Glossary
Second fermentation
A sealed bottling stage for kombucha or water kefir that builds carbonation and lets in added flavors.
Second fermentation is a follow-up stage used mainly for kombucha and water kefir, after the primary fermentation is done and the SCOBY or grains have been removed. The liquid is bottled, often with added fruit, juice, or herbs for flavor, and sealed in an airtight bottle for a short additional fermentation, usually a few days at room temperature.
Because the bottle is sealed rather than vented, the carbon dioxide produced during this stage has nowhere to go, which is what builds natural carbonation into the drink. This is very different from the primary, open-vessel ferment, where gas is meant to escape through an airlock or loose-fitting lid. Bottles designed with a swing-top or thick glass construction hold pressure more safely than thin, repurposed containers.
The biggest risk during second fermentation is overcarbonation, since sugar from added fruit continues to feed the culture and build pressure inside a closed bottle. Many home fermenters build a daily habit of briefly opening, or burping, each bottle to release excess pressure, or they move bottles to the refrigerator once the desired fizz level is reached, since cold temperatures slow fermentation dramatically.