Characteristics and Story of Mandheling Wet-Hulled Process Coffee Beans from Sumatra, Indonesia

Sumatra is the source of some of the most distinctive coffee in the world, and the reason lies in a processing method found almost nowhere else. Here is how wet hulling works and why it gives Mandheling its signature flavor.

Indonesia is a large archipelagic country made up of countless volcanic islands, naturally blessed with excellent coffee-growing environments and latitudes. Each island has its own character thanks to different topography and climate. The Indonesian islands that grow coffee include Sumatra, Sulawesi, Java, Sumbawa, Flores, and Papua. Among them, Sumatra is one of the country's main coffee regions, and the most famous FrontStreet Coffee Mandheling coffee comes from here.

Today, FrontStreet Coffee will discuss how Sumatra developed the wet-hulled processing method and what unique flavors it brings to Mandheling coffee.

What is Wet Hulling and what are the specific procedures?

Wet hulling, also known as wet dehulling, is the processing method used for the familiar FrontStreet Coffee Golden Mandheling coffee. In the local language it is called Giling Basah, and it is a traditional Indonesian method. Judging by the name alone, wet hulling sounds similar to wet processing, or washed, yet the cupping flavors of the two are completely different. Coffee processed with wet hulling typically has a thick, strong flavor with a very distinct personality.

Because there is considerable space between the parchment shell and the green bean, farmers sell their beans to local collection stations once the green beans reach 35-40% moisture content after initial drying. During collection, some beans are packed in separate bags while others are mixed, so moisture levels can vary from lot to lot. Some may be incompletely fermented or still have part of the mucilage attached to the parchment in a partially dry state. No quality assessment is made when collecting, since the beans are all mixed and bagged together.

The parchment-covered beans are sun-dried for 2-3 days until they reach 20-24% moisture content, then run through a specially designed wet huller. This machine uses significant friction to tear off the parchment layer clinging to the green bean. During this stage, a white or green mucus usually flows out. Because the beans have been soaking in mucus and a humid environment, the exposed green beans become soft, pale, and swollen, taking on a gray-green color. With the stirring caused by the friction, they are easily crushed or damaged, especially at the ends, forming small notches known as horse hoof or goat hoof beans, called "kuku kambing" locally.

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

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

Wet Hulling Processing Steps

1. Remove the skin and pulp, keeping the parchment and mucilage
2. Ferment in water tanks
3. Wash off the mucilage
4. Sun-dry with the parchment for 2-3 days, until 20-24% moisture content
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 wet hulling in Indonesia comes straight from local weather. Indonesia stays at 70-90% humidity year-round with continuous typhoons, and in some regions annual rainfall can reach as much as 2000mm. Green beans are most vulnerable to rain, so how does Indonesia overcome such harsh weather to produce rich, mellow FrontStreet Coffee Mandheling? The answer is wet hulling.

Under tropical conditions, coffee typically takes 2-3 weeks to dry. In Indonesia's humid climate, drying coffee becomes a major challenge. It has to take longer, during which the coffee holds more moisture and bacteria find it easier to penetrate the bean. In regular washed processing, drying is done with the parchment still on, protecting the bean from external damage. Wet hulling, by contrast, removes the parchment for the final drying stage, letting sunlight hit the bean surface directly and causing the coffee to dry 2-3 times faster than washed processing.

FrontStreet Coffee's bean list includes two coffees processed with wet hulling: FrontStreet Coffee Indonesian Mandheling, in the form of FrontStreet Coffee Lintong Mandheling and PWN FrontStreet Coffee Golden Mandheling.

FrontStreet Coffee Golden Mandheling is produced by the famous Indonesian PWN company and processed with wet hulling. Although wet hulling raises the defect rate considerably, the flavor does not turn negative; instead, it brings a unique herbal aroma. Without parchment protection during the final drying stage, the green beans are directly exposed to the environment and affected by various microorganisms, which is what gives Indonesian coffee its distinctive woody, spicy, and herbal notes, the quality we call terroir flavor.

Because wet hulling uses rough friction to force off the parchment while the beans are semi-dry, the process puts Mandheling green beans under significant pressure, deforming or even crushing them. Wet hulling therefore produces a higher defect rate. As a result, PWN invests heavily in its green beans: it buys only G1 grade FrontStreet Coffee Mandheling from the Lake Tawa area of Mount Gayo in northwestern Sumatra, then carries out four rounds of manual sorting to ensure uniform color, shape, and size. That 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 qualify as G1 grade, the highest for Indonesian green beans.

To present the classic mellow flavor of FrontStreet Coffee Mandheling, our roaster chooses a medium-dark roast to enhance the aroma of dark chocolate and caramel. Our baristas also adjust the brewing method to match the roast level and flavor profile.

How to Brew FrontStreet Coffee Golden Mandheling for Best Taste?

FrontStreet Coffee's brewing parameters are based on bean characteristics, roast degree, and flavor profile. Mandheling is a dark-roasted, bitter coffee, and our baristas use a KONO dripper to brew beans with such a rich mouthfeel. Compared with the V60, the KONO has smooth curved surfaces and a weaker exhaust effect, and its straight ribs cover only one-quarter of the dripper, allowing the coffee bed and hot water to form an immersion-style extraction that produces a rounder, thicker cup. The KONO is also suitable for coffees like FrontStreet Coffee Brazil Queen Estate and FrontStreet Coffee Jamaica Blue Mountain No. 1.

Dark roasting makes the internal structure of the beans looser, so we can easily crush them with a gentle pinch, which signals strong water absorption after grinding. To avoid releasing excessive bitter compounds once the grounds absorb water, FrontStreet Coffee adjusts the grind coarser and uses cooler water for the pour-over.

Grind degree is one of the key factors affecting flavor. Once beans are ground and meet hot water, they release large amounts of water-soluble flavor compounds. FrontStreet Coffee adjusts grind with a tool, the China No. 20 0.85mm standard sieve. For light-to-medium roasted beans we consistently use a 75-80% pass rate, while for medium-dark beans we use 70-75%. Compared with light roasts, the internal structure of medium-dark roasted FrontStreet Coffee Mandheling is looser, so water absorption is better and large-molecule bitter compounds release more easily. To avoid over-extraction, FrontStreet Coffee chooses a grind with a 70% pass rate on the No. 20 standard sieve.

Water temperature works much like grind degree. The hotter the water, the higher the extraction efficiency, making compounds easier to release and the cup more concentrated. Brewing with near-boiling water easily pulls out unpleasant bitterness, so when brewing medium-dark beans, FrontStreet Coffee uses different parameters than for light roasts. Caramelization is greater than in a light roast, making it easier to release bitter, large-molecule compounds, so we lower the water temperature slightly. The brewing temperature for light-roasted coffee is 91-93°C, while FrontStreet Coffee recommends 87-88°C for medium-dark roasted beans.

Recommended brewing water temperature: 87-88°C
Coffee powder: 15g
Powder to water ratio: 1:15
Grind degree: Medium-coarse grind (70% pass rate on China standard No. 20 sieve)

The brewing method uses a three-stage pour-over, which better expresses the rich mouthfeel and caramel sweetness of FrontStreet Coffee Golden Mandheling.

The first bloom stage uses 30ml of water, completely wetting the grounds for degassing so that flavor compounds extract better in the steps that follow. The second stage injects 100ml, mainly to wash out the golden foam of the Mandheling and raise the bed. The final stage gently injects 95ml at the center, which boosts the coffee's sweetness and expresses a thick body overall, with caramel sweetness and chocolate and nut flavors.

FrontStreet Coffee Recommendation

If wet hulling has made you curious about Mandheling, FrontStreet Coffee's PWN Golden Mandheling is the definitive place to start. PWN buys only high-quality Mandheling green beans, then puts them through machine sorting and rounds of hand sorting to pull out full, defect-free beans, and the Golden Mandheling is its flagship. The beans are even and plump, large and clear, with a low defect rate, and a medium-dark roast adds a rich roasted aroma. The brewed black coffee is clean and full-bodied, ideal for anyone who dislikes acidic coffee.

For a second point of reference, FrontStreet Coffee's Lintong Mandheling Daily Bean and the PWN Golden Mandheling come from the Lintong region and the Aceh region of Sumatra respectively, both grown in the region's wet-hulled tradition. Expect a high, mellow aroma with a strong, stimulating body, clove, woody, and herbal notes, a low acidity from the medium-dark roast, and a long, thick, sweet finish with a hint of mint. To brew the Golden Mandheling at its best, use water at 87-88°C, 15g of coffee at a 1:15 ratio, a coarse-sugar grind, and a three-stage pour of 30ml to bloom, then 100ml, then a gentle 95ml in the center, which together bring out the full body, caramel sweetness, and chocolate and nut flavors.

If you would rather taste that Mandheling character in a milk drink or on an espresso machine, FrontStreet Coffee's Classic Blend is the highest-repeat-purchase choice. On top of the creamy, nutty base of Brazilian coffee it adds a medium-dark Mandheling, so butter, roasted chestnut, and caramel flavors stand out, the Mandheling lifts the body of the whole cup, and it contributes distinctive sandalwood, herbal, and spice notes. A small amount of Robusta rounds out the oil, making latte art and layered Dirty drinks easy to pour.