What is a drupe in coffee?

A drupe is the botanical term for a type of fruit that has a fleshy outer layer surrounding a hard stone or pit that contains the seed. Coffee cherries are drupes - the outer skin and pulp form the fleshy layer, and the seed inside (the coffee bean) is analogous to the pit of a peach, plum, or cherry.

Understanding the coffee cherry as a drupe helps explain the structure of the coffee processing chain. Each layer of the drupe - the outer skin (exocarp), the fruit pulp (mesocarp), the mucilage (a layer of pectin-rich material), the parchment (endocarp), the silverskin (tegument), and finally the seed (endosperm) - corresponds to a processing step. Pulping removes the exocarp; fermentation and washing remove the mucilage; hulling removes the parchment and silverskin.

Most coffee cherries contain two seeds facing each other, which is why standard coffee beans have a flat face. When only one seed develops - the other failing to fertilise - it grows into a round peaberry. The drupe structure explains both the typical flat bean and the occasional round peaberry as products of the same underlying fruit biology.