Know Your Coffee: Methods of Processing
Comparing speciality grades of arabica coffee is a highly subjective topic. Although ultimately a roaster's coffee roasting profile will likely have the greatest impact on flavour and aroma, many consumers might look at the country of origin of the crop as the next relevant indication of what to expect in their coffee.
Geographical provenance is possibly the most widely (or easily) grasped point of distinction by the coffee swilling masses, it is nevertheless the "Processing Method" of the freshly picked coffee fruits that might actually have the greatest effect on coffee's taste.
In Bonaverde's speciality coffee Marketplace, you can filter your selection by processing method -- so once you learn about how this might impact flavour, you should give it a try!
What is Coffee Processing?
Coffee beans, as seeds of coffee cherries, need to be extracted from the fruit mucilage and pulp, prior to the subsequent processes which transform these into the more familiar green coffee beans.
The term coffee processing entails the removal of the cherry flesh prior to drying beans to that *magic number* of around 11% of their original moisture content.
The manner in which a coffee fruit is processed ultimately has a marked impact on the final flavour of that coffee as a beverage -- the choice of method is on one hand a combination of experimentation, cultures & tradition, education, the specific requirements of the location (aka "what works best"), the financial, mechanical, and natural resource availability at the specific processing site; and on the other hand dependent on the coffee cherry itself: varietal/cultivar, pigmentation, nutrient content, plant health, and of course its degree of ripeness.
It is interesting to note the role fermentation plays, and by a similar token, the role the amount of mucilage left to remain on the bean prior to and during drying, and its subsequent effect on the flavour profile of coffees.
As a fore note, understand that below outlines only the general approach to processes in a simplified manner -- there are in practice, infinite shades of grey, hybrids and blurring of lines (or dare we say completely novel approaches) amongst these core processes.
On that note, this blog does not cover quality control measures such as 'winnowing' and 'floating' vs 'sinking' of ripe coffee prior subsequent extraction of green coffee from the cherry through processing.
The 3 Primary Coffee Processes:
'Natural' or 'dry' processing is the opposite end of the spectrum from a washed coffee: the coffee cherry fruit mucilage and pulp is not removed prior to drying the coffee (it is instead removed via fermentation and to a degree mechanically with friction as a final processing stage if necessary). As a result, this process is notably more time-intensive than washed or semi-washed methods.
Natural processing is common in areas where water access is more restricted or impractical, or where the costs of the expensive water-washing machinery is prohibitive. There are two variations within Natural processing:
As is sometimes seen in Brazil natural processes, ripe coffees are left on the trees to dry, whereas in other areas such as Ethiopia (and far more commonly practiced overall) the ripe coffees are picked, sorted and dried on patios or raised beds for nearly 3 weeks -- a time-intensive process by comparison. Early in the drying stage, the combination of a humid environment as moisture is lost, heat, and the inherent sugars within the cherries creates a rigorous environment for the naturally occurring wild yeasts present on coffee to begin fermenting. There is much complexity imparted from the concentrated and caramelisation of sugars, pectin acids in natural processing.
Though this process is perhaps the most uncontrolled of the 3 outlined in this article (wild yeasts are almost a topic unto themselves, and vary in terms of efficiency), coffee connoisseurs might characterise Natural coffees as highly complex and multi-levelled flavour palate of naturally processed coffees due to the prominent role of the cherry flesh itself: the development of its sugars and the role fermentation to plays through the drying stage.
When done right, natural coffees are often -- but not exclusively -- characterised as chocolatey, nutty, and with lower acidity.
This method is perhaps the most common in the speciality coffee industry, and involves removing the cherry's pulp and mucilage via water washing and pulping early on in the processing stage prior to drying.
Coffee cherries are washed in water and soaked in large tanks. Once drained and dried, a cherry pulper removes all pulp (i.e. skin) and mucilage (i.e. fruit) from the green beans before they are either again submerged in water, or left for 1-2days to ferment of remaining cellulose, or potentially mechanically scrubbed. The short anaerobic environment of wet and natural approaches ferments off any remaining mucilage and sugary parchment, the freshly exposed green beans are once again drained and sun dried until approximately 11% moisture content (drying can often take two weeks or more, depending on climactic conditions) with constant rotating on drying patios.
As can be imagined, the speedy removal of the cherry mucilage results in a focus on inherent taste properties of purely the bean itself, with little presence of classically described fruity or fermented flavour notes as would occur had the pulp left more intact during cherry processing.
Washed coffees are generally recognised for clean brightness, clarity and defined vibrant tasting notes.
Semi-washed coffees are known under a plethora of names, including pulped natural, miel and honey. Commonly practiced throughout Brazil and Costa Rica in particular, it's a hybrid between washed, and natural processing: where the outer pulp (i.e. the cherry's skin) is removed, but the sticky mucilage (i.e. the fruit flesh) is left still on the green bean.
As with fully washed coffee, ripe cherries are normally floated as a quality measure, and a calibrated pulper first strips the skin pulp from the cherry but leaves a proportion of mucilage (and occasionally parchment) still on the bean. Foregoing a secondary fermentation wash stage, it is instead immediately dried on patios in sunlight (1-2 weeks, to approximately 11% moisture content). As such, drying of course occurs marginally quicker than a fully washed process -- given the prior removal of the coffee pulp layer, and the absence of water soaking stages.
During drying, the remaining sugary mucilage dries into the bean's parchment giving it a caramelised appearance -- hence the term 'Honey Washed'.
There are various degrees within the semi-washing coffee spectrum -- often termed "Red", "Yellow" or "Black" honey washed. The names correspond to the amount of mucilage allowed to be left remaining on the coffee prior to drying, and hence the resultant colour of the green beans once properly dried (the more mucilage left on the bean, the darker the end-colour).
Semi-washed coffees are regarded as having a characteristic fruity sweetness, and this might vary dependent on the degree between red, yellow or black honey. As a hybrid process, it presents traits of purely washed coffees with its presence of mucilage during drying, whilst the speedier drying time and prior removal of cherry pulp still preserves the cleanness of the coffee bean itself.
NOTE: Again, despite bids to standardise it, describing (and favouring) certain coffees is a highly subjective issue. It's not the Bible on coffee processing, but you could take this article at minimum as encouragement for coffee lovers to explore and understand their taste preferences more; and at maximum, as a rather simplified 'quick start' guide to the three core groups of coffee processing.
There is "textbook" standard methodology of washed, natural and semi-washed processes, but keep in mind that on-farm scenarios (and hybrid experimentation) means the iterations processes are myriad.
If you're interested in really delving deep into coffee processing, we'd suggest the following two options for further reading:
James Hoffmann's The World Atlas of Coffee as a good start, or Coffee: Growing, Processing, Sustainable Production by Jean Nicolas Wintgens for incredible detail (normally reserved for true coffee professionals).