airlocks

[james_m_r]

Just wondered, what are the benefits of using an airlock and what are they for?

[Bigster]

Use them religiously when making wine but have never used since I came back to beer making last year. Just put the lid on and hope for the best. Not had a problem yet .....

The theory is that the co2 produced by the fermenting must pushes through airlock without any airborne nasties coming back the other way

[Scooby]

An airlock is a one way valve, allowing Co2 produced by the fermenting wort to be vented but keeping the wort safe and sterile away from contact with air

[SteveD]

With beer, during primary fermentation the yeast cap and copious CO2 production exclude air from the wort, so an airlock isn't essential. Traditional fermenters are open anyway. As the fermentation quietens down, less CO2 is produced and so there is less protection. Then, some will just loosely clap a lid on (me), others will transfer to a secondary fermenter fitted with an airlock and let it finish there, others will to start with have conducted the whole ferment in an airlockable fermenter and will just leave there until ready to barrel/bottle it once the ferment dies off. Horses for courses.

[tubby_shaw]

An airlock is a one way valve, allowing Co2 produced by the fermenting wort to be vented but keeping the wort safe and sterile away from contact with air

What Scooby says is correct, but, once fermentation has ceased and CO2 is no longer being produced, there is no longer a positive pressure in the FV.
So changes in atmospheric pressure can cause air to bubble into the FV from atmosphere