What is the potato defect in coffee?

The potato defect is a notorious quality issue found in green coffee from parts of East Africa - particularly Rwanda, Burundi, and the Democratic Republic of Congo. It produces a sharp, unmistakable raw potato smell and taste in brewed coffee, even when only a single affected bean is present in a batch.

The defect is caused by the Coffee Berry Borer or a related antestia bug species - specifically the bacteria Pantoea agglomerans, introduced when the insect punctures the coffee cherry. The bacteria produce isopropyl methoxypyrazine, the same compound responsible for the smell of raw potato, which is so volatile and potent that a single affected bean can contaminate an entire grind.

There is currently no reliable way to identify a potato-defect bean visually before roasting - the compound is colourless and the bean looks normal on the outside. This makes the defect particularly problematic in espresso, where a single bean can ruin an entire shot. Management focuses on pest control at origin (reducing antestia bug populations) and manual removal of damaged cherries at intake. Some East African lots are sold with a potato defect disclosure - honesty that experienced buyers appreciate.