Fermentation and Coffee Processing Methods

Text: Isabelle Huard

Fermentation is a crucial step in post-harvest processing. First used as an inexpensive way to remove the mucilage surrounding the beans, producers now use it to influence the development of aromas and flavours and to increase the quality of their coffees. It is essentially the process through which micro-organisms (bacteria and yeasts) take part in breaking down the sugars in the mucilage; while they play an important role by producing enzymes, alcohols and acids, it is rather the acidification of the environment that directly drives the breakdown of these sugars.

Fermentation can happen aerobically or anaerobically. In the first case, when the cherries or pulped beans are exposed to oxygen —on raised beds, drying patios or in fermentation tanks, for instance— the fermentation is described as aerobic. It is the traditional method par excellence. Because fermentation is, by definition, a process that does not require oxygen, the term “anaerobic” refers instead to the use of airtight bags or tanks from which oxygen is removed and cannot get back in.

Three main methods are used around the world: the wet method, the dry method and the so-called “honey” method.

THE WET METHOD

The wet method, whose coffees are called “washed”, consists of completely removing the pulp and mucilage before drying the beans. Fermentation takes place once the pulped beans are placed in fermentation tanks. The term “washed” can be confusing, since it doesn’t necessarily involve water; fermentation can also happen without the beans being submerged. Dry fermentation is faster, considerably reduces the amount of water used and lessens the problems associated with disposing of wastewater. A coffee is called “fully washed” when, after pulping, the beans are submerged in water tanks for fermentation. Washed coffees are known for a very clean and nuanced cup, with a vibrant acidity and delicately sweet and floral notes.

Wet method. Gold Mountain Coffee Growers.

THE DRY METHOD

The dry, or “natural”, method consists instead of keeping the skin and pulp on during fermentation. Since the cherries are full of sugars, this method can considerably amplify the aromas of the beans. Traditionally, the cherries are dried on raised beds —African beds— or drying patios, and fermentation continues until they are completely dry. Today, with the many innovations in processing, new techniques are being tested; more and more producers are fermenting cherries in airtight bags or tanks. This method must be well controlled: while it offers producers the opportunity to improve the quality of their coffees, poor control can have the opposite effect.

Dry method. Gold Mountain Coffee Growers	Dry method. Gold Mountain Coffee Growers

THE HONEY METHOD

The so-called “honey” method is a process we owe largely to Costa Rica, and it is a hybrid of the two previous methods: the cherries are pulped and set to dry with part or all of the mucilage left on the bean. There are a few variations —white, yellow, gold, red, black— depending on the percentage of mucilage left on the beans during drying, the period during which fermentation takes place. Moisture content and drying time are crucial variables, influenced by the climate and the amount of light the beans are exposed to. Some honeys dry faster in the sun (white, yellow), while others are dried in the shade, extending the drying time and allowing more sugars to develop in the beans (gold, red, black). The honey method requires constant monitoring to avoid unwanted over-fermentation.

Honey method. Gold Mountain Coffee Growers.	Black honey. Gold Mountain Coffee Growers.

Brazil developed a process almost identical to honey called “pulped natural”: the skin is removed, but the pulp is kept on the bean. The drying process is therefore faster than for natural coffees, with the dry climate of some Brazilian regions helping to speed up this step.

All these methods, first used as an economical way to remove the mucilage from the beans, now allow producers to put their knowledge to work, to experiment and to stand out by playing with the many possibilities that a well-controlled fermentation offers.