So what happened to the potato defect?

​Potato defect, quakers, anthracnose, immature cherries, coffee berry borer. All of them are real threats to your Rwandan, Burundian or Congolese cup. But of all those defects, the potato defect is the worst, because you can't see it. A bean with the potato defect looks exactly like a clean one. Same colour, same size, same density. It only shows up when you roast and grind: that foul smell of raw, earthy potato. And worse still, a single bean is enough to ruin a cup.

​This has an uncomfortable consequence for the whole industry: sorting, however meticulous, doesn't solve the problem. For some years now, the best Rwandan cooperatives — the ones producing the best lots — have been sorting wet parchment after washing and fermentation. When the parchment is translucent with moisture, immature beans, floaters and insect-damaged beans stand out far more clearly. The brown streaks left by antestia are plainly visible at that stage, and those beans come off the raised bed. But that removes part of the visible damage, not the invisible defect.

So why do cooperatives like Gasharu have such a low incidence, under 1%? Because the battle was fought in the field, not at the washing station. Systematic pruning — antestia prefers dense bushes. Applications of pyrethrum and alpha-cypermethrin. Weekly collection of eggs and damaged plant material. Farmer training. It's less photogenic work than a raised bed, but it's what actually moves the needle.
And there's a recent twist: research published this year by the Rwanda Agriculture Board shows that mechanical damage can produce the defect without any antestia involved. In other words, post-harvest handling — how the coffee is pulped, how the parchment is moved — matters too. The story is still being written. What this means for you as a roaster: the risk isn't eliminated, it's managed.