Introduction to Sumatra Wet Hulling Process, Sumatra Coffee Varieties, and Using Mandheling for Espresso

As the largest island in Indonesia, Sumatra is bordered by the Indian Ocean to the west and the South China Sea to the east, with the equator running directly through it. Additionally, it is situated right on the Pacific Ring of Fire, dotted with volcanoes that have brought fertile volcanic ash soil to this land. The mountainous areas, often shrouded in clouds and mist, have considerable elevation, abundant rainfall, and comfortable temperatures, making this land rich in products, with coffee being one of the most suitable crops for cultivation.

The Origin of Indonesian Coffee

The beginning of Indonesian coffee dates back to the late 17th century when the Dutch conquered Java Island, turning it into the headquarters of the Dutch East India Company in the East. The Dutch successively introduced two batches of Typica seedlings from India to Java for breeding. Before the 1870s, coffee growing areas were established in Sumatra, Bali, Sulawesi, and Timor, soon expanding from the original Java Island throughout Indonesia. However, this prosperity was short-lived, as the sudden outbreak of coffee leaf rust in 1869 swept through the entire Asian coffee producing region, including Indonesia, causing large areas of Typica trees to wither and die.

To save the devastated coffee plantations, the Indonesian government introduced the more disease-resistant Robusta variety from Africa and other places. Until now, Robusta remains the main variety in Indonesia. To ensure plant survival rates and increase coffee production, Indonesia continuously introduced various disease-resistant varieties from other countries. Among them, the Tim Tim, Bor Bor, and Catimor series Ateng, which are hybrids of Robusta and Arabica, have adapted to Sumatra's terroir due to their excellent disease resistance and abundant yields.

Soon, Ateng and Tim Tim gradually replaced the original Typica varieties, becoming the main varieties in regions like Sumatra. The most famous representative among these specialty coffees is Mandheling. Mandheling is mainly produced in the two major lakes within Aceh Province and North Sumatra Province: Lake Toba and Lake Tawar.

Unique Growing Regions

Lake Toba is the world's largest volcanic lake and also Indonesia's largest highland freshwater lake. The average annual temperature here is around 22°C, belonging to a tropical rainforest climate. The rainy and humid environment, combined with the mountainous elevation of about 1,400 meters, produces various spices, timber, palm oil, and other agricultural products, including coffee of course. Under the cloudy and rainy weather, most fruit trees grow on the slopes at higher elevations, covered by several types of ferns that provide good sun protection for the coffee, so growers do not need to adopt shade cultivation.

The Gayo Mountains, located in the central part of Aceh Province in Sumatra, have a very unique microclimate environment, near the famous volcanic Lake Tawar. Here, the lake water is clear, the natural environment is excellent, and coffee trees are planted on mountains 1,200 to 1,800 meters high. The area is shrouded in mist all year round, with large temperature differences between day and night. The volcanic soil is rich in minerals and nutrients, and fallen fruit serves as fertilizer. Farmers widely use organic methods to grow coffee, coupled with a long history of cultivation, making the coffee produced here of higher quality and yield. Many green coffee merchants also place more importance on this area, including the locally famous Pwani Coffee Company (PWN).

From September to April each year is the coffee fruit ripening season, when you can see many farmers busy picking in the mountains. The traditional Lintong Mandheling is cultivated by the Mandailing and Batak ethnic groups, while the "rising star" Lake Tawar Mandheling is cultivated by the Gayo people.

Unique Processing Method: Wet Hulling

The biggest difference from many coffee-producing regions is that Indonesia mostly has humid weather, with annual humidity between 70-90%, and typhoons can come at any time, with annual rainfall even reaching over 2000mm. Such harsh weather conditions simply cannot allow for the sun-drying method used in Africa for 2-3 weeks. The washed process is not only time-consuming but also consumes more labor costs and water resources, which Indonesia, with its focus on mass production, cannot afford. Therefore, a unique semi-washed method was developed - the wet hulling process.

Coffee cherries with their skin and pulp removed are directly poured into a pool for a brief fermentation of several hours. At this point, a small amount of mucilage still remains on the parchment shell, which is removed through brushing. Next, the parchment beans are dried, with the first drying only needing to reach 30-40% moisture content, about 2-3 days, after which the coffee farmers sell the green beans to buyers. The buyers will directly use machines to hull the purchased semi-dry parchment beans to obtain the innermost green beans.

Because the hulling machine uses greater friction to tear off the parchment layer tightly attached to the green beans, at this stage, white or green mucus usually flows out. Due to long-term mucus soaking and humid environments, the final green beans become soft, pale, and swollen, forming a gray-green color. Through the stirring of friction, the green beans are also more easily crushed or squeezed, especially at both ends, forming small gaps and creating "elephant beans" or "broken beans." The "naked beans" without the parchment layer dry very quickly, while also being directly exposed to the humid environment, where various fungi grow. This gives the coffee a unique woody, spicy, and herbal aroma - what we call the "Mandheling flavor."

FrontStreet Coffee's Mandheling Selection

FrontStreet Coffee hopes everyone can experience the classic Mandheling flavor. The two Mandheling coffees sold daily are both medium-dark roasted, including the Lintong region's Mandheling daily bean and the Aceh region's PWN Gold Mandheling coffee. Deep-roasted Mandheling coffee typically presents aromas of dark chocolate, nuts, caramel, and cream, first reminiscent of the rich and mellow taste of Italian espresso.

However, we should know that since Italian espresso is extracted under fine grinding, high water temperature, and 9 bar pressure, the high concentration of coffee concentrates multiple flavors, including the woody and herbal notes mentioned earlier. Various flavor compounds impact our taste buds exponentially, making the resulting Americanos and lattes likely not very appealing.

Therefore, rather than risking the waste of high-quality coffee beans, FrontStreet Coffee suggests choosing Italian espresso blend coffee beans that are more conducive to extraction, such as FrontStreet Coffee's own roasted classic Italian espresso blend, which combines 50% Brazilian coffee, 30% wet-hulled Mandheling coffee, and 20% Robusta coffee. The resulting espresso not only has rich caramel and cream aromas and solid chocolate richness but also carries Mandheling's unique spice and pine aftertaste, creating a very rich layering.

For professional coffee knowledge exchange and more coffee bean information, please follow Coffee Workshop (WeChat public account: cafe_style).

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