Why Indonesian Coffee Beans Use Wet Hulling Processing and How This Method Affects Flavor Profile

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Do You Truly Understand the Meaning of "Wet Hulling"?

Indonesia is a country with year-round humidity between 70%-90%, with frequent typhoons in certain regions and annual rainfall exceeding 2000 millimeters.

Why do Indonesian coffees have a higher defect rate? How has nature endowed them with such intense and mellow undertones?

If your answer to any of the above questions is affirmative, then reading this article carefully may provide you with clarity. This article covers all aspects of Indonesian coffee's "wet hulling" process, including its pros and cons, and its significance for coffee roasters and brewers.

What Exactly is "Wet Hulling"?

All coffee beans must be processed to transform from coffee cherries to dried green beans before being roasted and consumed. There are many methods to transform coffee from cherries to dried green beans, but they can all be classified into three categories: Natural, Wet Processing, and Honey Processing. "Wet hulling" is one method for removing the coffee seed's parchment, and it's the traditional method used in Indonesia. Although the processing method has similarities to the wet process, their nature is completely different. Therefore, do not confuse wet hulling with the wet process.

What is the Processing Flow of "Wet-Hulled Coffee Beans"?

Inside each coffee cherry, from inside to outside, there are: coffee seeds (Green Beans), silver skin, parchment (also called endocarp), gelatinous mucilage, and outer skin (exocarp).

Hulling actually refers to removing the parchment to obtain the green beans.

Step One:

Remove the outer skin. After removing the red outer skin of the coffee cherry, the seeds are still wrapped in parchment and gelatinous mucilage. At this point, fermentation must be performed first, typically in water tanks or plastic bags overnight. This helps break down the pectin components in the gelatinous mucilage, making the flesh easier to remove during the subsequent washing steps.

Step Two:

On the following day, the coffee cherries that have fermented overnight need to be washed to remove the gelatinous mucilage. The washing process requires large amounts of water. If the coffee plantation doesn't have enough water, these first-fermented cherries with mucilage will be sent to processing plants for washing. After the washing process, we obtain "wet parchment coffee."

Step Three:

The "wet parchment coffee" obtained after washing needs to be dried for 2-3 days based on sunlight and humidity conditions (not exceeding 3 days), reducing the moisture content to 20%-24% during the drying period to achieve the hardness needed for parchment removal. So far, there seems to be no difference between "wet hulling" and the wet process. If we choose to continue with wet processing for the coffee, these "wet parchment coffee" beans would need to continue drying until the moisture content drops to 11%. This would make the parchment brittle and easily removable during the hulling process. However, if we choose to use "wet hulling" to remove the parchment from "wet parchment coffee," it means we must perform the hulling process when the moisture content is still at 20%, when the "wet parchment coffee" is still very soft and the parchment is only semi-dry. Therefore, we must use a special wet hulling machine because it requires greater friction to remove the semi-dry parchment.

As you can imagine, using "wet hulling" to remove parchment is not as clean as the dry milling process. High moisture content means the parchment and coffee seeds (Green Beans) are not fully separated, so the resulting green beans typically retain some parchment residue. Throughout the entire hulling process, extreme caution is required—if touched with slight force, the soft, wet coffee beans will crack.

Due to the relatively low density of the coffee beans, they are prone to cracking after the hulling process, becoming defective. These cracked beans are called "kuku kambing" or "goat hooves."

Step Four:

The hulled beans continue to be dried until the moisture content reduces to 12%-13%. During the day, the green beans are sun-dried on the drying patio, but at night they must be stored indoors, bagged, and undergo further fermentation.

This process repeats for several more days. After the final drying process is complete, the green beans reach the finished product standard with a deep green color. At this point, the coffee can be packaged and prepared for export.

Why Did "Wet Hulling" Become Indonesia's Traditional Processing Method?

"Wet hulling" became Indonesia's primary coffee processing method for two reasons: history and environment. Coffee was first introduced to Indonesia by Dutch colonists in 1699 for economic profit. From their profit-oriented perspective, the longer coffee remained on the farm, the greater the loss. At the same time, using "wet hulling" after coffee picking could significantly shorten the time required for coffee processing, allowing them to see returns faster and reduce labor costs.

However, even if the Dutch weren't motivated by profit, they might have had to adopt "wet hulling" for coffee processing anyway, because the extremely humid climate makes the entire processing process challenging. The coffee drying process takes approximately 2-3 weeks in warm climates. But most regions of Indonesia have high rainfall and humidity.

If using conventional processing methods, the time required would be even longer. The large amount of bacteria that would grow during this period would be sufficient to spoil the coffee beans before they could dry sufficiently. The parchment provides protection for the coffee beans and makes the inner beans more stable during drying, which is why conventional processing methods dry first and then hull. "Wet hulling," however, removes the parchment first and then dries. Although it carries the risk of a higher defect rate, the sun's heat can act directly on the hulled green beans, shortening the drying time by 2-3 times.

How Does Wet-Hulled Coffee Taste?

Indonesian wet-hulled coffee is renowned for its full, mellow undertones and denser mouthfeel, even reminiscent of tobacco's addictive sensation, while simultaneously possessing certain sweetness and acidity. Regarding Indonesian coffee, people have always had different opinions, believing that Indonesian coffee is of average quality and inconsistent in flavor.

Why Are Indonesian Wet-Hulled Coffee Beans Like This?

Firstly, the "wet hulling" method has a fast processing speed, which may result in less consistent quality. More importantly, quality equipment is relatively scarce in Indonesia, and wet beans are often dried on concrete patios, mud floors, or even roads. These are clearly not optimal conditions for drying coffee. In some coffee-producing countries, increasing demand for high-quality coffee is driving transformation in processing methods.

In Indonesia, however, the situation is different. Most Indonesian coffee is used for blending and sold to average commercial coffee shops. Therefore, producing high-quality coffee seems to offer little benefit to locals. However, there are exceptions. The number of coffee farms is slowly growing, especially in Sumatra and Sulawesi, where coffee farmers have begun to strengthen their care for coffee beans.

What Is the Significance of Indonesian Wet-Hulled Coffee Beans for Roasters and Brewers?

For Roasters:

For a coffee roaster, the most important question is: even though wet-hulled coffee is often so inconsistent and full of defects, why should we roast it?

The answer is its mellow undertones. Indonesian coffee is popular because even a small amount can greatly enhance the layers and body of coffee. Even blending only 10% Indonesian wet-hulled coffee will have a significant impact on the flavor of the entire cup. It's worth noting that adding too much Indonesian wet-hulled coffee will give the brewed coffee an earthy taste. Finding the most suitable ratio requires continuous exploration and experimentation by roasters.

Wet-hulled coffee beans are relatively more difficult to roast than other coffee beans. One of the most important steps in roasting coffee is dehydrating the beans to ensure they crack at the appropriate time. The moisture content of wet-hulled coffee beans is generally between 10.7%-11%. This approximately 1% difference in moisture means that when roasting coffee, wet-hulled coffee requires relatively longer dehydration time. If this dehydration step is not done well, the resulting coffee will taste inconsistent, unbalanced, and overly acidic. Another point very important for making blended coffee is to roast the coffee one degree darker or more to balance the maturity of other beans. The simplest method is to roast different coffees separately.

For Brewers:

Please remember this point: wet-hulled coffee has a very heavy body. During extraction, the water temperature should be slightly lower, and the brewing time should not be too long to avoid destroying the coffee's rich layered texture. Strive to balance the coffee's flavor to highlight its advantages.

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