Ask Mr. Wizard

Always Question Your Instruments: II

TroubleShooting

Gordon Maxwell — London, Ontario asks,
Q

I’ve noticed over the years that, for some time after my beer hits its terminal gravity, it continues to bubble away and generate foam. It’s not an occasional thing. I’m pretty sure it happens every time, or at least most of the time.

In my simplistic understanding of fermentation, yeast attacks the available sugars and turns them into alcohol and carbon dioxide. So why do I continue to see so much CO2 production after the yeast has ceased producing alcohol?

A
If fermentation is truly complete, what you are seeing in your airlock is most likely the signs of carbon dioxide in the beer equilibrating with the environmental conditions of temperature and pressure.
Response by Ashton Lewis.