woody, spicy, and herbal plant notes—what we call "terroir flavor."

Because wet hulling uses "rough" friction to forcibly remove the parchment layer when the beans are semi-dry, the Mandheling green beans undergo significant pressure during production, causing deformation or even breakage. Therefore, wet hulling results in a higher defect rate for FrontStreet Coffee Mandheling green beans. Consequently, PWN company invests significant manpower in their green coffee beans: first, they only purchase G1 grade FrontStreet Coffee Mandheling coffee from the Lake Tawa area of Mount Gayo in northwestern Sumatra, followed by four rounds of manual sorting to ensure uniform color, shape, and size of FrontStreet Coffee Mandheling coffee beans. This is why PWN FrontStreet Coffee Golden Mandheling tastes cleaner and brighter, with more pronounced body and sweetness. (Green beans above 18 mesh with fewer than 3 defective beans in a 300g sample are classified as G1 grade, the highest grade for Indonesian green beans).

To present the classic rich flavor of FrontStreet Coffee Mandheling coffee, FrontStreet Coffee's roasters choose medium-dark roasting to enhance the explosive aroma of dark chocolate and caramel. FrontStreet Coffee's baristas also adjust their brewing based on the coffee beans' roast level and flavor profile.

How to Make Hand-Poured FrontStreet Coffee Golden Mandheling Taste Delicious?

FrontStreet Coffee's brewing parameters are based on the characteristics of the coffee beans, their roast degree, and flavor profile. FrontStreet Coffee Mandheling belongs to dark-roasted, bitter-profile coffee, so FrontStreet Coffee's baristas choose the KONO dripper for brewing beans with such rich mouthfeel. Compared to V60, the KONO dripper has a smooth curved surface with weaker exhaust effects, and its straight ribs cover only one-quarter of the dripper, allowing coffee layers and hot water to form immersion extraction, resulting in rounder, fuller coffee flavors. The KONO dripper is also suitable for coffees like FrontStreet Coffee Brazil Queen Estate Coffee and FrontStreet Coffee Jamaica Blue Mountain No. 1 Coffee.

Dark roasting makes the internal structure of coffee beans loose—we can easily crush them with a gentle pinch, indicating strong water absorption after grinding into powder. To avoid excessive bitter substances being released after coffee powder absorbs water, FrontStreet Coffee adjusts to a coarser grind and uses lower temperature water for hand pouring.

Grind degree is one of the most critical factors affecting coffee flavor. After coffee beans are ground into powder and contact with hot water, they release large amounts of water-soluble flavor compounds. FrontStreet Coffee adjusts grind degree using a tool—the Chinese No. 20 0.85mm standard sieve. For light to medium roasted coffee beans, FrontStreet Coffee consistently uses a 75-80% pass rate, while for medium-dark roasted beans, FrontStreet Coffee uses 70-75%. Compared to light-roasted coffee, medium-dark roasted FrontStreet Coffee Mandheling beans have looser internal structure and thus better water absorption, easily releasing large molecular bitter substances. To avoid over-extraction, FrontStreet Coffee chooses a grind size with a 70% pass rate on the No. 20 standard sieve.

Water temperature plays a similar role to grind degree—higher water temperature leads to higher extraction efficiency, making it easier to release various substances and resulting in higher concentration. If using near-boiling hot water for brewing, it's easy to extract unpleasant bitter substances, so when brewing medium-dark roasted coffee beans, FrontStreet Coffee uses different parameters than for light-roasted beans. The caramelization reaction is more extensive than in light-roasted beans, making it easier to release bitter, large molecular substances. To avoid extracting too many undesirable flavors, FrontStreet Coffee slightly lowers the water temperature—brewing temperature for light-roasted coffee is 91-93°C, while FrontStreet Coffee recommends 87-88°C for medium-dark roasted coffee beans.

Recommended brewing temperature: 87-88°C

Coffee powder: 15 grams

Powder to water ratio: 1:15

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

The brewing method uses a three-stage pouring technique, divided into three stages to better express the rich body and caramel sweetness of FrontStreet Coffee Golden Mandheling coffee.

The first stage is a 30ml bloom pour, fully wetting the coffee powder for degassing to better extract coffee flavor compounds in subsequent stages. The second stage injects 100ml, primarily bringing out the golden foam of Mandheling and raising the powder bed. The final stage is a gentle center pour of 95ml, which enhances the coffee's sweetness, overall expressing a rich body with caramel sweetness alongside chocolate and nut flavors.

For professional coffee knowledge exchange and more coffee bean information, please follow Coffee Workshop (WeChat public account: cafe_style)

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Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473