Ask Mr. Wizard

How can you prevent a vacuum from forming in an airlock during fermentation?


Larry Silbernagel asks,

I brewed a batch of lager (a three-gallon batch in a five-gallon carboy) and instead of the normal CO2 venting out of the air lock, a vacuum formed. I used a wort chiller and pitched the yeast at 72° F. At 18 hours the cap was sucked down onto the vent tube with liquid (vodka) suspended in the vent tube (wort temperature 64° F sitting in my basement). I moved the carboy to a refrigerator sitting in a cold garage and the wort temperature dropped to 54° F over the next 18 hours. The same vacuum was present at 36 hours. Finally after 48 hours a slow fermentation was going with the cap raising, but it was three-plus days before a strong fermentation was present. Could the rapid temperature change or poor yeast start have caused the vacuum? What happened and what can I do to prevent this in the future?


What happened to your fermenter was due to the relation between gas temperature, volume and pressure. A sealed container will exhibit a decrease in gas pressure if the container is cooled and
Response by Ashton Lewis.