Demijohn Fermentation Air Bulb

A demijohn with airlock, in the cellar during wine fermentation. The fermentation lock or airlock is a device used in beer brewing and wine making that allows carbon dioxide released by the wine to escape the fermenter, while not allowing air to enter the fermenter, thus avoiding oxidation.

There were until recently two main designs for the fermentation lock, or airlock. These designs work when half filled with water. When the pressure of the gas inside the fermentation vessel exceeds the prevailing atmospheric pressure the gas will push its way through the water as individual bubbles into the outside air.