The Characteristics and Story of Indonesian Aged Mandheling Wet-Hulled Coffee Beans

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As the single-origin coffee with the darkest roast level on FrontStreet Coffee's menu, Mandheling coffee possesses rich roasted aromas, with a solid and mellow mouthfeel, low acidity, and a long-lasting bitterness when deep-roasted. We can experience both its lingering bitter notes and elegant sweetness. Through both smell and taste, we can simultaneously perceive its unique earthy aroma, which is truly captivating.

At this point, some might ask: among all medium-dark roasted coffee beans, why does only Mandheling have this unique flavor?

FrontStreet Coffee's Mandheling comes 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. The biggest difference from many coffee-producing regions is Indonesia's variable weather, which is humid and rainy, with humidity levels between 70-90% year-round. Sometimes it's sunny in the morning and heavy rain in the afternoon, with typhoons arriving unexpectedly, and annual rainfall even reaching over 2000mm.

Wet Hulling Processing Method

Under such harsh climatic conditions, drying the 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 quickly sell it for cash: the Wet Hulling method.

Wet Hulling, known as "Giling Basah" in Indonesian, is a processing method that, like traditional methods such as washed, natural, and honey processing, has existed and been practiced in coffee production for many years. Currently, Wet Hulling coffee is still mainly produced in Indonesia, particularly in the northern regions of Sumatra and the Sulawesi producing areas.

Initially, sun-drying was the primary processing method here, but quality was inconsistent, so it was changed to a semi-natural method similar to Brazil's. Farmers 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 take out the beans that sink to the bottom and place them in a bucket for brief fermentation. The specific fermentation time depends on the situation but generally lasts only a few hours. After fermentation, a small amount of mucilage remains on the parchment shell, which is cleaned off before placing the parchment beans outdoors for drying.

However, unlike the arid climate of Brazil, it's impossible to dry the sticky parchment beans outdoors for several days here. Doing so in the hot, rainy environment of Indonesia 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 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 the processing plant to have the hulls removed with specialized machinery to prevent over-fermentation and sour odors.

Since hulling is more difficult under 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 pale and swollen, forming a gray-green color. The stirring from friction also makes the green beans more prone to being crushed or squeezed, especially at the ends, forming small notches shaped like goat hooves. This is why Sumatra coffee has a higher proportion of "goat hoof" beans.

It's important to know that coffee beans have four protective layers: skin, mucilage, parchment, and silver skin. Wet Hulling removes the third and fourth layers midway, meaning that Wet Hulling coffee beans are directly exposed as "inner beans" to receive sun bathing.

After the parchment beans have their hulls removed 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 40 and 80 kilograms 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, resulting in very rapid drying. Therefore, whether in terms of fermentation time or drying time, Wet Hulling is shorter than traditional washed processing, thus saving significant labor.

Characteristics and Defects of Wet Hulling

Although Wet Hulling shortens the drying time of green beans, the roughness of the production process greatly increases the defect rate. First, natural defects that occur during coffee cherry harvesting are unavoidable. Next is the "crushing" during hulling, plus moisture absorption during the transport of wet parchment beans, and damage from sunlight and humidity to the exposed green beans. Finally, uneven drying caused by irregular weather also detracts from the flavor. This is why high-grade Mandheling requires so many rounds of manual sorting.

Operations involving direct exposure to humid environments inevitably cause various microorganisms to grow on the inner beans, thus affecting the final coffee quality. Interestingly, however, "contamination" is not entirely a bad thing, as some microorganisms bring unique flavors to Indonesian coffee. Academic research suggests that the woody, herbaceous, 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 the coffee its highly recognizable earthy aroma—what we call "terroir flavor."

From this, we can understand that the unique herbal and aged wood notes of FrontStreet Coffee's Mandheling coffee result from the high local humidity, which forces the entire post-processing to be divided into three different stages for drying coffee beans. If you want to experience Mandheling's unique flavor, you can purchase it directly from FrontStreet Coffee's Taobao store or FrontStreet Coffee's Tmall flagship store.

Aged Mandheling Processing

First, freshly harvested coffee cherries must undergo regular processing. For example, FrontStreet Coffee's aged Mandheling coffee beans are processed using the Wet Hulling method. During this process, carefully selected good beans (full, large particle size, defect-free) are chosen for aging. After screening the coffee beans, they are stored in warehouses for aging.

Aging processing must be conducted in the country of origin, placed in warehouses with temperature and humidity control or good ventilation. During the aging period, the beans must be regularly turned and checked for two purposes: first, to ensure the coffee makes even contact with air for consistent dryness; second, to check for moldy or insect-damaged beans to prevent widespread infection.

The current aging time for aged Mandheling is generally 2-3 years. During this period, the acidity of the coffee beans gradually diminishes and converts to sugars, and the color becomes deeper, mostly yellowish-brown or dark brown, making them highly recognizable.

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