What is silverskin in coffee?

Silverskin - also called chaff - is the very thin inner membrane adhering to the coffee bean beneath the parchment layer. It's the seed coat of the coffee bean, present from cherry all the way through to roasting.

During roasting, silverskin detaches from the bean as it expands and becomes friable - floating off as dry, papery flakes that collect in the chaff collector or cyclone of a drum roaster. Managing chaff build-up is an important fire safety and hygiene consideration; accumulated chaff is a combustion risk.

In green coffee, silverskin is visible as a pale coating in the crease on the flat side of a bean. Coffee that's been heavily polished at the dry mill has had the silverskin mechanically removed for appearance - producing a shinier, more uniform look. Polishing has no meaningful effect on cup quality, but the presence or absence of silverskin in green samples can be a useful freshness indicator.