Coffee Bean Moisture Measurement
shipment, coffee is dried and a coffee moisture meter is used to measure coffee bean moisture. Coffee must be dried from approximately 60% moisture
content to 11-12% moisture content. Coffee is typically
dried on large patios made of asphalt or cement and then
transferred to mechanical dryers. The coffee on the drying
patios is shifted every 30-40 minutes and is shaped into
long rows of no more than 5 cm in height. Next to each row
is open ground, which is warmed and dried by the sun. The
coffee is then shifted onto the dry portion of the patio,
and the section where it was previously is now allowed to
dry in the sun. This helps accelerate the coffee drying process
and prevents fermentation and moldy beans from developing.
This method is widely used in Brazil, but less widely used
in Guatemala or Costa Rica where the coffee is more often
piled perpendicularly to the old piles. Drying coffee solely
by patio takes 6-7 days for washed coffees, 8-9
days for pulped naturals (semi-washed), and 12-14 days for
natural (dry-processed) coffees. This is why coffee beans are
typically dried on a patio until they reach a moisture content
of 15% and are then transferred to mechanical dryers. Once
the coffee reaches a 25% moisture content or less, it can be piled
at night and covered with cotton cloths to allow the coffee
to breath. If it rains, these piles can also be covered with
plastic. Coffee should not be covered with burlap sacks
since this will impart a distinct burlap flavor and aroma
to the coffee.
Coffee Drying Stages
In a study done in Kenya, Kamau
reports that there are six stages to drying coffee.
1) Skin drying. Moisture 55-45%.
2) White Stage drying. Moisture 44-33%.
3) Soft Black stage. Moisture 32-22%.
4) Medium Black Stage. Moisture 21-16%
5) Hard Black Stage. Moisture 15-12%
6) Fully dry coffee and conditioning. 11-10%.
In this study he found that sun drying coffee for stage three
is mandatory for coffee quality. He also reports that as
long as the temperature was between 40-50°C--which means
a bean temp of 35°C--then the coffee quality will not
be seriously compromised during the other coffee drying stages.
The final two coffee drying stages (15-11% moisture) take just six hours
at 40°C in a mechanical dryer.
Coffee Drying Equipment
There are several coffee dryer
systems available. Many older dryers are converted grain
dryers that are not as efficient as the new horizontal barrel
dryers. The new coffee dryers are designed to mix the coffee evenly
to ensure uniform drying. Drying coffee by using mechanical dryers
accelerates the slowest part of the coffee drying process (15-11%)
and helps prevent fermentation. In some environments that
have a high humidity the entire drying process must take
place in mechanical dryers. Mechanical dryers should never
be set higher than 40-45°C and this question should
be asked before buying any coffees that have been mechanically
dried. At higher temperatures the germ is killed and the
flavor potential of the coffee is ruined. At extreme temperatures
the bean crystallizes, and when smashed with a hammer, will
break like glass.
Other Machinery for Drying Coffee
best, but least utilized method of drying coffee is by using
drying tables. In this method the pulped and fermented coffee
is spread thinly on raised beds, which allows the air to
pass on all sides of the coffee. The coffee is mixed by
hand and the drying that takes place is more uniform and
fermentation is less likely. Most coffee from Africa is
dried in this manner and select coffee farms around the
world are following their lead.
A relatively new method of drying coffee is solar drying. Solar coffeee dryers are more economically sound than mechanical coffee dryers because they don't rely on fossil fuels or electricity. They are also more efficient than the patio drying technique because their hotter drying temperatures dry coffee beans at a faster rate. While solar coffee dryers have great potential for saving energy, they are not widely used today. For more information about solar coffee drying systems, visit EcoVentures International or the MesoAmerican Development Institute.
showing patio drying: medium
(4.6 Mb) or small
of mechanical drying: medium
(1.57 Mb) or small (736
Characteristics Due to Processing