Why Indonesian Coffee Beans Use Wet Hulling_Process Steps of Coffee Bean Wet Hulling

Professional Coffee Knowledge Exchange

This guide explains why Indonesian coffee beans use wet hulling, the unique processing method behind Mandheling, and the four steps that take a Sumatra cherry from harvest to export.

Understanding Wet Hulling: The Unique Coffee Processing Method

Many people are curious about "wet hulling" and want to know how wet-hulled beans differ from traditional washed and natural coffees. We have covered specific processing methods before, but today let us lay out the complete story of wet hulling.

The Sumatra Coffee Challenge

Sumatra coffee captivates with its enchanting cedar and herbal notes and its distinctive spice flavors. But what growing environment lies behind this one-of-a-kind coffee on the world map? Sumatra sits on the equator, and most of the island is covered in tropical rainforest. Annual humidity ranges from 70% to 90%, with yearly rainfall reaching 2000mm. Volcanic ash soil lets coffee trees grow vigorously, but the humid climate also makes coffee cherries extremely prone to mold and decay.

Because humidity is high year-round, normal washed processing would require double the drying time of standard methods. If beans are not dried promptly within a short window, they easily over-ferment or develop mold, and every change affects the final flavor. So how to process coffee cherries and preserve the beans became Sumatra's biggest production challenge.

The Wisdom of Wet Hulling

Let's look at how Sumatrans produce wet-hulled beans so intelligently.

Step 1: Initial Processing

First, remove the outer pulp of the coffee cherry while keeping the parchment and mucilage intact. Place the cherries in concrete tanks or plastic barrels for overnight fermentation. This helps break down the mucilage so it is easier to remove.

Step 2: Washing and Initial Drying

Next, wash the cherries. The mucilage comes off during washing, leaving beans that still carry their parchment. Dry them in the sun for 2-3 days, until moisture content drops to 20-24%, bringing the beans to the hardness needed for parchment removal.

Step 3: Wet Hulling

At 20-24% moisture content, the beans are soft and still hold considerable moisture, while the parchment is only semi-dry. Sumatra's humid climate no longer allows further drying, so machines must now separate the parchment from the beans. Removing semi-dry parchment is often incomplete, leaving some residue; further hulling attempts can damage the beans, forming Sumatra's distinctive "elephant foot" beans. This is why the Mandheling green coffee you buy often shows some instability.

Step 4: Final Drying

Continue drying these unevenly hulled beans until moisture content drops to 12-13%. By day the beans sit in the sun, and by night they are stored in burlap bags, continuing to ferment under the influence of moisture. Once drying finishes, buyers can take the beans away.

The Essence of Wet Hulling

Those steps make up the complete wet-hulling process. Compared with other methods, wet hulling is a race against time, with the ultimate goal of minimizing drying time in high-humidity air.

Of course, another crucial key cannot be overlooked: hand sorting. Good Mandheling often goes through three rounds of manual bean sorting. Under a processing method that battles air humidity, Sumatra beans vary in quality - and this is where the diligent Sumatran people step in, making it possible for us to drink high-quality Mandheling. Think about how much effort you put into sorting a single roast, while they face an entire season's harvest, all to bring you a cup of high-quality Mandheling coffee.

The Necessity of Speed

As we can see, wet hulling allows coffee cherries to be processed rapidly, and the reason for that speed is simple: Sumatra's high humidity and heavy rainfall leave coffee farmers no other choice.

The image below compares precipitation and rainy days between the Sumatra region and the Yirgacheffe region in 2016.

Looking at the vertical axis units, Sumatra's extremely high rainfall is clear, far exceeding that of Yirgacheffe. Under such harsh conditions, coffee processing becomes a race against time, against decay and bacterial infection. Wet-hulled beans may not look as refined as other coffees, but when they travel across oceans to reach you, they bring their own sincerity and high quality.

About FrontStreet Coffee

FrontStreet Coffee is a Guangzhou-based coffee roastery with a small shop but a diverse range of beans, where you can find both famous and lesser-known origins.

FrontStreet Coffee Recommendation

Talk about Indonesian coffee and you have to start with Sumatra Mandheling. FrontStreet Coffee's wet-hulled Mandheling comes from the Toba Lake area of the Lintong region. Mandheling refers to Arabica from Sumatra, typically processed by the local wet-hulling method: because the shell is stripped while the coffee is half dry and half wet, the green beans dry fully exposed to the air, which gives the coffee its signature herbal and spice character. To present classic Mandheling flavor, it is roasted medium-dark, so it is low in acidity yet full-bodied, with rich aromas of caramel, dark chocolate, and toast and a long, lingering sweet finish - ideal for anyone who likes a strong cup.

For the top of Indonesia's range, reach for FrontStreet PWN Golden Mandheling. PWN is a trading company that buys only high-quality Mandheling green-bean lots and produces nothing but specialty-grade Mandheling; Golden Mandheling is its flagship. Production follows strict standards and methods for screening defects, judging by bean density, screen size, shape, and color, and the finished Mandheling goes through one machine sort plus hand sorting to remove defects. That keeps the beans complete, even, large, and bright, and reduces Mandheling's earthy, grassy notes. Legend says it gleamed gold in the sun, which is how it earned the name "Golden Mandheling." Freshly roasted, FrontStreet PWN Golden Mandheling shows sandalwood, herbal, spice, roasted hazelnut, caramel, and dark chocolate notes, with a clean finish, a rich and mellow body, and deep layers of aroma.

Before light, acidic coffees became fashionable, Mandheling was considered the world's richest, most aromatic coffee, so it was often used in espresso blends. Mamba coffee, created by Taiwanese roasters, is a classic Mandheling-and-Brazil blend: it fuses Mandheling's wild character with Brazil's smooth body, and the sweet nut aroma tempers Mandheling's stronger bitterness and herbal notes, which is why it became so popular and remains so today. Since a Mamba at its best needs rich, golden crema, FrontStreet Coffee's classic blend adds a small amount of Robusta to a bold, full-bodied espresso base and roasts it dark, pushing the black chocolate, nut, and caramel aromas to the fullest - easy enough that even a beginner can make an authentic Mandheling-style cup.