e. Disc Pulpers
The same concept is involved with the disc pulper except that the pulp is removed using abrasion by a rough disc.
e1. Mucilage removal. The amorphous gel of mucilage around the bean consists of hemicelluloses, pectic substances and sugars and is insoluble in water. This mucilage can be removed by chemical methods or warm water but, for small-scale units, the only feasible method is fermentation. Fermentation involves the beans being placed in plastic buckets or tanks and left until the mucilage has been broken down by natural enzymes and bacteria. The coffee should be stirred occasionally and every so often a handful of beans should be tested by washing them in water. If the mucilage can be washed off and the beans feel gritty rather than slippery, the beans are ready.
e2. Drying. The beans should then be washed and dried immediately to prevent spoilage and the development of off-flavors. To prevent cracking the coffee beans should be dried slowly to 10% moisture content (calculated on a wet-basis) using the same drying methods described above.
e3. Hulling and cleaning. After drying the coffee should be rested for 8 hours in a well ventilated place. The thin parchment around the coffee is removed either by hand, in a pestle and mortar or in a small huller. The hulled coffee is cleaned by winnowing.
Roasting and Grading
The final flavour of the coffee is heavily dependant on how the beans are roasted. Roasting is a time- and temperature-dependant process. The roasting temperature needs to be about 200ºC.
The degree of roast is usually assessed visually. One method is to watch the thin white line between the two sides of the bean, when this starts to go brown the coffee is ready. As preferences vary considerably from region to region, the locally-acceptable degree of roast will need to be assessed.
Coffee beans can be roasted in a saucepan as long as they are continually stirred. A small improvement is made by roasting the coffee in sand, as this provides a more even heat.
A roaster will produce a higher quality product. The simplest roaster is basically a tin can with a handle so that it can be rotated slowly over a fire. There are various other roasters suitable for larger scale units.
Coffee is graded by size, shape, odor, density and color.
It is easy to assess the quality of whole coffee beans. However, ground coffee beans produce a brown powder which can be easily adulterated. Because of this there is often market
resistance to ground coffee. This market resistance can only be overcome by consistently producing a good product. There are basically two types of grinders – manual grinders and motorized grinders.
a. 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. Motorized grinders are available for larger scale production units.
b. 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.
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.
a. 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.
b. 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.
c. Percolation batteries. This is the most common method. The roasted and ground 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.
d. 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.
f. Slurry extraction. Coffee and water are agitated together in a tank and separated using a centrifuge.
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