Characteristics of Sumatra Wet-Hulled Coffee Beans_Differences Between Indonesian Wet-Hulled, Washed, and Natural Coffee

Sumatra Mandheling Harvest Season and Processing

March to May and September to December are the harvest seasons for Mandheling in Sumatra. Most coffee farmers now harvest only fully red cherries. After collecting the cherries in the morning, they remove the fruit skin and pulp in the afternoon. Sumatra's natural environment is superior, and most of the water used comes from mountain springs.

The Wet-Hulling Process

1. Removing Coffee Fruit Skin
After removing the fruit skin, the parchment beans are placed in large buckets or troughs filled with water, and any defective parchment beans floating on the surface are scooped out.

2. Fermentation
The dense parchment beans that sink to the bottom are briefly washed, then placed in buckets or plastic bags for a short dry fermentation. This allows the mucilage sugars on the seed coat surface to ferment and develop flavor. Generally, the longer the fermentation, the stronger the acidity. The duration varies with individual preference, typically lasting only a few hours. Some estates, however, skip the dry fermentation stage and sun-dry the parchment beans directly to reduce acidity and enhance the viscous mouthfeel, letting the mucilage sugars ferment fully to increase flavor. The usual fermentation time is between 12-36 hours, depending on the specific conditions.

3. Sun-Drying and Hulling
The parchment beans are sun-dried for one to two days until the moisture content reaches 30%-50%. The beans remain semi-hard and semi-soft, and then the hulling machine is used to remove the seed coat before further drying, which accelerates the process. After about two days, when the moisture content reaches 12%-13%, the beans are collected into woven bags, typically 40 and 80 kilograms per bag, and sent to coffee processing factories for hulling. The entire process takes about four days.

Hulling Process
Hulling involves grinding off the bean husks with a hulling machine, then continuing to dry until the moisture content reaches approximately 12%-15%. The beans are then sent for mechanical sorting to remove impurities and are classified by particle size.

Potential Issues in Wet-Hulling

Unexpected Challenges
During the hulling process, the coffee bean temperature can rise to 30-60 degrees Celsius, completely destroying the parchment layer and potentially triggering bean germination.

Wet-hulled coffee beans may also have higher rates of defective beans, such as moldy beans, than washed and natural coffees. This is because the fruit skin is removed during processing, allowing the beans direct contact with the air.

The Role of Mold in Flavor Development

Mold Creates Good Flavors
Coffee beans have four protective layers: fruit skin, mucilage, parchment, and silver skin. In washed processing, only the first two layers are removed, preserving the parchment and silver skin for sun-drying. Wet-hulling, however, removes the third and fourth protective layers midway, exposing the beans directly to sunlight, which is why Sumatra green beans have a blue-green color. While wet-hulling shortens drying time, it also significantly increases the chances of contamination by mold, fungi, and yeast.

Interestingly, contamination is not necessarily bad; it depends on the specific mold species. Dr. Martha Taniwaki of the Brazilian Food Technology Research Institute once experimented with different mold species to infect coffee beans, producing flavors including spoilage, rancidity, moldy odor, woody, iodine, caramel, chocolate, and floral notes. Academics believe that Sumatra coffee's woody, herbal, earthy, vanilla, and spice flavors are likely created by mold.

Roastery managers often report to academia that some beans infected with certain molds taste better after thorough cleaning, while others become unpalatable, and this depends on the mold species. Perhaps it is possible to inhibit bad molds and introduce beneficial ones, similar to the winemaking concepts of "controlling wild yeast" and "cultivating elegant yeast." Which molds help enhance coffee flavor and which are harmful, however, remains to be researched further.

High Incidence of Elephant Beans

Wet-hulling may trigger the germination mechanism of green beans, affecting flavor. Because beans stripped of their four protective layers are fully exposed, they are more likely to germinate than washed beans that lose only two layers. This activates the metabolism of sugars, proteins, and fats, all of which are precursor aromatic compounds in coffee.

In addition, the friction generated during hulling raises the bean temperature to 30°C-60°C, which also promotes germination and mold growth. Moreover, the semi-hard, semi-soft moist green beans are easily damaged by mechanical force during parchment removal, causing the beans to crack open in a hoof-like shape, the so-called elephant beans. This is why Sumatra has a higher incidence of elephant beans, though whether elephant beans are good or bad remains inconclusive to this day.

One expert joked: "With proper control, what's released is rich fruit aroma and sweetness. The key is keeping equipment and beans clean. Once hulled, drying must be quick to produce the mellow, low-acidity, sweet Sumatra flavor; if control is poor, you may get bland Sumatra coffee with moldy and earthy flavors. If you prefer stronger acidity, Gayo can also undergo washed processing, depending on customer needs."

Summary of Wet-Hulling Characteristics

Flavor Profile
To summarize the flavor of wet-hulling, setting aside subtle differences, wet-hulled beans are generally considered to carry earthy, smoky, and chocolate flavors. The acidity is described as "low acidity," and the body is mellow yet somewhat heavy.

The low acidity comes from the shorter and less effective fermentation and the longer drying time. As for the "earthy taste" of wet-hulled beans, opinions vary widely. Some say it relates to the bean varieties selected in wet-hulling regions (Timor and Catimor, with their Robusta lineage), others say it results from the organic interaction between green beans and the external environment, and some even attribute it to too many defective beans.

There are numerous sources of defective beans in wet-hulled coffee. First, natural defects from the fruit stage are inevitable. The hulling process contributes as well, along with corrosion during the transport of wet parchment beans, damage to exposed green beans from sunlight and moisture, and irregular weather causing uneven drying speeds that detract from flavor. This is exactly why Mandheling requires so many manual selections.

Comparison of Green Bean Processing Methods

Acidity: Washed > Semi-washed > Honey > Wet-hulled > Natural

Sweetness: Natural > Honey > Wet-hulled > Semi-washed > Washed

Production Risk: Wet-hulled / Natural > Washed / Honey > Semi-washed

Equipment Cost: Washed / Semi-washed > Honey / Wet-hulled > Natural

Water Usage: Washed > Semi-washed > Honey / Wet-hulled

FrontStreet Coffee Recommendation

Wet-hulling gives Sumatran coffee its heavy body and low acidity, and FrontStreet's Sumatra Mandheling is the classic place to start. FrontStreet's Lindong Mandheling and PWN Golden Mandheling come from the Lindong region and the Aceh region of Sumatra, respectively, and both are roasted medium-dark so that the dark chocolate and caramel character shows while the wild herbal side is preserved, a fragrance that comes directly from the growing environment and processing method. Mandheling's signature is a heavy, low-acid cup with notes of clove, woody aroma, and, at medium-dark roast, a spice and mint impression and a strong, lasting, mellow sweetness.

For the highest grade, FrontStreet's PWN Golden Mandheling is the top choice. PWN mainly buys high-quality Mandheling green beans, and after purchase it applies machine sorting and hand sorting to pick out only full, flawless beans, all of them specialty grade, with the Golden Mandheling as its flagship product. The beans are uniform, plump, large, and clear with a low defect rate, and after a medium-dark roast they carry a rich roasted aroma and brew into a clean, full-bodied cup that suits anyone who dislikes acidic coffee. FrontStreet recommends a pour-over at 87-88°C with 15g of coffee at a 1:15 ratio and a coarse, granulated-sugar grind, using a three-stage pour: a 30ml bloom, then 100ml to lift the golden foam and raise the bed, and finally a gentle 95ml center pour to raise the sweetness, bringing out the thick body and the caramel, chocolate, and nut flavors.