Introduction to Sumatra Coffee Semi-Washed Processing: Why Sumatra Coffee Uses Semi-Washed Method

Sumatra's Wet-Hulling Method

Most coffee is processed one of two ways: natural or washed. As producers kept experimenting, honey processing appeared, followed by more elaborate techniques such as anaerobic fermentation and wine-style fermentation. None of those, however, belongs to Indonesia. Indonesia has a processing method all its own: wet hulling.

Indonesia sits in the tropical rainforest, where rainfall is heavy and dry, sunny days are scarce. That shortage of reliable sunshine pushed farmers to develop the world's only wet-hulling method, and it is largely responsible for the distinctive character of Indonesian coffee.

Wet hulling, also called wet dehulling, is known locally as giling basah. The method yields raw beans with a dreamlike, jewel-like blue-green cast, and it is why Indonesian coffee keeps its woody fragrance without the grassy notes found elsewhere.

Farmers pick fully ripe cherries in the morning. After the fruit is gathered, the skin and pulp are removed that same afternoon, leaving the parchment and mucilage intact. The parchment-covered beans are then placed in water, usually in large barrels or sinks, and any defective beans floating on the surface are skimmed off. Sumatra's natural environment is generous, and most farmers use mountain spring water for this step.

The dense beans that sink to the bottom are scooped out, rinsed lightly, and packed into barrels or plastic bags for a short dry fermentation that lets the pectin and sugars on the bean surface develop flavor. The more sour the target, the longer the fermentation; fully developing the pectin and sugars generally takes 12 to 36 hours, depending on conditions.

The parchment-covered beans are then sun-dried for 2 to 3 days, until they reach a moisture content of 20-24%. At that point a hulling machine strips away the parchment to speed up drying, and the beans continue to dry until they reach 12%-13% moisture, which completes the process.

Wet hulling shortens the fermentation period considerably. It cuts acidity sharply while pushing caramel and fruit aromas to the foreground. This is exactly the classic "taste of terroir" that defines Mandheling coffee.

Outside Sumatra, though, the same method cannot be called authentic. The island's climate is what makes wet hulling the best possible choice.

FrontStreet Coffee Recommendation

Because wet hulling is what gives Mandheling its earthy, herbal depth, the best way to taste this method is through a bean built around it. FrontStreet Coffee's Mandheling comes from the Lintong growing region near Lake Toba in Sumatra. It is roasted medium-dark to present the classic Mandheling profile: low acidity, a full and heavy body, and layered notes of caramel, dark chocolate, and toasted bread with a long, sweet finish. It suits anyone who prefers a strong, bold cup.

For a step up, FrontStreet Coffee's PWN Golden Mandheling represents the top of the category. PWN is a trading company that sources premium lots of Mandheling green beans, and Golden Mandheling is its flagship. The beans are screened to strict standards covering density, size, shape, and color, then passed through one machine sort and a manual hand sort to remove defects, leaving an even, large, bright bean. Freshly roasted, it shows sandalwood, herbs, spices, roasted hazelnut, caramel, and dark chocolate, with a clean finish and a rich, sweet, strongly layered body.

When you brew either bean, expect the wet-hulled character to come through in the cup: low in acidity and heavy on the palate. Both make a fine everyday coffee and an equally good gift.