Characteristics and Story of Wet-Hulled Indonesian Coffee Beans: What Makes Mandheling Coffee Special?

For more premium coffee knowledge, please follow the WeChat official account: FrontStreet Coffee

As the darkest roast single-origin coffee in FrontStreet Coffee's bean selection, Mandheling coffee possesses rich roasted aromas, a solid and mellow mouthfeel, and low acidity. When dark-roasted, we can experience both its lingering bitter notes and its elegant sweetness. Through both smell and taste, we can simultaneously appreciate its unique earthy aroma, which is truly intriguing.

At this point, some friends might ask: why does Mandheling alone have this unique flavor when all are medium-dark roast coffee beans?

FrontStreet Coffee's Mandheling comes from the mysterious island of Sumatra, Indonesia. The entire island of Sumatra is divided into eight provinces, with only the high-altitude Aceh and North Sumatra provinces engaging in large-scale cultivation of Arabica coffee. Unlike many coffee-producing regions, Indonesia experiences variable weather with high humidity and rainfall throughout the year, with humidity levels between 70-90%. Sometimes it's sunny in the morning and raining heavily in the afternoon, with typhoons arriving unexpectedly, and annual rainfall can even exceed 2000mm.

The Wet Hulling Process

In such harsh climatic conditions, it's very difficult and time-consuming to dry the parchment beans to the specified moisture content. However, Indonesia is a major coffee-producing country focused on mass production. To shorten the coffee bean processing time and quickly sell them for cash, people developed a unique semi-washed method: Wet Hulling.

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

Initially, this region also used natural processing, but the quality was unstable, so they switched to a semi-natural method similar to Brazil's. Farmers first use machines to remove the outer skin of the coffee cherries, then pour them directly into a water tank. They skim off the defective parchment beans floating on the surface and then take out the sunken beans, placing them in a bucket for brief fermentation. The specific fermentation time depends on the situation, usually just a few short hours. After fermentation, a small amount of mucilage remains on the parchment shell, which is brushed clean before placing the parchment beans outdoors to dry.

However, unlike the arid climate of Brazil, it's impossible to dry the sticky parchment beans outdoors for several days after removing the coffee fruit skin in Indonesia's high-temperature, rainy environment. Doing so would only cause them to mold directly, so they can only be勉强 dried for one to two days.

When the moisture content of the parchment beans drops to 30-50%, they are purchased by brokers. At this point, the mucilage layer has not yet solidified, and the bean body is still semi-hard and semi-soft, but they need to be promptly taken to the processing plant and hulled with specialized machines to avoid over-fermentation and sour odors.

Hulling undried green beans is more difficult, so requires more powerful hulling machines

Due to the greater difficulty of hulling in wet hulling, 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 usually flows out. Due to long-term immersion in viscous liquid and the humid environment, the final green beans become soft, pale, swollen, and form a gray-green color. Through the agitation of friction, the green beans are also more easily crushed or squeezed, especially at both ends, forming small gaps resembling sheep's hooves. This is why Sumatra coffee has a higher proportion of so-called "sheep's hoof beans."

It's important to know 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, meaning that wet-hulled coffee beans directly expose their "inner beans" to receive sunlight.

After the parchment beans are hulled by machines, they continue to be dried until the moisture content reaches 12-15%. At this point, the coffee beans can be collected into woven bags, typically 40kg and 80kg per bag. Finally, these green beans are uniformly sent for mechanical sorting to remove various impurities and classified by particle size.

Moreover, because all four protective layers are removed before drying, the beans are completely "exposed" to intense sunlight or humid air, making the drying process very fast. Therefore, whether in terms of fermentation time or drying time, wet hulling is shorter than traditional washed processing, which also saves a lot of labor.

Quality Challenges and Unique Flavors

Although wet hulling shortens the drying time of green beans, the rough production process greatly increases the defect rate. First, natural defects from coffee fruit harvesting are inevitable. Then there's the "crushing" during hulling, moisture absorption during transportation of wet parchment beans, and damage to exposed green beans from sunlight and humidity. Finally, irregular weather带来的 uneven drying also detracts from the flavor. This is why high-grade Mandheling requires so many rounds of manual sorting.

Operations directly exposing beans to humid environments inevitably allow various fungi to grow on the inner beans, thus affecting the final coffee quality. Interestingly, however, "contamination" is not entirely bad. Some fungi actually bring unique flavors to Indonesian coffee. Academic research suggests that the woody, herbal, earthy, and spicy notes of Sumatra coffee are likely created by mold. Finally, through a series of Maillard reactions and caramelization reactions, substances transform into aromas like caramel and dark chocolate, giving coffee a highly recognizable earthy aroma—what we call "terroir."

From this, we can understand that FrontStreet Coffee's Mandheling coffee's unique herbal and woody notes are all due to the high local humidity, which forces the entire post-processing to be divided into three different stages for drying the coffee beans.

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473