What is Sumatra Wet Hulling? Introduction to the Characteristics of Wet-Hulled Coffee Beans

Understanding Wet Hulling: Indonesia's Unique Coffee Processing Method

Wet hulling is a green coffee processing method invented by Indonesian coffee farmers to adapt to the local extreme climate. Not only does it shorten the drying time of coffee beans, but it also creates the unique style of Mandheling coffee. Today, wet hulling has become the most common post-processing method for Indonesian coffee. FrontStreet Coffee will now introduce this unique green coffee processing method to everyone.

What is Indonesia's Most Famous Coffee?

As the largest island within Indonesia, Sumatra is bordered by the Indian Ocean to the west and the South China Sea to the east, with the equator running through its center. Additionally, it happens to be situated on the Pacific Ring of Fire, with volcanoes scattered throughout, bringing fertile volcanic ash soil to this land. The mountainous areas, perpetually shrouded in clouds and mist, have considerable altitude, abundant rainfall, and comfortable temperatures, making this land rich in products, with coffee being one of the most suitable crops for cultivation.

As the representative coffee of Sumatra Island, Mandheling was originally a very small place name, which later gradually evolved into a designation for an ethnic group—the Mandheling people. Mandheling is mainly produced from the two major lakes within Aceh Province and North Sumatra Province: Lake Toba and Lake Tawar.

Lake Toba is the world's largest volcanic lake and also Indonesia's largest alpine 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 mountain altitude 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 higher altitude slopes, covered by several types of ferns, which together provide good sun protection for 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. The lake water here is clear, and the natural environment is excellent. Coffee trees are planted on high mountains of 1,200-1,800 meters, with year-round mist, large temperature differences between day and night, and volcanic soil rich in minerals and nutrients. Fallen fruits serve as fertilizer. Farmers widely use organic methods to grow coffee, and with a long history of cultivation, the quality and yield of coffee produced here are higher. Many green coffee merchants also value this area more, including the locally famous Pawani Coffee Company (PWN).

From September to April each year is the mature season for coffee cherries, and you can see many farmers busy picking in the mountains. 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.

Why Use Wet Hulling?

The biggest difference from many coffee-producing regions is that Indonesia mostly has humid weather, with humidity between 70-90% throughout the year, typhoons can come at any time, and annual rainfall even reaches over 2000mm. Such harsh weather conditions simply cannot adopt 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, they developed a unique semi-washed method—wet hulling.

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

Since the hulling machine uses greater friction to tear open the parchment layer tightly attached to the green beans, white or green mucus usually flows out during this stage. Due to long-term soaking in mucus and humid environments, the green beans eventually become soft, pale and swollen, forming a gray-green color. After being stirred by friction, the green beans are also more easily crushed or squeezed, especially at both ends, forming small gaps and creating "elephant beans." The "naked beans" without the parchment layer dry very quickly, while also being directly exposed to humid environments, subject to the growth of various microorganisms. This gives the coffee unique woody, spice, and herbal plant aromas—what we call the "Mandheling flavor."

Which Grade of Mandheling is the Highest?

The Indonesian local green coffee merchant PWN Company mainly focuses on acquiring high-quality batches of Mandheling green beans, producing only specialty grade Mandheling. "Golden Mandheling" is their exclusive signature product. At that time, the Japanese took a fancy to Sumatran coffee and planned to purchase it long-term, gradually discovering that the quality of local beans was often inconsistent. Therefore, they paid more attention to Mandheling production and began to formulate strict screening standards and methods for defects, starting with values such as bean density, specifications, shape, and color. The produced Mandheling requires machine and manual removal of defects to ensure that the produced Mandheling particles are complete, uniform, large, and translucent, while also reducing the earthy and grassy flavors of Mandheling. It is said that it appears golden in the sun, hence the name "Golden Mandheling."

It is worth mentioning that in order to protect the prestigious brand of "Golden Mandheling," the forward-thinking PWN Company quickly registered "Golden Mandheling" as an exclusive trademark, so only their Golden Mandheling coffee beans are genuine. Unlike many local green coffee merchants in Indonesia, PWN Company is the exclusive supplier of high-grade Golden Mandheling coffee, contracting the best Mandheling coffee producing areas in Indonesia, and the produced Mandheling beans are all superior products. The Golden Mandheling coffee that consistently occupies the upper right corner of FrontStreet Coffee's bean list is produced by PWN Company, and the Dongshankou store also displays the burlap bags used for green bean packaging and certification certificates.

Wet hulling saves a lot of time and labor costs for green bean processing, but the rapid process also brings a higher defect rate. Therefore, only through strict manual selection can defects that cannot be screened by machines be removed. PWN Company subjects the purchased Mandheling green beans to one machine selection plus three manual selections, with quality far exceeding Indonesia's local highest G1 grade. To meet PWN Golden Mandheling production standards, the following points must be satisfied:

• Green bean size: 18-19 mesh or above after first manual selection
• Less than 3 defective beans in a 300-gram green bean sample
• Green coffee bean grade is G1
• Color is dark green, uniform in size, and regular in shape

Before launching a coffee bean, FrontStreet Coffee will always taste this coffee through cupping. Cupping is an evaluation of the quality and flavor of coffee beans. Unified standards can more objectively identify the strengths and weaknesses of the flavors presented by the beans, as well as their characteristics. Through cupping, FrontStreet Coffee found that PWN's wet-hulled Golden Mandheling not only has aromas of dark chocolate, nuts, and caramel, but also flavors of herbal spices and pine, with the aftertaste of dark chocolate, full-bodied lingering finish, high cleanliness, and a light fruit acidity as the temperature decreases.

Important Notice :

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