After the cherries are harvested from the coffee trees, they’re taken to a processing facility of some kind. Through any type of processing coffee must go from 65% moisture down to 9-12% moisture without any negative qualities being imparted (i.e. mould, rot, etc). Below are the main ways of doing this along with some insight into how each process impacts cup. Coffee is processed in the following ways:
The oldest and simplest processing method available because it’s demand on technology & resources is minimal. Being that coffee is grown in mostly developing nations, it’s often adopted where water is too precious to waste or of too poorer quality to be used. Coffee is received to the patios and undergoes an optional flotation (rinse) to remove foreign matter. After this the coffee is taken to one of a few types of drying facilities, most commonly:
- Patio: Concrete or clay patios, on which the coffee is placed and turned frequently to ensure even drying as airflow to lower layers is minimal to none.
- Drying beds: Frames of wood with mesh screens to suspend the coffee of the floor, allowing greater circulation of air during drying.
Held together by the skin, each cherry as it dries is a closed fermentation tank, the fermentation of sugars and alcohols creates very distinct cup profiles:
- Fermented sugars (syrupy sweetness)
- Thick body
- Wild and intense fruit qualities
As this is such a ‘natural’ system (minimal input from humans) care must be taken when selecting naturally processed coffees as ferment can quickly turn to rot. Strict green analysis and vigorous cupping is a must when looking to purchase.
Washed processing is largely responsible for the greater clarity we now expect from Specialty Coffee. In this process it is of utmost importance that all mucilage (cherry flesh) is removed to maintain absolute clarity of cup and ensure no rot/mould can develop. While this would have initially been done very primitively (squeezed off by hand), the majority of farmers now use depulpers that, if carefully tuned, accurately remove majority of the flesh and skin leaving the parchment coffee.
Then all washed coffee must go through fermentation. This can happen a number of ways and with various quantities of water (or none). Natural yeasts and bacteria eat away the remaining mucilage in a process not dissimilar to the fermentation that happens during beer brewing. Depending on the amount of remaining mucilage, weather, temperature, and humidity fermentation ranges from 6 to 72 hours with varying impact to cup profile. After this all the parchment coffee is removed a washed through tracks of water and taken to be dried.
It’s through this fermentation and washing process that this coffee exhibits:
- Clarity of terroir (all aspects of coffees growth)
- Crisp acidity
When buying washed coffee it’s these components we look for first, and prize so highly.
SEMI WASHED/PULPED NATURAL/HONEY
Similar to washed, semi washed is depulped, removing cherry skin and varying degrees of flesh (mucilage) but from here it changes. Instead of undergoing fermentation* the parchment coffee with mucilage is placed straight on the drying patio/beds.
Drying of semi washed coffee can be a scary prospect. The fermenting sugars can quickly turn mouldy without the skin of the cherry as protection. Farmers processing this way must pay close attention during drying, as one mould could mean an entire crop lost. The coffee is laid out and raked constantly to ensure even air flow until fully dried. The parchment coffee of the semi washed process has a brown/reddish (honey) colour due to the natural caramelisation of sugars (looks like praline).
A well processed semi washed should exhibit a blend of natural and washed processing:
- Intense sugary sweetness (natural)
- Heavier body (natural)
- Crisper acidity (washed)
- Clean cup (washed)
A lot of risk is taken when processing this way so it’s less common. Countries like Brazil, Costa Rica and Bolivia are amongst the few who risk the process but deliver high quality due to their attention to detail throughout the whole process.
Some coffee does go through fermentation, but not for long enough to remove mucilage. This fermentation acts as a stimuli for the breakdown of sugars.
Classified as fully washed Kenyan prep is actually a little different so deserves its own name.
Kenyan coffee factories require coffee to be sorted by farmer to remove under/over ripes & visible defects. Once sorted the coffee is pulped to remove most if not all the mucilage. The standard pulper used in Kenyan factories (McKinnon) uses floatation to separate density into different channels. Water is drained off and usually recycled once, the coffee is dry fermented (aerobic fermentation) for 12-24 hours to loosen the remaining mucilage.
After fermentation the coffee is washed to remove the loose mucilage and any remaining fermentables along with the yeast responsible. Soaking/Secondary Fermentation takes place to ensure all fermentation has or can take place. This usually requires another 12-24 hours. The coffee is rinsed then for a final time and dried slowly over 2- 3 weeks where the moisture content is reduced to 9-12%.
EXPERIMENTAL FERMENTATION / HYBRID
As we all know fermentation (when done correctly) can yield some extremely unique flavours in coffee. Today there are many different forms of controlled or experimental processing. As most of these techniques are fairly modern we are still learning the science behind the method. But by controlling the rate the sugars break down in the processing of coffee you can achieve more intense flavours, mouth feel, and overall cup quality.
These methods can be controlled in many ways, for example having the coffee's placed in anaerobic conditions allowing the sugars to break down slower to increase certain flavours in the cup. Or by the introduction of certain acids, or bacteria in the fermentation process. For example: lactic fermentation.
Lactic fermentation allows for the growth of lactic acid bacteria under anaerobic conditions with constant measurement of oxygen level, sugar content, and pH levels. The bacteria feed on the sugars in the mucilage, generating a high concentration of lactic acid that impacts the coffee's flavour profile, this will increase the coffee's body giving it a more "creamy" mouth feel. After reaching the desired pH levels, the coffee's are then soaked in clean water to stop the growth of bacteria, washed, and dried on raised beds.