Coffee Fermentation Methods

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by Adobestock/Subbotina Anna

Coffee is one of the most heavily traded export commodities in the world. It grows in about 70 countries, within a relatively narrow area just north and south of the equator. Broadly speaking, two species of coffee are commercially cultivated: Coffea arabica (Arabica) and C. canephora (robusta). Arabica represents approximately 60 percent of the world’s coffee production, and it also represents most of the coffee used in the specialty coffee industry.

berries on green leaf tree

Coffee’s ripe cherry-like fruits have to go through a series of steps to get to the seeds (what we see as raw green coffee) that are shipped later. Three different methods are used to take coffee from the fruit to the exportable green coffee: the wet process, the dry process, and the semi-dry process. These processes distinguish themselves from each other by the internal bean’s contact with the outside fruit; water usage; factors that influence a farmer’s processing options, such as water availability, organic controls, and, more recently, wastewater management; and flavor or cup profile. Within each process, fermentation plays a slightly different role in removing the fruit from the seed.

All three processes can affect a coffee’s physical and chemical composition; processing time; and attributes, including acidity, body (richness), flavor complexity, and sweetness.

Coffee Cherry Anatomy

A coffee bean, or “cherry,” of either Arabica or robusta coffee is actually a berry…–…some definitions identify it as a “drupe.” Either way, coffee cherries are composed of multiple layers, and each one performs specific functions during the fermentation process.

coffee bean structure illustration

The outer skin (“exocarp”) of an individual coffee cherry consists of a single layer of porous cells that are permeated with a waxlike substance. Underneath is the cherry’s pulp (“mesocarp”) and mucilage (a sticky substance common to all plants), which contains mostly water. The pulp is also pectin-rich. Pectins are important for growth and for maintaining the physical structure of the cherry’s cell walls.

Under the mucilage, a parchment-like covering (“endocarp,” or “hull”) surrounds a thinner layer, the “silverskin,” which itself covers the two coffee beans. For wet-processed coffees, the parchment layer is removed by hulling after the beans have been fermented, washed, and dried; and the silverskin is removed by the roasting process. For dry-processed coffees, the mucilage and parchment layer are removed together. For semi-dry processes, the removal of these layers falls somewhere in the midst of processing.

The Wet Process

The wet, or “washed,” process is the most accepted in specialty coffee, and it can produce coffees with clarity (distinguishable flavors and tastes), flavor, and more pronounced acidity than other processes. In general, the wet process will create a more consistent coffee.

person riding bike with filled hopper of coffee berries

After harvesting the cherries, producers begin a series of separation processes, removing light debris from cherries via winnowing (separating with a sieve) and then using hydraulic separation to remove heavier debris. Then, machines remove the coffee’s outer skin and pulp.

The cherry, with its mucilage layer intact, is then placed into cement water tanks and allowed to ferment for a few hours to a few days, depending on other factors, such as climate. This controlled fermentation breaks down the cherry’s mucilage layer, utilizing enzymes that naturally occur in the cherry and microflora from the environment. (Producers may also use starter cultures to add more control and improve the coffee’s attributes during the fermentation process.) Pectins and starches are split into simpler sugars through a process called “hydrolysis,” and those sugars then soften the mucilage and allow it to be removed with screens or mechanical scrubbing after fermentation. The choice between these two removal processes depends on how much water is available, as using screens involves more water than mechanically scrubbing the mucilage away.

white cement tanks

Fermentation can begin when cherries are still on the tree, depending on the humidity, so they need to be pulped within hours of harvesting to prevent over-fermenting. The cherries’ tendency to begin fermenting while on the tree means the wet process requires a central washing station or pulping equipment. Some of the modern eco-pulpers don’t require any fermentation at all…–…they operate much faster on a fraction of the water…–…but more traditional methods call for 12 to 24 hours of fermentation to remove the mucilage between pulping and washing.

several large cement tanks

Not only does wet processing require equipment and access to water, but it also requires space for drying. However, wet-processed coffee requires the least drying time out of the three processes: a recommended 12 to 14 days to reach 10 to 12 percent moisture.

The Dry Process

In dry processing (sometimes called the “dry method,” “natural method,” or “unwashed method”), the whole cherry remains intact during drying. First, heavier debris can be removed from ripe cherries by a hydraulic separation process…–…much like wet processing…–…or by winnowing. Whether or not this is done, producers spread the cherries in sunlight on patios or raised mats to dry and ferment, in the same way that grapes are dried into raisins.

large farm view of beans layed out to dry

Ripe cherry selection and consistent drying are important to prevent uneven fermentation between the cherries. Rather than being placed into a water-filled vat to encourage fermentation, the cherries are allowed to ferment naturally. As they dry, the outer skin, pulp, and mucilage attach to the parchment from the outside in, and the mucilage ferments around the parchment. After drying down to 11 to 12 percent moisture and temporarily being stored in bags or silos, the dried cherries are sent to the mill, where all exterior layers are removed via hulling, and preparation for shipment begins.

picked coffee berries in sacks and barrels

Dry processing is the oldest and simplest coffee-processing method because it requires no additional equipment or access to water, only space for drying. However, drying the coffee takes longer: a recommended 22 to 24 days, versus 12 to 14 days for wet-processed coffee. Because the cherries take twice as long to dry, they require double the drying space than what’s needed for wet-processed coffees.

Dry-processed coffees tend to possess more body, as well as fruity, fermented notes. They can come from all over the world, but perhaps the best-known come from Ethiopia, where elevations sometimes surpassing 6,300 feet provide the ideal conditions for growing coffee, and where dry processing has been practiced for centuries. Dry-processed coffees from Ethiopia can taste tart, with blueberry, and even raspberry, notes.

The Semi-Dry Processes

The semi-dry process may refer to “pulped natural,” “honey-processed,” or “wet-hulled” coffee. In all three of these semi-dry methods, over-fermenting is a common defect because of the extended drying time.

Pulped natural is a common processing method used in Brazil, which supplies about a third of the world’s coffee and harvests over a relatively short harvest season (June through September), given its volume. Pulped natural involves removing only the outer skin from the coffee, leaving the pulp on the beans. This process skips deliberate fermentation altogether, and instead dries the coffee immediately. But there’s still moisture, as well as sugars, in the pulp, so some amount of fermentation does happen, even at lower humidity levels. Brazil’s pulped natural coffees, in general, have a creamy body with caramel, chocolate, and nutty notes.

The honey process, in which both the skin and pulp are removed, leaves on some of the sticky mucilage…–…hence the name “honey process.” Variations exist within this process, including removing specific percentages of mucilage, and changing the amount of fermentation they undergo by the amount of mucilage left on the bean. This process was popularized in Costa Rica, and the variations in the amount of mucilage and the amount of light coffee receives as it dries alter the flavors produced.

Yellow coffee has the least mucilage and is dried in full sun, so the mucilage only oxidizes to a yellow or gold color by the time the beans are sufficiently dry. Red has more mucilage and is typically dried during cloudy periods or under light shade, and black has the most mucilage and is dried under an opaque tarp, allowing the mucilage to oxidize until it’s very dark. The darker the coffee, the greater the fermented sweetness, acidity, and depth of fruitiness. Yellow coffee receives the most light and dries the fastest, then red, then black. The “darker” the coffee is from yellow to black, the more you can expect increases in ferment-level sweetness, acidity, and darkness of fruit.

The wet-hulled process…–…additionally called giling basah (“wet grinding”)…–…is used in much of Indonesia, predominantly in Sumatra. Cherries that undergo this process are de-pulped right away by small pulping machines or by hand. Then, the beans ferment overnight to wash off any remaining mucilage. After that, they’re dried to approximately 30 to 50 percent moisture. Collectors pick up just enough of this coffee to wet-hull at a time, and then take it to processing centers, where millers will finish drying it. Coffees from Indonesia, in particular, are grown over a longer harvest season (January through October) and are characterized by low acidity; heavy body; and earthy, hoppy, and occasional fruity notes.

Before it ever makes it into your cup…–…or even to grocery store shelves…–…coffee goes through an extensive production process that influences its flavor profile, body, acidity, and more. With this knowledge, you can enjoy the luxury of choosing the coffee you’ll most relish, and appreciate the journey it took to get to you.

Cold Brew, Just Right

In the United States, more than 300 million cups of coffee are consumed every day. Most of those 300 million cups are hot, but cold-brew coffee’s popularity has snowballed in the past five years, becoming a menu fixture in many coffee shops. Cold-brew coffee differs from iced coffee in several ways, all of which influence the finished cup. Iced coffee is typically brewed hot, chilled, and then served over ice, whereas cold-brew coffee undergoes an extended 12-hour (or longer) brew time at room temperature.

Cold-brew coffee with orange juice and ice on green bokeh background

Hot water is a more efficient extractor; it produces a different chemical reaction in brewing and forces out a different range of flavors and far more aromatics. However, cold-brewing results in a concentrated beverage, offering a more tempered acidity, generally lower caffeine content, and predominantly sweeter, chocolatelike flavors.

Changing the temperature at which coffee is brewed affects its aroma and flavor characteristics, caffeine content, and the content of the more than 1,000 other chemical compounds in roasted coffee. Different compounds are extracted to different extents at different ends of the temperature spectrum. As you increase the temperature at which coffee is brewed, extraction becomes more efficient. Therefore, cold-brewing at room temperature will take less time than cold-brewing in the refrigerator.

While extended brew times can increase body, sweetness, and intensity of the flavors, cold-brewing coffee in the refrigerator can produce a sour, tinny taste (because of a preponderance of citric and malic acids). However, the problem with extended brew times at room temperature is that coffee can start to spoil. Changes in room temperature or humidity can also accelerate spoilage. When coffee is hydrated, it becomes susceptible to yeast, bacteria, and molds, just like any other perishable product. Extended brew times at room temperature can result in a coffee that tastes musty.

However, extended brew times at a cellar temperature of around 55 degrees Fahrenheit can produce clean-tasting and full-flavored cold-brew coffee. If you have a wine refrigerator or a wine cellar, try keeping your cold brew there for better flavors.

Conscientious Choices for Coffe Purchases

As coffee consumers, we have the opportunity to invest in companies that sustainably produce coffee and support farmers with living wages and humane working conditions. But how can you determine if the coffee you buy is rooted in these ethical practices?

hands harvest coffee bean ripe Red berries plant fresh seed

One important way is to look for legitimate certifications on coffee products. These labels mean that coffee production has met strict governmental and organizational requirements. Here are a few of these labels and brief summaries of what they mean:

Certified Organic

Overseen and verified by the U.S. Department of Agriculture (USDA), this certification means the product is produced in a way that minimizes synthetic material usage, conserves biodiversity, maintains and improves soil and water quality, and promotes ecological balance, among other requirements. USDA Organic

Fair Trade Certified

Products that are officially certified by a fair trade organization means the company meets standards that support safe working conditions and sustainable livelihoods for farmers and producers, as well as standards that protect the environment.

Bird Friendly Certified

This is a coffee-specific certification from the Smithsonian Migratory Bird Center, which requires that coffee be grown under shade trees to protect habitats and sequester carbon.

Bird Friendly Certified:

Scott M. Spilger is a certified Q Arabica grader and the coffee quality manager for Just Coffee Cooperative, a roaster based in Madison, Wisconsin. He has more than two decades of experience in the specialty coffee industry. He lives with his best friend and administrative assistant, Robin, a Catahoula Leopard mix he rescued from Louisiana in 2019.

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