Q: After fermenting my beers I have always “cold crashed” the carboy or bucket in my temperature-controlled chest freezer or outdoors (during the cooler months) for a few days before bottling. This gives me a nice, compact yeast cake and seems to clear the beer very well, especially if I add gelatin. However, when I do this, my airlock works in reverse and seems to suck in plenty of air. I have become concerned about this method introducing oxygen into my finished beer. Do you think this is something I should be worried about?
— Adam Sickmiller • Cincinnati, Ohio

A: This is a fairly common question and is clearly a concern to many homebrewers. Although I have answered a similar question before (in the October 2012 BYO), I will give a slightly different spin on this topic. The general rule is that there is only one place in the brewing process where oxygen should be introduced, and that is during wort aeration. After this point, beer oxidation can result from air pick-up.

The good news about homebrewing is that yeast is normally present and even yeast that is not actively fermenting absorbs oxygen and helps to protect beer from oxidation. This is why bottle conditioned beers are known to have better shelf life than filtered beer, especially when air pick-up during filling is high. The flow of air into your fermenter certainly would not make my top 10 list of problems homebrewers should worry about.

What does concern me a bit more is the tendency of some airlocks to suck liquid into the fermenter when they operate in reverse. Even if you use an airlock design that does not allow liquid to be sucked into your fermenter when cooled, the air that is sucked could potentially contaminate your beer. And adding alcohol to the airlock as an air sterilizer is not very effective since gas flows through in large bubbles and has a very short contact time.

I suggest using cotton batting to filter air as it flows into the fermenter headspace upon chilling, and then replacing the airlock after the beer has cooled. If you are really concerned about the air pick-up, consider racking into a keg and applying top pressure prior to crashing. Or you can introduce a slow flow of carbon dioxide into the headspace of a glass fermenter that has a cotton plug or airlock with a second hole to provide a slow flow of cover gas during cooling. This latter method will be difficult to control without a special regulator and is really a method that I would only use for experimental purposes.