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ON-FARM PROCESSING

There are two main processing methods:

  • The unwashed (or dry) process;

  • The washed (or wet) process.

DRY PROCESS

In the dry process the harvested cherries are spread out evenly on concrete and paved ratios, or trays, or simply on the ground. Drying on bare ground is highly inadvisable as the cherries become stained with dust and earth and often acquire an earthy smell and taste.

Drying  in  the sun takes 2 to 4 weeks, depending  on the thickness  of the layers  of cherries,  the ambient  temperature   and  the  amount  of daily  sunshine.  Drying is complete when the moisture content of the dried cherries is around 12%. At this stage the outer shell, by now dark brown, has become brittle; the beans rattle inside the husk.

WET PROCESS

PULPING

The wet process is much more sophisticated and complex than the dry process. Fresh  ripe  cherries  are fed  into  a pulper  which  usually  consists  of rotating  disks each fitted  with  adjustable  knives.  These  are designed  to separate  the  bean  from the flesh  of the fruit  while  leaving  each bean intact  in  its  parchment envelope.  The pulp  is  discarded,  and the parchment is  collected  either  in  washing  channels  or fermentation tanks.

WASHING

After pulping, it is important to separate out the unpulped beans, light beans (floaters), and waste pulp not removed during pulping, as they all have an adverse effect on the liquor. Light beans (and immature) take less time to ferment than sound, heavy beans and their presence during fermentation   cause off flavor to develop.

The washing process is rather simple, and it can be done either in washing channels or in fermentation tanks. Using the first method, the pulped beans are forced into a washing channel fitted at regular intervals with sluices.  These are designed to retain the heavy and sound beans and to allow light and unpulped beans to float until they are collected in a separate fermentation tank or are repassed through a low capacity single-disk pulper. Lights, floaters, and the product   of the repasser should never be mixed with batches of first-grade parchment because they are of inferior quality and would negatively affect the liquor.

FERMENTATION

Although  the original  objective  of fermentation  was not to develop  flavor  but to facilitate   and  accelerate   drying,   correctly   fermented   coffee  does  gain   much  in finesse and appearance.

During   fermentation,   the  slippery   mass  called   mucilage   which   adheres  to  the parchment after  it  has been pulped  is  broken down by enzymes  and leaves  the parchment  clean    and  crisp    when squeezed   in    the  palm    of   the  hand.  The fermentation process normally takes 12 to 36 hours depending on the ambient temperature and other factors.

Cleanliness is of utmost importance at the fermentation stage. Beans left to over-ferment in washing channels, ducts or fermentation tanks cause irreparable damage to the end-product. Indeed, a stinker bean (a stray bean that has been left to rot during wet processing and that has been accidentally mixed in an otherwise sound parcel of coffee) can ruin the whole consignment.  Receiving  tanks, pulpers, sieves,  ducts, washing  channels  and fermentation  tanks must therefore  be cleaned daily  with  water to ensure that  all  material  that may ferment  is  dislodged  and discarded.

DRYING

Once the  surface  water has been drained,  the wet parchment still  contains  over 50% moisture,  most of which  has to be eliminated  to permit  long-term  storage. It is generally agreed that parchment should be dried to a moisture content of about 11%. If arabica  is  over-dried  to a level  of less  than 10%, it  loses  it  bluish-green color,   becomes  brittle   and  acquires   a  common taste. Over-drying   also causes unnecessary weight loss. 

Parchments are dried, as cherries are, either on concrete or paved floors,   or on drying   trays or tables.  Mechanical   dryers are frequently employed on plantations or in collecting centers where large volumes of coffee are handled, or where rain is a regular hazard during the harvesting seasons.