What are the specific steps of the Indonesian wet hulling process? Why does Golden Mandheling have a unique herbal flavor?

The Distinctive Character of Mandheling Coffee

As the darkest roasted single-origin coffee in the selection, Mandheling coffee boasts rich roasted aromas, with a solid and mellow mouthfeel, low acidity, and a lingering bitterness complemented by elegant sweetness when dark roasted. Through both smell and taste, one can experience its unique stuffy aroma profile, which is truly intriguing.

Why Mandheling's Unique Flavor?

At this point, some might wonder: among all medium-dark roasted coffee beans, why does only Mandheling possess this distinctive flavor?

Mandheling originates from the mysterious island of Sumatra in Indonesia. The entire island of Sumatra is divided into eight provinces, with only the high-altitude Aceh and North Sumatra provinces cultivating Arabica coffee on a large scale. Unlike many coffee-producing regions, Indonesia experiences variable weather patterns - humid and rainy, with annual humidity between 70-90%. Sometimes sunny mornings give way to afternoon downpours, with typhoons appearing unexpectedly, and annual rainfall can even exceed 2000mm.

The Wet Hulling Method

Under such harsh climatic conditions, drying parchment beans to the specified moisture content is extremely difficult and time-consuming. As Indonesia is a major coffee-producing country focused on mass production, people developed a unique semi-washed method to shorten coffee processing time and enable quicker sales: the Wet Hulling method.

Wet Hulling, known as "Giling Basah" in Indonesian, has existed and been practiced in coffee production for many years, alongside traditional methods like washed, natural, and honey processing. Currently, wet-hulled coffee is still primarily produced in Indonesia, particularly in the northern regions of Sumatra and the Sulawesi growing area.

The Processing Journey

Initially, this region also focused on natural processing, but due to inconsistent quality, they switched to a method similar to Brazil's pulped natural process. Farmers first use machines to remove the outer skin of coffee cherries, directly pour them into a water pool, skim off the defective parchment beans floating on the surface, then scoop out the sinking beans and place them in a bucket for brief fermentation. The specific fermentation time varies depending on circumstances but typically lasts only a few hours. After fermentation, a small amount of mucilage remains on the parchment shell, which is thoroughly cleaned before the parchment beans are placed outdoors for drying.

However, unlike arid Brazil, this region cannot dry the sticky parchment beans outdoors for several days after removing the coffee fruit skin. In the hot and rainy conditions of Indonesia, doing so would cause them to mold directly, so they can only be dried with difficulty for one to two days.

When the moisture content of parchment beans drops to 30-50%, they are purchased by middlemen. At this point, the mucilage layer has not yet solidified, and the beans are still semi-hard and semi-soft, but they need to be promptly taken to processing plants to use specialized machines to remove the parchment shell to prevent over-fermentation and souring.

Hulling undried green beans is more difficult, requiring more powerful hulling machines

Due to the greater difficulty of hulling in the wet hulling method, machines need to use greater friction to tear away the parchment layer tightly attached to the green beans. During this stage, white or green viscous liquid typically flows out. Due to long-term immersion in this liquid and the humid environment, the green beans eventually become soft, turn white and swollen, and form a gray-green color. Through the stirring action of friction, the green beans are also more easily crushed or bruised, especially at both ends, forming small notches resembling goat's hooves - this is why Sumatra coffee has a higher proportion of "goat hoof" defects.

It's important to understand that coffee beans have four protective layers: fruit skin, mucilage, parchment shell, and silver skin. Wet hulling removes the third and fourth protective layers midway through the process, meaning wet-hulled coffee beans directly expose their "inner beans" to receive sunlight.

After the parchment beans have their shells removed by machines, they continue to be dried until moisture content reaches 12-15%, at which point the coffee beans can be collected into woven bags, typically 40 and 80 kilograms per bag. Finally, these green beans are uniformly sent for mechanical sorting to remove various impurities and classify them by particle size.

Advantages and Disadvantages

Moreover, because all four protective layers are removed before drying, the beans are completely "exposed" to intense sunlight or humid air, resulting in very fast drying speeds. Therefore, whether in terms of fermentation time or drying time, wet hulling is shorter than traditional washed processes, thus saving considerable labor.

Although wet hulling shortens the drying time for green beans, the roughness of the production process significantly increases the defect rate. First, natural defects present from coffee cherry harvest are unavoidable, followed by the "crushing" during hulling, moisture exposure during transport of wet parchment beans, and damage to exposed green beans from sunlight and humidity. Finally, irregular drying speeds caused by variable weather also detract from the flavor. This is why high-grade Mandheling requires such extensive hand-picking.

The Origin of Unique Flavors

Operations directly exposed to humid environments inevitably allow various microorganisms to grow on the inner beans, thereby affecting the final coffee quality. Interestingly, however, "contamination" is not entirely bad - some microorganisms bring unique flavors to Indonesian coffee. Academic research suggests that Sumatra coffee's woody, herbaceous, earthy, and spicy notes are likely created by mold. Finally, through a series of Maillard reactions and caramelization reactions, substances transform into aromas like caramel and dark chocolate, presenting coffee with a highly recognizable stuffy aroma profile - what we call "terroir flavor."

Conclusion

From this, we can understand that Mandheling's unique herbal fragrance and沉木味 (沉木味 refers to aged wood fragrance) are due to the high local humidity, which forced the entire post-processing to be divided into three different stages for drying coffee beans.

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