• QA 175
    Does polishing add value to quality coffee?
    Why would green coffee be polished and is this needed for quality coffee?
    Asked by:
    Student (Agriculture) - Ethiopia

    For arabica polishing as such adds nothing to the inherent quality but the process can improve the green bean appearance. In some instances this may improve saleability but generally speaking it is doubtful that the process adds real value. It is therefore advisable to establish first whether the polishing of a particular type of coffee makes commercial sense.

    Usually, polishing machines are used to remove the silverskin that clings to the beans even after hulling. Friction is applied, mostly using phosphor bronze bars that rotate inside a cylinder through which the hulled beans are passed. These bars are softer than steel and do not damage the green beans whereas the bronze tends to impart a bluish patina to the beans. This improves the appearance but, unless the coffee is excessively coated (much silverskin adhering), polishing does not materially affect marketability. However, some types of hullers, impact hullers for example*, can produce an untidy green bean appearance which polishing can improve.

    Coated coffees not only do not look attractive but, they are also difficult to sort electronically. Furthermore, experienced coffee buyers know coatedness can be caused by drought or over-bearing, or by under-fermentation during wet processing. Entirely coated beans may originate from unripe cherries. None of this is positive for quality…

    Polishing needs to be carried out correctly, using machinery that suits the type of coffee that is to be polished. If too much heat is generated** then the coffee will lose both colour and quality: only cool polishing is acceptable, i.e. the finished product is not warm to the touch at the polisher exit.

    Note also that whilst polishing can improve green appearance, in certain coffees this may encourage fading to set in earlier. Fading is the loss of colour that forms part of the ageing process whereby the colour and taste of green coffee slowly deteriorate.

    For dry processed robusta some mills routinely polish coffee because silverskin adheres strongly to a dry processed coffee bean. This is not to be confused though with the so-called washing and cleaning of robusta, using moist parchment skins to provide friction. This is sometimes done to obtain a shiny, greenish bean.

    See also topic 11.05.06 of the Coffee Guide itself.

    *   Impact hullers rely on impact rather than friction to separate the green bean from the parchment. However, the coffee to be hulled must absolutely and uniformly be at the correct moisture content.
    ** Often caused by incorrect machine settings that cause a build-up of beans, and therefore pressure, inside the polishing cylinder.

    Posted 10 January 2008

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