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What it does :
Airlocks are used in closed-vessel fermentation. They are typically made of clear plastic to permit visibility of the water level in the airlock and bubbling activity.
What it is:
An airlock, also called a fermentation lock, is a device that allows one-way escape of gases. It is similar in action to a sink drain trap, though certainly more clean.
How it works:
These airlocks employ a water trap to allow fermentation gases to escape, but prevent outside air from entering the fermentation vessel. They are typically installed with a bung to ensure a snug fit with the fermentation vessel, whether glass carboy or plastic bucket.
Airlocks are used during the cold side of brewing (e.g. fermentation, bottling, and packaging). They are directly exposed to the beer during the critical fermentation stage. So, airlocks should be sanitized before use to prevent contamination.