Wetting the appetite for washed Arabicas.
Much time and effort goes into producing red ripe coffee cherry and the roast of the ground coffee product. In between is the less glamorous and talked about on-farm processing stage, when coffee cherry is put through a sequence of drying, fermentation, washing, and cleaning depending on type and origin. Getting things right during this stage is crucial when fruits are detached from the tree, exposed, and susceptible to the full force of the environment and microbial activity. Many bean defects showing up downstream are due to poor and sloppy on-farm processing. Water brings coffee to life with cool clean rain to swell red ripe cherry and hot steaming liquid to draw out coffee chemicals and produce the cupped infusion. High value mild Arabica is wet processed (washed) with fermentation. Robusta coffees (with a few exceptions) and most Brazilian Arabica coffee (unwashed Arabica) are dry processed. Dry processed coffee cherry is literally 'hung out to dry' whereas wet processed coffee beans are given a taste of 'aqua pura' to wet the appetite before the ultimate steps to a superior cup of coffee.
Dry processing is a relatively simple method in which whole coffee fruits (cherries) are dried straight after harvest by natural or artificial methods. All that remains is the final curing process. Batches of beans are re-dried to a moisture content of about 11% w/w for easier cleaning (dehusking to crack the dried outer layers or husk and polishing), and to ensure storage stability of green coffee. Dry processing offers many advantages to the coffee grower especially when labor is in short supply. The process can run with a mixture of ripe and unripe fruits. This minimizes the number of harvesting runs by using strip picking and is likewise more versatile and tolerant of variable coffee cherry yielded by mechanical harvest. The whole process is generally cheaper and less likely to cause have any errors.
That said, coffee picked and processed in this way, with a high proportion of unripe coffee berries, cannot produce a fine brew as a stand-alone origin however good the roasting process may be. The roaster can only work with what he or she is given and a high proportion of unripe berries and accordingly immature beans will not provide the same tight aroma and flavor precursor profile as beans processed from uniformly red ripe cherry.
Wet processing of coffee is the complete antithesis and requires uniformly red ripe cherry, more labor, and expense, which is more complicated with greater chance of something going wrong. But the results are worth the extra time, expense, and trouble for mild Arabica coffees sold at more expensive princes.
Origin of Wet Processing
Wet processed coffee underpins the top end of taste and quality in today's coffee market but came about more by accident than design some 300 years ago. Coffee was taken from its natural home in the Horn of Africa and the Middle East (Ethiopia and Yemen) during the early part of the 18th century and into the islands of the West Indies and East Indies, where there was a good climate for growth of coffee trees but not for curing coffee beans. The dry process was of little use because coffee was harvested during the wet and humid part of the year when soft ripe cherries became moldy and went rotten before they could be fully dried out in the sun.
Labor was no problem in those days so coffee beans were traditionally squeezed out of the cherry by hand or by treading in the same way that grapes are treated for winemaking. The beans were removed easily enough but a layer of mucilage remained on the parchment. Mucilage is rich in pectin and sugar offering an ideal substrate for microbial growth, as well as making rapid efficient drying of parchment coffee very difficult to achieve. Around 1750 the Dutch in Java first used the growth of microbes (yeasts and bacteria) to ferment and break down the mucilage.
The idea caught on in the West Indies and was carried along with the spread of coffee growing spread into Latin America. Wet processing rapidly became the chosen method for on farm processing of high quality mild coffees in many New World countries including Jamaica, Costa Rica, El Salvador, Guatemala, Colombia, and only to a very limited extent in Brazil. Similarly the wet method of processing accompanied arrival and spread of coffee growing in to Africa south of the Equator in Kenya, Tanzania, and neighboring countries that happened much later towards the end of the 19th century.
With adoption of wet processing came the responsibility to nurture and protect Arabica coffee beans with high quality potential. They are exposed to a sequential multi-stage process demanding considerable care because beans are more susceptible to spoilage during the wet method.
After all pulped cherries are immersed in water for microorganisms to grow on and ferment the mucilage. Not to specifically induce the production of flavor and aroma compounds as in cocoa fermentation but purely for the purposes of biological cleaning, otherwise leaving the bean physically and chemically unchanged.
Fermentation is an exact science requiring conditions to encourage the right microorganisms to grow and ferment the mucilage for optimum duration. Any slippage allowing the wrong microbes to proliferate or the biochemical process to overrun will results production of spoilage acids and other chemicals that ruin the batch long before it enters the roaster. Every stage of wet processing, from harvesting cherry to bagging of cleaned green or parchment coffee requires close attention focus and care to maintain bean quality.
Mechanics of the Wet Processing
Wet processing requires coffee cherry to be uniformly ripe which generally means picking by hand to harvest the fruits. Added bonus of hand picking is low frequency of debris and sub standard cherry thus reducing requirement for pre-process cleaning. Picked ripe cherry must be taken to the processing facility without delay, otherwise the bean mass will start to heat up and sweat causing damage and defects like discolored beans and stinker beans. Crops are unloaded into receiving tanks allowing dried up or insect damaged cherry and debris to float off by overflow and 'good' cherry to sink and be siphoned off for pulping via a funnel shaped outlet. The brief wetting loosens the outer coverings and allows easier pulping (drum or disc) to remove the outermost layer (red epicarp) and most of the sticky mesocarp. What remains is a bean with silverskin (testa) and parchment (endocarp) still intact but sticky due to a residual layer of mucilage (remains of the mesocarp) on the external surface of the bean. This is removed biologically by enzyme action during fermentation.
Fermentation is typically carried out in water filled concrete tanks by the most experienced workers and for just long enough to loosen the mucilage for rapid easy removal during the subsequent washing stage.
Mucilage comprises one third pectin, the sticky substance that cements plant cells together and gives jam its consistency.
Pectinase enzymes in the cherry hydrolyze (breakdown) and loosen the mucilage. The reaction is pushed harder by yeasts, which have their own pectinolytic activity, but care must be taken to maintain pH within the moderately acid 5.5 to 6.0 range to encourage and maintain yeasts, while discouraging other microbes like molds and aerobic coli bacteria. Conditions (including over fermentation) that allow other microbes to predominate produce unwanted chemicals causing taints and off flavors.
Length of fermentation is largely a function of temperature and condition of the crop and essentially down to experience of those in charge. Twelve to 36 hours is the normal duration. Enzyme action increases with temperature up to a maximum of 40[degrees]C or thereabouts after which the enzyme is progressively destroyed. Overripe cherry ferments rapidly while unripe cherry requires longer. Under-fermentation leaves some mucilage sticking to the bean and a 'green fruity flavor' in the coffee. Over- fermentation is just as damaging, causing 'foul fruity' and 'sweaty onion' flavors.
The difference between success and failure is narrow. When fermentation is carefully monitored and stopped at the appropriate time the coffee estate is rewarded by pristine beans with crisp fruity acidity and aromatic high notes that single out the world's best-washed coffees. Allow fermentation to overrun and the result is one of the worst of all bean defects dubbed 'ferment' and said to produce coffee tasting like the latrine.
The importance of knowing when to stop fermentation is rivaled only by knowing exactly when to remove beans from the roaster. By taking a handful of wet parchment from the fermentation tank, washing in water and feeling the texture will indicate whether or not fermentation is complete. If the beans wash clean and feel gritty then fermentation is complete but if they still have a slimy feel more time is required.
Once fermentation is complete manual washing in a channel or mechanized washing using custom-designed machines must be carried out without delay or else the loosened mucilage will continue to ferment with resulting off flavors. Wet parchment coffee is now ready for draining to reduce water content from 60 -53% followed by drying down to at least 12% bean moisture content as quickly as possible. Final hulling stage to remove parchment and silverskin followed by polishing produces clean green coffee and usually takes place away from the estate.
Bean Fermentation and Flavor
Conventional wisdom is that fermentation just removes mucilage from the coffee beans leaving them chemically unaltered. Fermentation of cocoa beans similarly removes mucilage but in the process develops flavor compounds and precursors in the beans. Fermentation of cocoa beans lasts considerably longer and much higher temperatures generated by intense microbial activity over a longer period of time.
Fermentation is a biological process, changing chemicals and producing new ones, so who is to say fermentation doesn't leave chemical 'calling cards' in the beans. Coffee bean fermentation produces the same winy smell reminiscent of grapes fermenting in vats. Under and over-fermentation is known to produce distinctive (unwanted) flavors so it seems reasonable to expect correctly timed fermentation to leave its own positive signature.
Expert coffee tasters differentiate between closely related origins in the same way that wine tasters distinguish vintage, because each has its own unique taste flavor and aroma profile. Coffee variety, soil type, and growing condition is clearly the basis but fermentation may be adding an edge to acidity and pushing flavor and aroma notes that bit higher. Washed coffees are brighter and produce cleaner and more consistent flavors than dry processed origins but the process is longer and more expensive.
In an effort to obtain the best of both worlds big producers like Costa Rica have substituted traditional wet processing with a method called Aqua Pulping. They dispense with fermentation and washing and simply pulp, rinse, and dry the beans. Top experts within the specialty coffee industry claim coffees wet processed by this short cut method cannot express the highest notes and subtle characteristics of traditionally wet processed beans.
Dr. Terry Mabbett is a technical writer with a PhD degree in Tropical Agriculture. He has worked in crop production and processing throughout the tropics--India, South East Asia, West Africa and the Caribbean--and in his home country of the U.K. Dr Mabbett has been writing professionally for over 20 years.
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|Title Annotation:||The Wetting Process|
|Comment:||Wetting the appetite for washed Arabicas.(The Wetting Process)|
|Publication:||Tea & Coffee Trade Journal|
|Date:||Jun 1, 2007|
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