The Difference Between Wet Hulling and Wet Processing - Why Use Wet Hulling for Mandheling Coffee Beans?

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Are Wet Hulling and Wet Processing Methods Similar Yet Different?

In traditional wet processing methods, fresh coffee cherries are pulped and left to ferment overnight in concrete pools, undergoing acidification and pectin decomposition. Next, the green beans with parchment are dried in the sun for about 20 days until reaching 11% moisture content. They can then be packaged, with the parchment layer removed, and the coffee is sorted by density and size and hand-graded for export.

The green beans continue to "wash" within the parchment layer. Protected by the parchment, the green beans themselves are placed in a relatively clean microenvironment that can stabilize moisture, moderate temperature fluctuations between day and night, and reduce the effects of direct sunlight.

So far, wet hulling and wet processing are basically the same.

As mentioned above, in wet processing, beans in this state are typically dried to 11% moisture content. The green beans become what we know as dry green beans, with thin parchment that can be easily removed.

Next, you'll see the different side of wet hulling.

What Exactly is Wet Hulling Processing? What Are the Specific Procedures?

Wet hulling, also known as Wet Hulling, is called Giling Basah in the local language and is a traditional Indonesian coffee processing method. Judging by the name alone, wet hulling seems similar to wet processing (washed), however, these two processing methods produce distinctly different cupping flavors. Coffee processed by wet hulling is typically full-bodied and intense, with a very distinct character.

Because there's considerable space between the parchment shell and green beans, farmers sell the green beans to local collection stations that purchase wet-parchment coffee when the beans first reach 35-40% moisture content. During the collection process, some are packaged in separate bags, some mixed, so the moisture content of the green beans may vary. Either due to incomplete fermentation forms or mucilage still partially adhering to the parchment layer in a partially dry state. However, quality assessments are not made when collecting green beans; they are all mixed and bagged together.

Dried with parchment in the sun for 2-3 days until reaching 20-24% moisture content, they are processed through a specially made wet hulling machine. This machine uses significant friction to tear away the parchment layer tightly attached to the green beans. At this stage, white or green mucus typically flows out. Due to prolonged mucilage soaking and humid conditions, the resulting green beans are very soft, pale and swollen, forming a gray-green color. Through friction stirring, they are easily crushed or squeezed, especially at the ends, forming small notches that create "horseshoe beans" 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 parchment protection, the green beans dry very quickly, finally drying to 12-13% moisture content. At this point, the green beans appear dark green or blue-gray. The green beans are then sorted and bagged for export preparation.

Without parchment protection, 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 may further lead to inconsistent roasting levels). Excessive heat during the hulling process can also cause rapid coffee aging, resulting in the woody, earthy flavors we often taste in Sumatran coffee.

Wet Hulling Processing Steps

Remove skin and pulp, keeping parchment and mucilage

Pool fermentation

Wash away mucilage

Sun-dry with parchment for 2-3 days until reaching 20-24% moisture content

Hull off the parchment

Dry green beans to 12-13% moisture content

Prepare for export

Why Use Wet Hulling?

Climate Reasons

The tradition of using wet hulling in Indonesia must be understood in the context of local weather. Indonesia's humidity remains between 70-90% year-round with continuous typhoons, and in some regions, annual rainfall can reach as much as 2,000mm. Knowing that green beans fear rain most, how does Indonesia overcome such harsh weather conditions to produce the rich and full-bodied Mandheling coffee? That's where wet hulling comes in.

Under tropical climate conditions, coffee takes an average of 2-3 weeks to dry. In Indonesia's particularly humid climate, drying coffee becomes a major problem. Coffee drying must take longer, during which time the coffee maintains higher humidity, making it easier for bacteria to penetrate the green beans. In ordinary washed processing, the drying process is carried out with parchment to protect the green beans from external damage. However, we can note that wet hulling removes parchment for the final drying step, allowing sunlight to directly hit the green bean surface, causing coffee green beans to dry quickly, 2-3 times faster than washed processing.

Economic Benefits

Dutch colonists first introduced coffee to Indonesia in the 17th century. These colonists pursued more and faster economic returns. Wet hulling processing greatly reduced time spent on farms and significantly lowered labor costs. This aligns with the investor mentality of pursuing quick profits and reducing costs. This is also a driving force behind wet hulling processing.

Defects and Kuku Kambing (Goat Hoof Beans)

When wet hulling removes parchment, the coffee's moisture content is still as high as 20-24%, whereas in general processing methods, coffee moisture content is reduced to 10-12% before hulling. In the "semi-dry" state, parchment tends to adhere to the green bean surface, making hulling more difficult than "fully dry" hulling, requiring greater friction. However, on the other hand, "semi-dry" coffee green beans are very fragile at this point, with hardness far below that of "fully dry" green beans, so the beans are more easily squeezed, forming small notches. This is what we commonly call "horseshoe beans" or "goat hoof beans" (locally called kuku kambing).

The ultra-fast speed of wet hulling processing also results in higher defect rates in wet hulled coffee beans. Employees are arranged at processing plants for manual defect sorting, typically including DP (double picking) and TP (triple picking). Mandheling that undergoes triple picking will have better defect rates than double picking.

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