Degassing
The release of carbon dioxide from coffee in the days after roasting.
Freshly roasted coffee gives off CO2 for one to three weeks. Too fresh and the gas can make espresso erratic; too old and the coffee tastes flat.
Storage containers with one-way valves let gas escape without letting fresh air in.
Why it matters for small spaces
Understanding degassing explains why coffee often tastes best a few days to a couple of weeks after the roast date.