Air lock

What does an airlock do?
Filling the airlock with a little water makes it act as a siphon;
the water closes the beer or wine off from the outside air;
hence preventing outside bacteria from entering.
However it allows the carbon dioxide formed during fermentation to escape through the airlock because of the excess pressure.

Twin-bubble airlocks are the cheapest, but these are harder to clean than the cylindrical models. With volumes of 50 l or more, the use of cylindrical airlocks is essential for the increased flow of carbon dioxide to be processed. The water in the airlock evaporates; regularly checking and refilling is necessary.
The airlock can also be filled with a disinfecting solution instead of ordinary water.