Fermentation Lock, for fermentation without foreign bacteria
If you seal a mash barrel with a fermentation lock, the carbon dioxide that is generated during fermentation can escape from the barrel in a pressureless way, but unwanted microorganisms and vinegar flies (fruit flies) are unable to get into the barrel.
The stopper that goes with the fermentation lock is food safe and resistant to alcohol as well as the other constituents of the mash. The outside is very conical, with the outside diameter ranging from 3.5 to 5.9 cm. Hence, a matching hole can always be drilled in the lid of the fermentation vessel using conventional compass saws or core drills (available in DIY stores as a set with different diameters).