The moment I brought my nose to the cup, a smell like medicine, like smoke, rose up. No need to drink. Somewhere in the fermentation of this lot, something happened. But I do not call this smell simply a "failure". A defect is less a villain than "the answer sheet of the process". Know which defect comes from where and you can fix next year's processing. Today I sort the defects you can see from the defects you only find by drinking, and map where the main off-flavours come from.
Defects divide broadly into two. One is physical defects: black beans, unripe beans, insect damage, breakage, mouldy beans and so on, visible to the eye. The other is cup defects: beans that look normal but reveal a taste defect when drunk, divided into taints, which cover the whole, and faults, which are fatal. The troublesome ones are the latter. Even among the clean-looking beans that passed sorting, taste defects hide. The medicinal smell at the start came from beans that looked clean. So in the end they can only be caught by tasting (cupping).
In recent years, visible defects have become readable by machine. A study that detected eight kinds of green-bean defect by deep learning reached 95.2% accuracy, and 100% for presence or absence alone (Huang et al., 2021). Insect damage can be identified non-destructively with 95% accuracy by visible and near-infrared hyperspectral imaging (Lien et al., 2020). Invisible taste defects, meanwhile, come from the process. In a study using starters in fermentation, markers of good and bad aroma were identified; guaiacol (medicinal, smoky), for example, is an indicator of off-flavour when certain bacteria have worked to excess (Martins et al., 2019). The smell at the start was exactly this. Go as far as over-fermentation and acids such as butyric and propionic acid appear, edging toward onion and vinegar. A taste defect is also the sign that fermentation could not be stopped "just right".
Line up the well-known ones by source and it looks like this. Over-fermentation (fermented, alcoholic, vinegar): when the stopping point of fermentation was passed. Mouldy: when water activity stayed high for too long in drying or storage. Phenolic, medicinal: excess or contamination by particular microbes. Potato and earthy: insect damage or particular bacteria. Bleaching, fading: appearance defects arising from storage and drying. All can be read as a record that "something happened somewhere in processing, drying or storage". Put the smell into words and you can guess the cause.
"We do not hide defects as something shameful. They are valuable material, used to check the answers of the process. A lot with the smell of over-fermentation gets an earlier stop next year. If mould appears, we ask where in drying it was left at high water activity. A defect is information for fixing next year."
"A defective bean is the answer sheet of the process. Rather than hide it, use it for the next year."
For the boundary between over-fermentation and fruity see Ferment or fruity: where is the line; for bleaching and fading, How green coffee degrades; for bean-side causes that cannot be seen, The other reason your coffee is sour. For stopping fermentation at the right point, The four stages of fermentation.
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