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  • Defects in robusta coffees

     
     
    Improper processing techniques, including use of incorrect equipment and improper handling, contribute to defects in quality. In washed robusta the major off-tastes caused by improper processing techniques are raw/green, fruity, overripe, fermented, medicinal, chemical, stinkers, stale, earthy, baggy/oily, spicy and to an extent metallic.

    Unwashed robusta coffee is less susceptible to quality deterioration. Off-tastes such as raw/green, fruity/fermented, overripe, medicinal, chemical and stale occur mainly because of negligence during processing.

    Impact of immatures/greens, brown beans, fruity/overripe, fermented and medicinal off-tastes. Selective picking of cherries is essential for the production of high grade robusta. The quality of the bags or baskets used for collection during harvesting should also be carefully checked. Cherries collected in fertilizer bags or in bags previously used for chemicals could absorb an off-taste, especially when such bags are tightly tied and left unattended for a length of time or directly exposed to strong sunshine.

    The raw/green off-taste in robusta coffee has been attributed to incorrect harvesting techniques. For economic reasons, selective picking may not be practised. This results in unripe cherries being pulped or dried along with ripe cherries. The green or immature beans present among the unripe cherries give a raw or green off-taste to the cup.

    On the other hand, the presence of brown beans and an off-taste of overripe could occur when the cherries have been picked in an overripe or even already dried condition. Where feasible, growers are advised to sort the cherry after picking, to ensure that the coffee to be pulped (or dried in the cherry for natural preparation) does not contain unripe or overripe cherries that lower the cup quality.

    Fermented and 'medicinal' off-tastes have been observed in natural (dry processed) robusta. The cause for these could be delays in spreading the cherries for drying or the deterioration of overripe cherry. (Late harvesting means general overripeness = poor cups.)

    Causes of pulper-nipped beans/cuts, stinkers, putrid/rotting off-taste.
    Invariably, not all coffee cherries will be the same size. If they are not sorted on size, with the help of mechanical cherry sorters, hand sieves or flotation then it is very likely that the beans will be cut during pulping. This can also happen if the pulper has not been suitably adjusted or fitted with flexible chops. Micro-organisms can enter pulper-nipped beans through the injury and cause the formation of stinkers or black beans, adversely affecting quality.

    Causes of earthy, fruity and fermented off-tastes. The water used for washing, as for all the stages of processing, should be clean to ensure the quality of the end product. Unclean water or water contaminated with fine silt, and recirculated water with a high solid content, could cause earthy, fruity or fermented and other off-tastes.

    Causes of mouldy and faded beans and impact of improper drying and storage: During the preparation of natural robusta, spreading the cherries in thick layers with no or inadequate stirring and raking could result in mould formation. This can adversely affect the visual appearance and the cup quality of the cherry beans. Lack of protection from rain and night dew during drying can also cause mould growth. For more on this go to Chapter 12 Quality control issues.

    The fading (of the colour) of coffee and the cup being described as stale could be the result of inadequate drying facilities, storage of beans with a high moisture content, or the storage of well dried coffee in improperly ventilated warehouses. Stale cups can also be caused by improper storage on the farm, at the curing factory or at warehouses awaiting sale. Storage of coffee on the drying yards, inadequate covering of coffee stacks, poorly ventilated warehouses, or stacking coffee in a haphazard manner up to the ceiling of the warehouse can all cause a stale off-taste to develop in the cup.

    Spicy and chemical off-tastes could be due to packaging in poor quality bags or bags in which spices or fertilizers have been packed earlier. Storing coffee with spices, chemicals, fertilizers or fungicides could also cause these off-tastes. Remember that coffee beans readily absorb taints and odours that could lower their aromatic quality and, therefore, value.
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