• QA 195
    What could cause 'occasional' foul cups in apparently sound green coffee?
    We have a problem with a parcel of green coffee, most of which has been returned by our buyer because during roasting he twice found a 'foul cup'. We have tasted various samples of this coffee but cannot find any foul cups, nor do we see any off beans in the green. Our supplier also maintains there is nothing wrong with the quality. Yet we have no reason to doubt our buyer's statement and we wonder if you might have any thoughts on the possible origins of such a problem?
    Asked by:
    Importer - Europe

    Apparently sound coffees that nevertheless throw a very occasional foul cup may contain what some call hidden or invisible stinkers. Such beans are extremely hard to spot, if at all, which is why conventional sorting (both manual and electronic) tends to miss them.

    Without access to the actual coffee it is of course difficult to be precise but the following may assist you and your supplier.

    Hidden or invisible stinkers are beans that have been over-fermented during wet processing but not to the point where actual decomposition sets in, i.e. they maintain a bluish-green appearance. Or, they are beans that have suffered insect stings or minute cracks that allowed fermentation water to enter and so continue the process….  Or, they can be beans that have been affected by unsanitary conditions or mould infestation during processing and drying.

    One potential way to determine whether such beans are present is to prepare a number of individual sample roasts and taste these over 6 to 12 cups each. But this does not assist in terms of identifying the offending beans. One possible way to do this is to carefully examine individual green beans for slight differences in appearance: colour, the odd black spot, small signs of insect damage, equipment damage like pulper nips or minute cracks…  Cut or crush such beans and immediately check the pieces for any offending smell. Speed is of the essence here as, usually, the smell tends to dissipate very quickly.

    The chemical composition of seriously unclean cupping beans is different from sound beans. Over-fermentation changes the chemical composition of the bean. Chemical change is also the case with other types of serious taste problems such as phenolic. Bacterial infections can lead to yet other off tastes like potato flavour or peasiness - this can happen when cherry is stung whilst on the tree. When exposed to Ultra Violet light (UV) this type of bean tends to fluoresce and can be seen amongst the other beans. However, this only works for fresh coffee because, as coffee ages (during storage, shipment etc), so does its chemical composition change as well. Over time most beans begin to fluoresce, rendering the exercise pointless.

    Your experience points first of all to the importance of tracing the history of individual batches that are processed at origin. The area, field or block the coffee came from, when it was picked and processed, was the cherry quality checked/recorded, did any unusual circumstances occur, was the water quality checked, how long did the drying process take, etc.

    You might also recommend your supplier to look into acquiring a UV sample-viewing box that should help him verify whether any such invisible stinkers are present in future batches. For an introduction to the Satake Company  who manufacture such equipment visit www.satake-usa.com.  Alternatively contact vision@satake-usa.com to enquire about UV sorting equipment.

    Posted 25 June 2008

    Related chapter(s):
    Related Q & A:
    Q&A 060, 093, 099