Coffee Milling

After drying, processing operations are essentially the same whether the coffee is wet-processed, dry-processed, hand-picked or machine-harvested. Appropriate storage conditions for parchment coffee (and green bean) are relative humidity less than 65% and temperature less than 25°C. Higher temperatures and/or humidities will result in loss of bean colour and flavour and increase the potential for mould and fungus development. In bulk storage situations, potential for ‘hot-spots’ must be avoided; e.g. silo exposed to direct sunlight. Provision of aeration may be necessary in certain environmental conditions. Typically, milling and grading of wet-processed coffee is conducted immediately before marketing. However, dry-processed product will usually require milling soon after drying.

There is a large and specialised range of commercial milling and grading equipment available and it is beyond the scope of this article to discuss its use and suitability in any detail. However a brief overview of dry-factory operations follows.

Hulling and polishing

Hulling is the first step and refers to the removal of parchment from parchment coffee, or the removal of skins (de-husking) and parchment from dry-processed coffee. Different hulling machines are used for hulling of parchment coffee and de-husking of dry-processed coffee though they are essentially variations of the same principle. These machines usually incorporate an additional polishing chamber where the silverskin is removed. Silverskin is more difficult to remove from dry-processed coffee. Hulls, parchment and silverskin are separated from beans in the huller-polishers by grates and aspiration.

Parchment and dry cherry must be at the correct stage of dryness for hulling and polishing. If bean is overdry it may shatter, while underdry bean will bruise and deform, causing overheating of bean and machine blockages. Dry-processed cherry that is too wet is extremely difficult to hull efficiently.

Grading

It is necessary to grade the green bean. This provides a uniform product for marketing and allows even roasting. Additionally, price premiums are usually obtained for certain classifications of graded green bean.

Size grading is the principal operation and is achieved using perforated screens with round holes for normal beans and slotted holes for pea-berry (whole round beans). The proportion of bean in the respective size grades will depend on the cultivar and growing conditions. The size grades used for marketing range from No.20 (20/64″) to No.10 (10/64″) with the bean held on each screen being designated 20 and above, 19s, 18s, 17s, 16s etc.

Additional gravimetric and colour-based sorting may be conducted using density tables, catadors (aspirators) and electronic colour sorters. This equipment is used to remove extraneous matter (e.g. sticks, stones and soil clods) and beans which are light, deformed, discoloured or insect damaged. If your operation is small, manual sorting may be appropriate and is greatly assisted by sorting tables and belts.

Quality Assessment

Specification systems are used to provide information about a particular consignment of green bean. Ultimately, price is determined by four factors.

Liquor quality

This is the most important criterion on which coffee quality is judged. Assessors use many attributes to describe liquor quality but it is principally determined by acidity, body and the presence or absence of any faults or off flavours which downgrade quality. Roasting and aroma characteristics are generally included in an assessment of liquor quality.

Size

Although liquor quality differences may be slight between size grades, price premiums are usually paid for the larger bean sizes and pea-berry.

Colour

Wet-processed arabicas are generally blue-green to grey-green and uniform in colour. The blue-green colour is especially well-regarded and sun-drying has historically been considered to enhance this. Dry-processed bean tends to be brownish and less uniform. Green bean colour fades (becomes pale and whitish) with prolonged storage.

Appearance and defects

Beans should appear uniform, intact, well-formed and free of defects. Defects are more common in dry-processed coffee. A quantitative assessment of defective beans and extraneous matter in a given sample weight is the usual way in which the defect level is quoted. Defects include: dried whole cherry

• black bean derived from bean which has died within the cherry on the tree

• insect-damaged bean

• immature (quaker) bean

• parchment bean

• broken bean (greater than half)

• bean fragment (less than half)

• floater bean (whites and pales)

• shells (husks)

• husk fragments

• parchment fragments

• twigs

• stone/earth

Source

Craig Lemin