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

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Sumatra Mandheling Harvest Season and Processing

March to May and September to December are the harvest seasons for Mandheling in Sumatra. Currently, most coffee farmers only harvest fully red coffee cherries. After collecting coffee 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 coffee fruit skin, place the parchment beans in large buckets or troughs filled with water, and scoop out any defective parchment beans floating on the surface.

2. Fermentation
The dense parchment beans that sink to the bottom are briefly washed, then placed in buckets or plastic bags for a brief dry fermentation process. This allows the mucilage sugars on the seed coat surface to ferment and develop flavor. Generally, the longer the fermentation time, the stronger the acidity. The fermentation duration varies by individual preference, typically lasting only a few hours. However, some estates skip the dry fermentation stage and directly sun-dry the parchment beans to reduce acidity and enhance viscous mouthfeel, allowing the mucilage sugars to fully ferment and increase flavor. The usual fermentation time is between 12-36 hours, depending on 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, then the hulling machine is used to remove the seed coat before further drying, accelerating the drying process. After about two days, when the moisture content reaches 12%-13%, the coffee 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
The hulling process involves grinding off the bean husks with a hulling machine, then continuing to dry until the moisture content reaches approximately 12%-15%. Then, the coffee beans are sent for mechanical sorting to remove various 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, compared to washed and natural processing methods. This is because the fruit skin is removed during processing, allowing the beans direct contact with 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 the washed processing method, only the first two layers are removed, preserving the parchment and silver skin for sun-drying. However, wet-hulling 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 mold, fungi, and yeast contamination.

Interestingly, contamination is not necessarily bad—it depends on the specific mold species. Dr. Martha Taniwaki from the Brazilian Food Technology Research Institute once experimented with different mold species to infect coffee beans, producing various 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 coffee beans infected with certain molds have better flavor after thorough cleaning, while others become unpalatable—this depends on the mold species. Perhaps it's possible to inhibit bad molds and introduce beneficial ones, similar to the concepts in winemaking of "controlling wild yeast" and "cultivating elegant yeast." However, which molds help enhance coffee flavor and which are harmful remains to be further researched.

High Incidence of Elephant Beans

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

Additionally, the friction generated during hulling causes the bean temperature to rise 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 (elephant beans). This is why Sumatra has a higher incidence of elephant beans. However, whether elephant beans are good or bad remains inconclusive to this day.

He joked: "With proper control, what's released is rich fruit aroma and sweetness. The key to control lies in maintaining clean equipment and beans. Once hulled, drying must be quick to produce the mellow, low-acidity, and sweet Sumatra flavor; if control is improper, it may result in bland Sumatra coffee with even 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, excluding subtle differences, we generally believe that wet-hulled beans carry earthy, smoky, and chocolate flavors. The acidity is described as "low acidity," and the body is mellow yet somewhat heavy.

The low acidity originates from the shorter and less effective fermentation process and longer drying time. If you forget about wet-hulling fermentation, go back and review. As for flavors like the "earthy taste" of wet-hulled beans, opinions vary widely. Some say this is related to the bean varieties (Timor & Catimor: with Robusta lineage) selected in wet-hulling regions, others say it's the result of organic interaction between green beans and the external environment, and some even attribute it to too many defective beans! Of course, there are quite a few...

There are numerous sources of defective beans in wet-hulled coffee. First, natural defects from the coffee fruit stage are inevitable. Another contributor to defects is the hulling process, along with corrosion during transportation of wet parchment beans, damage to exposed green beans from sunlight and moisture, and irregular weather causing uneven drying speeds that detract from flavor. Why does Mandheling require so many manual selections? There's definitely a reason...

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

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Important Notice :

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