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  • 11.5.9-COFFEE QUALITY-BEAN DENSITY

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  • Bean density

     
     
    Lights, shells or ears, brokens are all beans or parts of beans that are notably lighter in weight (i.e. less dense) than the average bean in a particular size grade. Note this distinction: although a small but solid bean will weigh less than a large one, it does not automatically follow that it is a light bean. Lights usually have natural causes such as drought, stress, or picking of immature cherry. All of these result in misshapen, shrivelled and soft beans. The breaking up of beans during hulling and other processing actions (including over-drying) results in shells, ears, brokens, chips and so on. Such beans and bits and pieces detract from the green appearance. They cause similar roasting problems to smalls in large bean grades, and they very definitely depress the cup quality.

    Not only do light and broken beans reduce the flavour, acidity and body of a coffee, but they often also introduce a flattish, common or ordinary taste. They can turn a potentially fine cup into a mediocre one. Proper density separation is therefore of extreme importance, especially when the coffee beans to be dealt with are also somewhat heterogeneous (uneven) by nature.

    Lights and brokens are removed pneumatically using strong airflows (catador), or by a fluidization process (gravity table). Both separate coffee by density but catadors are usually less accurate than gravity tables.

    Catadors are most useful for the initial clean-up of a coffee, directly after hulling (and polishing if installed). The strong air current removes most chips and small lights that would otherwise complicate or slow the subsequent processing. Gravity tables on the other hand are at their most efficient when the coffee has already been size graded. This is because the size grader will have removed most of the remaining smalls, and the product to be separated is therefore already of reasonably uniform density.

    Catador and gravity table settings must be based on the type and quality of the coffee under process and on the desired result. Constant, well informed supervision is essential, especially if the product is not homogeneous, for example if there has been no prior size grading. Again, an intermediate buffer silo or feed hopper permitting variable feed can ensure that the intake flow is correctly set. This is essential if optimal results are to be achieved.

    This applies to all grades of coffee, not only whole bean grades, because the value of the small bean coffees that are an inevitable by-product of the larger, whole bean, grades must also be maximized. Small lights, ears and chips in a grade of whole but small beans (C grade, pea-berry, screen 15 and even screen 14) cause exactly the same problems: they make the coffee awkward to roast and degrade the liquor quality. There are good markets for decent grinders (used for roast and ground only) if the coffee is homogeneous and properly graded.