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Coffee Processing – What is it?

The fruit of the coffee tree is called the coffee cherry.  The coffee beans are the seed of that coffee cherry.  Two seeds normally grow within each cherry. The coffee cherry contains many layers that need to be removed prior to roasting, grinding and drinking that fresh cup of coffee.  These layers are the outer skin, mucilage,  parchment, and the silver skin.  Once the coffee cherry is harvested there are several procedures it goes through in order to remove these layers.

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After harvesting, the coffee cherry is sent to a mill where it processing begins.  There are three main ways coffee can be processed: washed, also known as wet processed,  natural, also known as dry processed, and pulped natural also known as semi-washed. 

During the washed process the beans are washed or soaked in water to remove the outer pulp before drying, in natural processed the bean stays in the cherry for drying and in pulped natural the skin is removed from the cherry leaving the mucilage intact while drying.   Within each of these techniques are variations.

Wet / Washed Process

In the washed process, the fruit covering the beans is removed before being dried.  This requires the use of specific equipment and substantial quantities of water.  The skin of the coffee cherry and some of the pulp is removed by pressing the fruit in water through a screen. The bean will still have a significant amount of the pulp clinging to it that needs to be removed.

The fruit with the exposed pulp is then fermented in tanks where  bacteria and naturally occurring enzymes consume the pulp.   Fermentation takes between 24 and 36 hours,  depending on the temperature, thickness of the pulp and  concentration of the enzymes.

To determine the end of the fermentation process the beans are  assessed by feel.  The parchment surrounding the beans loses  its slimy texture and acquires a rougher “pebbly” feel. When the fermentation is complete, the  coffee is thoroughly washed with clean water in tanks or in  special washing machines.

After the pulp has been removed the coffee bean still has two layers surrounding it – the silver skin and the parchment.  Coffee beans can be dried in the  sun or by machine but in most cases it is dried in the sun.

When sun drying, the coffee is spread out in rows on large patios where it gets raked every six hours to promote even drying and prevent the growth of  mildew. Some coffee is dried on large raised tables and turned by hand. Drying coffee this way has the advantage of allowing air to circulate better around the beans promoting more even drying but increases cost and labor significantly.

After the drying process the parchment skin is thoroughly dry and crumbly, and easily removed by a machine called a huller that is used to crunch off the parchment skin before the beans are shipped.

The wet process generally guarantees a cleaner and more consistent flavor than the natural/dry processed or pulped natural method.

 Natural / Dry Processed

Dry process is also known as unwashed or natural processing.  It is the oldest method of processing coffee and is used in many countries where  water resources are limited. The harvested cherries are sorted and cleaned to separate out the unripe, overripe and damaged cherries. This can be done by winnowing , which is commonly done by hand, using a large sieve. Any unwanted cherries or other material not winnowed away can be picked out from the top of the sieve. The ripe cherries can also be separated by flotation in washing channels close to the drying areas.

The cleaned coffee cherries are spread out in the sun, either on large concrete or brick patios or on tables with matting raised to waist height.  As the cherries dry, they are raked or turned by hand to ensure even drying and prevent mildew. It may take up to 4 weeks before the cherries are dried to the optimum moisture content, depending on the weather conditions. On larger plantations, machine-drying is sometimes used to speed up the process after the coffee has been pre-dried in the sun for a few days.

The drying operation is the most important stage of the process, since it affects the final quality of the green coffee. A coffee that has been over dried will become brittle and produce too many broken beans during hulling.  Coffee that has not been dried sufficiently will be too moist and prone to rapid deterioration caused by the attack of fungi and bacteria.  

The dried cherries are stored in bulk in special silos until they are sent to the mill where hulling, sorting, grading and bagging take place. All the outer layers of the dried cherry are removed in one step by the hulling machine.   It is not practical to use the natural method in very rainy regions, where the humidity is high or where it rains frequently during harvesting

Pulped Natural / Semi – Washed

The pulped natural process is similar to washed in that the coffee cherry is removed before drying.  However, in pulped natural processing the bean is dried in the sun with the mucilage still clinging to the outside of the parchment.  In this processes the drying stage is very important and must be attended to constantly so that fermentation and rot do not set in.   During drying the mucilage dries onto the bean changing the flavor profile.

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