Reasons for High Defect Rate in Sumatra Coffee Beans and What Are the Defective Flavors

Why Does Mandheling Require So Many Hand-Sorting Rounds?

Today, let's explore the wet hulling processing method...

Sumatran Coffee Bean Processing - Wet Hulling Method

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
The coffee cherries are peeled, and the beans with parchment are placed in large water tanks or troughs. Defective beans floating on the surface are skimmed off and removed.

2. Dry Fermentation
The dense beans that sink to the bottom are briefly washed, then placed in buckets or plastic bags for a short dry fermentation period. This allows the pectin sugars on the seed coat surface to ferment and enhance flavor. Generally, longer fermentation time results in stronger acidity. The fermentation duration varies among producers, typically lasting only a few hours. However, some estates skip the dry fermentation stage and directly sun-dry the parchment beans to suppress acidity and improve viscous mouthfeel. Usually, fermentation time ranges between 12-36 hours, depending on specific conditions.

3. Wet Hulling and Final Drying
The parchment beans are sun-dried for one to two days until the moisture content reaches 30%-50%, when the beans are still semi-hard and semi-soft. A hulling machine is used to remove the parchment shell, then drying continues to accelerate the process. After about two days, when moisture content reaches 12%-13%, the coffee beans are collected into woven bags (typically 40kg and 80kg bags) and sent to coffee processing plants for final hulling. The entire process takes about four days.

The hulling process involves grinding off the bean shells with a hulling machine, followed by further drying until moisture content reaches approximately 12%-15%. The coffee beans are then sent for mechanical sorting to remove various impurities and classified by particle size.

Unexpected Challenges

Of course, unexpected issues can arise during processing.

During the hulling process, coffee bean temperatures can rise to 30-60°C, completely destroying the parchment layer and potentially triggering germination in the beans.

Wet hulling can also result in higher rates of defective beans, including moldy beans, compared to washed and natural processing methods. This occurs because the fruit skin is removed during processing, exposing the beans directly to air.

Mold Contribution to Flavor Development

Coffee beans have four protective layers: fruit skin, pulp, parchment, and silver skin. In the washed process, only the first two layers are removed, preserving the parchment and silver skin for sun drying. However, wet hulling removes the third and fourth layers midway through processing, leaving the beans exposed during sunbathing. This is why Sumatran green beans display a blue-green color. While wet hulling shortens drying time, it significantly increases the likelihood of contamination by mold, fungi, and yeast.

Interestingly, contamination isn't necessarily negative—it depends on the specific microorganisms. Dr. Martha Taniwaki from the Brazilian Food Technology Research Institute experimented with different fungal strains infecting coffee beans and found they produced various flavors, including rot, rancidity, mold, wood, iodine, caramel, chocolate, and floral notes. The academic community believes that Sumatran coffee's woody, herbal, earthy, banana, and spice flavors are likely created by mold.

Roasting plant managers often report to the academic community that some infected moldy coffee beans taste better after thorough cleaning, while others become unpalatable—this depends on the mold species. Perhaps it's possible to inhibit harmful molds while introducing beneficial ones, similar to the "controlling wild yeasts" and "cultivating elegant yeasts" concepts in enology. 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 in green beans, thereby affecting flavor. Coffee beans stripped of all four protective layers are in a more exposed state than washed beans (which only lose two layers), making them more susceptible to germination. 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 bean temperatures to rise to 30°C-60°C, which also promotes germination and mold growth. Moreover, semi-hard, semi-soft moist green beans are easily damaged by mechanical force during parchment removal. The beans tend to crack and split, resembling cloven hooves—hence the higher proportion of "elephant beans" in Sumatra. However, whether elephant beans are beneficial or detrimental remains controversial.

He laughed and said: "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 rapid to produce the mellow, low-acidity, and sweet Sumatran flavor; if control is inadequate, it may result in bland Sumatran coffee or even moldy, earthy flavors. If you prefer stronger acidity, Aceh can also undergo washed processing, depending on customer requirements."

Summary of Wet Hulling Flavor Profile

To summarize the wet hulling flavor profile, aside from subtle differences, we generally believe that wet-hulled beans carry earthy, smoky, and chocolate flavors, with what's considered "low acidity," a full body that can seem somewhat heavy or muted.

The low acidity originates from the shorter, less effective fermentation process and longer drying time—if you've forgotten about wet hulling fermentation, look back at the section above. As for the "earthy flavors" and similar characteristics of wet-hulled beans, opinions vary widely. Some say this relates to the bean varieties used in wet hulling regions (Timor hybrid & Catimor: with Robusta heritage), others claim it results from the interaction between green beans and external organic matter, and some even attribute it to excessive defective beans! Of course, there are quite a few...

There are numerous sources of defective beans in wet hulling. Natural defects present in coffee cherries are inevitable. Another contributor to defects is the hulling process itself, along with corrosion during transport of wet parchment beans, damage to exposed green beans from sunlight and moisture, and inconsistent weather leading to uneven drying speeds that detract from flavor. Why does Mandheling require so many hand-sorting rounds? There's certainly a reason...

Comparison of Green Bean Processing Methods

Finally, let's summarize the characteristics of various green bean processing methods:

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

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

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

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

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

Recommended Sumatran Coffee Bean Brands

FrontStreet Coffee's roasted Sumatran Mandheling coffee beans offer excellent guarantees in both brand and quality. More importantly, they provide exceptional value—a 227-gram package costs only 89 yuan. Calculating at 15 grams per cup, one package can make 15 cups of coffee, with each cup costing only about 6 yuan. Compared to café prices that often run tens of yuan per cup, this represents outstanding value for money.

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473