Airlock

An airlock is a small plastic or silicone fitting, filled with a bit of water or sanitizer, that sits in the lid or stopper of a fermenter. As fermentation produces carbon dioxide, gas pressure pushes bubbles up through the liquid inside the airlock and out, while the same liquid barrier stops air, dust, and airborne microbes from getting back in.

Bubbling through the airlock is a handy visual sign that fermentation is active, though it is not a perfectly reliable measure on its own, since a loose-fitting lid can let gas escape around the seal instead of through the airlock, making fermentation look slower or absent than it really is. For vigorous fermentations, many brewers switch to a wider blow-off tube running into a jar of sanitizer instead of a small airlock, since a standard airlock can clog or pop off under heavy krausen.

A common mistake is judging fermentation progress purely by airlock activity rather than gravity readings, since slowing bubbles do not always mean fermentation is finished, and a stalled-looking airlock with a properly sealed lid is not automatically a problem. Keeping the airlock filled to the correct line with sanitizer, rather than plain water that can get sucked back into the fermenter, also helps keep the barrier effective.