Degassing
What is degassing in coffee?
Degassing is the release of carbon dioxide from coffee beans after roasting. Freshly roasted coffee is full of gas, which bubbles up when hot water hits it. Many roasters suggest resting beans for a few days before brewing, and the bloom step helps release the remaining gas so water can extract evenly.
Coffee that barely bubbles during the bloom has usually lost most of its gas and much of its aroma. Bags with a one-way valve let gas out without letting air in.