Coffee : small-scale processing
Practical Action
producing a good product. There are basically two types of grinders - manual grinders and
motorized grinders.
Manual grinding mills An experienced operator can grind about 20kg in an eight hour day.
However, this is hard and tedious work. The grinding mills need to be set so that they produce
the desired degree of fineness of ground product which satisfies the end-user. For small-
scale production (up to 100kg/day) a series of these grinders is all that is needed. Motorised
grinders are available for larger scale production units.
Motorised grinding mills Horizontal plate, vertical plate or hammer mills are suitable for
grinding coffee. A grinding mill has to be placed in a separate and well-ventilated room
because of the fine particles of coffee dust generated during milling. These particles can be
an irritant.
Instant coffee
To produce an instant coffee, the soluble coffee solid and volatile compounds have to be
extracted and then dried into a powder or granules. The production of instant coffee is
unsuitable for small-scale enterprises as it requires expensive machinery such as an extractor
and a freeze- or spray-drier. However a general description is provided below.
Prestripping Sometimes the volatile compounds in coffee (of which there are over 700) are
removed before the extraction of soluble coffee solids. This is usually done by passing steam
through a bed of ground and roast coffee.
The initial steam pressure has to be high
enough for the steam to pass through a static
bed of coffee. The extracts and steam are
condensed to give a mixture of water and
volatile compounds. These compounds can be
condensed and collected using a tubular
condenser with chilled water flowing through
it.
Extraction of soluble coffee solids The
extraction of soluble coffee solids is usually
done using water as the solvent. Extraction is
continued until the solution obtained is
15-25% w/w coffee extract. The extraction is
usually done at 175ºC since at 100ºC the
extracted solids are difficult to dry. There are
three ways the solids can be extracted.
Percolation batteries This is the most
common method. The roasted and ground
Figure 4 : Percolation batteries
coffee is held in a series of 5-8 vessels. Hot
water is passed through the vessels and, when the soluble coffee solids have been fully
extracted from each vessel, it is isolated from the battery and spent coffee discharged. A new
vessel replaces this exhausted vessel (see Figure 4). As the extraction takes place at 175ºC,
the system needs to be under pressure. A solution of 15-25% w/w of soluble material is
produced which can then be dried.
Countercurrent system Coffee is fed continuously into the bottom of an inclined cylindrical
vessel and moved upwards by two helicoidal screws rotating at 10-22 revolutions per hour.
Hot water enters the top and the extracted solids in solution exit at the bottom. The vessel is
pressurised and kept at 180ºC by the use of temperature jackets.
Slurry extraction Coffee and water are agitated together in a tank and separated using a
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