Coffee is a widely consumed beverage and many are unaware that it is traditionally a fermented product. The coffee bean lies inside of a coffee cherry, and the cherry itself contains many fermentable sugars like glucose, fructose, sucrose but also pectins which are easily consumed by various microorganisms present in the environment. The coffee cherry has various layers, of which the exocarp, mesocarp and the mucilage are mainly available for fermentation. Once these sugars are consumed by microorganisms, flavor compounds are produced which then transfer into the bean due to its porous structure, effectively “infusing” the coffee.
These microorganisms can originate from the water used, soil, air or the coffee fruit itself and can therefore be highly varied. Generally, three types of coffee processing methods are used to obtain dry green coffee beans: wet processing, dry processing, and semi-dry processing. These may also be referred to as “washed”, “natural” and “honey” processing, respectively.
Wet Processing
During wet processing, the coffee is submerged into water in either cherry or pulped state, and finally is “washed” by having the mucilage removed. During this type of processing, the coffee comes in contact with the microbes present in the water, which then initiate fermentation. However, when the coffee comes in contact with water, an exchange of compounds occurs, since coffee is porous, the coffee bean and cherry “leak” compounds into the water, potentially diluting the end product. Depending on the coffee, this can be a positive, since harsher notes are diluted too, potentially creating a “cleaner” cup. Additionally, the coffee bean will undergo germination which causes the metabolism to shift due to the lack of oxygen. This results in the coffee bean consuming the sugars present in the coffee beans at a faster rate resulting in less “sweet” coffee, but additional flavor compounds will be created, which can be a positive or a negative depending on the coffee.
Dry Processing
Dry processing, however, involves drying the coffee in the cherry state. The coffee cherry may be dried immediately after selecting for defects, or it may undergo a separate fermentation step, either in a tank or in the open environment. When it is sent for drying immediately, the cherry will undergo simultaneous fermentation and drying, until the water content is too low for microbes to thrive. When it is fermented in the cherry state under dry conditions, the fermentation is impacted widely by not only the temperature, but also by the presence, or lack of, oxygen. When oxygen is present, it shifts the microbial metabolism to respiration, which is more efficient from a microbial growth perspective, but often produces less desirable flavour compounds.
Semi-dry Processing
Semi-dry processing is more similar to dry processing, but the coffee needs to be dried with the mucilage intact. Before that happens, it may undergo similar processing as in dry or wet processing, giving it the processing effects of other methods. However, since the coffee must be dried with the mucilage intact, it undergoes simultaneous fermentation and drying just like in the dry processing. Key difference is that there are less sugars present, and a low moisture content is attained much faster than during dry processing, and that there is a degree of maillard reaction occurring, depending on the level of mucilage left after pulping. The latter which are often referred to by producers as: “white”, “yellow”, “red” or “black” honey.
These methods only take into consideration in what state the coffee was dried in, which currently is the deciding factor for how the coffee is named. There are still more aspects to coffee processing, like the fermentation and drying protocol, which we will cover in the next upcoming articles.
