What is a sour bean in green coffee?

A sour bean is a primary defect in green coffee - a bean whose internal colour has turned light brown or yellowish, indicating excessive fermentation or enzyme activity that degraded the seed. Sour beans develop when cherries over-ferment on the branch, fall to the ground before harvest, or spend too long in fermentation tanks.

On the SCA grading form, a full sour bean counts as a primary defect - the most serious category alongside full black beans. Even a small number in a lot can significantly damage cup scores: they produce a sharp, vinegary, or fermented off-flavour that stands out clearly in the cup.

Sour beans can be partially screened through float separation - they have lower density than healthy beans - and are removed during colour sorting and hand sorting. Prevention is most effective at origin: selective picking and tight fermentation time management are the best defences.