Why does Indonesian coffee beans origin taste so earthy?

Mandheling's low acidity, heavy body and herbal, woody aroma come largely from Indonesia's wet-hulling method, not from soil and climate alone. Sumatra's Aceh and North Sumatra provinces grow Arabica at high altitude, where humidity stays between 70 and 90 percent, afternoon rain is routine and annual rainfall passes 2000 millimeters. Parchment beans rarely dry fully under those conditions, so producers hull them early.

Wet hulling removes the parchment while the bean still holds high moisture, then drying continues on the exposed green bean. The humid environment and slow drying encourage mold activity and Maillard reactions, which build the musty, regional character. Mandheling is roasted dark, the darkest single origin on the list, giving a tight mouthfeel, lasting bitter aftertaste and a sweet return. The same process also produces peaberry, or sheep hoof beans.

In the coffee world, Indonesian Mandheling stands out with its solid, mellow body and unique herbal, woody aromas. As the darkest roast single origin on the bean list, Front Street Mandheling presents a personality of low acidity, high body, and a long, intertwining bitter sweetness. This highly recognizable flavor is due not only to the soil and climate conditions of Sumatra's high-altitude growing regions, but more crucially to the local unique wet-hulling method. This article will provide an in-depth analysis of the complete wet-hulling process, from the climatic background and semi-washed operations to the formation of peaberry beans, and explore how the humid environment, through mold and Maillard reactions, ultimately creates Mandheling's signature musty aroma and regional flavor.

When it comes to Indonesia, coffee enthusiasts will most likely think of the name Mandheling, which can be considered the tough guy representative of the coffee world. Front Street Mandheling coffee beans have low acidity, prominent body, and a hint of herbal and woody notes. Among the factors that create this bean's special flavor, the growing environment is certainly key, but more central is a traditional processing method unique to Indonesia—wet hulling.

On Front Street's bean list, Mandheling is the darkest roasted single origin. It has a full roasting aroma, a tight and heavy mouthfeel, and weak acidity. After deep roasting, one can feel the lasting bitter aftertaste while also tasting an elegant sweet return, with both smell and taste simultaneously capturing that unique musty aroma, thought-provoking.

Some may wonder: among beans of the same medium-dark roast level, why is it only Mandheling that carries this special taste?

Mandheling comes from the mysterious island of Sumatra in Indonesia. Sumatra is divided into eight provinces, but only the higher-altitude Aceh and North Sumatra provinces cultivate Arabica on a large scale. What is completely different from most coffee-producing regions is that Indonesia's weather is capricious, humid, and rainy, with year-round humidity maintained between 70% and 90%. It is often sunny in the morning, then pouring rain in the afternoon, typhoons can come at any time, and annual rainfall even exceeds 2000 millimeters.

Under such harsh climatic conditions, getting the parchment beans to fully dry to the target moisture content is both difficult and time-consuming. And Indonesia is a coffee powerhouse oriented toward mass production. In order to compress the processing cycle and sell for cash as quickly as possible, the locals developed a unique semi-washed solution: wet hulling.

Wet hulling is called "Giling Basah" in Indonesian. Like washed, natural, and honey processing, it has been used in coffee production for many years. At present, wet-hulled coffee still mainly comes from Indonesia, with the northern Sumatra and Sulawesi producing regions being the most concentrated.

Originally, natural processing was also dominant here, but quality fluctuated greatly, so it later shifted to a semi-natural method similar to Brazil's. Farmers first use machines to remove the outer skin of the coffee cherries, then pour the beans into a water tank, scoop out the defective parchment beans floating on the surface, and then take out the beans that sink to the bottom and put them into barrels for short fermentation. The specific duration depends on the situation, usually only a few hours. After fermentation ends, a small amount of mucilage still remains on the bean shell, which must be brushed clean before the parchment beans are placed outdoors to dry.

But Indonesia is not as dry as Brazil. After peeling and extracting the sticky parchment beans, they cannot be dried outdoors for several consecutive days, because in the local high-temperature and rainy conditions doing so would only make the beans mold directly, so at most they can barely be dried for one or two days.

When the moisture content of the parchment beans drops to 30% to 50%, they are collected by middlemen. At this point, the mucilage layer has not yet solidified, and the beans are in a half-hard, half-soft state. They must be promptly sent back to the processing plant to have the seed shell removed with professional machinery, otherwise excessive fermentation will lead to sour spoilage.

Hulling green beans that have not yet dried is more difficult, so a more powerful huller is needed. Under wet hulling, hulling is more laborious, and the machine must apply greater friction to tear open the parchment layer tightly clinging to the green beans. At this stage, white or green viscous liquid usually flows out. Because they have been soaked in mucilage for a long time and because the environment is humid, the green beans eventually become soft, white, and swollen, appearing grayish-green. Under the stirring of friction, the green beans are also more likely to be crushed or pinched, especially at both ends, forming small gaps shaped like sheep hooves, which is precisely the reason for the high proportion of peaberry beans in Sumatra coffee.

You should know that coffee beans originally have four layers of protection: skin, mucilage, seed shell, and silver skin, while wet hulling removes the third and fourth layers midway. In other words, coffee beans processed by wet hulling are directly exposed to sunlight in their "inner bean" naked state.

After the seed shell is removed from the parchment beans by machine, they must continue to be dried until the moisture content drops to between 12% and 15%. Then the coffee beans can be packed into woven bags, usually 40 kilograms or 80 kilograms per bag. Finally, these green beans are uniformly sent for machine sorting to remove various impurities and grade them by bean size.

Moreover, because all four layers of protection have been removed before drying, the beans are completely "naked" under the scorching sun or in humid air, and the drying speed is extremely fast. Therefore, whether in fermentation or drying, wet hulling takes less time than traditional washing and also saves a great deal of labor.

Although wet hulling compresses the drying time of green beans, its rough production stages also cause the defect rate to rise sharply. First, the natural defects carried by coffee cherries at harvest are unavoidable, then there is the "crushing" at the hulling stage, as well as moisture damage during transport of wet parchment beans, and damage to the exposed green beans from sunlight and moisture. Finally, uneven drying caused by irregular weather will also drag down the flavor. This is the main reason why high-grade Mandheling needs multiple rounds of hand sorting.

Operations directly exposed to a humid environment will inevitably cause the inner beans to pick up various strains of bacteria, thereby affecting final quality. But interestingly, "contamination" is not entirely a bad thing; certain strains instead bring unique flavors to Indonesian coffee. Some academic research suggests that the woody, grass jelly-like, earthy, and spicy flavors of Sumatra coffee are very likely the result of mold action. Subsequently, the coffee beans undergo a series of Maillard reactions and caramelization reactions, and substances are transformed into caramel and dark chocolate-like aromas, giving the coffee a highly recognizable musty aroma, which is what we often call "regional flavor."

From this, it can be seen that the roots of Mandheling's unique herbal aroma and aged wood taste lie in the local high humidity, which forces the entire post-processing procedure to be divided into three different stages to dry the coffee beans.

The above content is compiled by CoffeeHunters, a coffee news website.