Carbonating a beer today, I realized something about priming sugar calculation. I’ve been using the priming sugar calculator at Brewer’s Friend and like all, it asks for the temperature of the beer.
Beer, as it ferments, produces carbon dioxide. In fact, fermented beer is a saturated carbon dioxide solution. As beer warms, carbon dioxide becomes less soluble and is driven off, a fact that explains why sometimes the airlock on a finished beer will start to bubble again if a beer is allowed to warm. If the beer warms then cools, if the cooling is not extensive, it may not take up the carbon dioxide into solution again or it may dissolve air instead. Either way, the warmed beer doesn’t have as much CO2 in solution as it had before warming.
The priming sugar calculator calculates based on two values, the amount of CO2 predicted to be in solution at the given temperature and the amount of additional CO2 produced by the yeast. The yeast are going to produce about the same amount of gas regardless, it’s the dissolved CO2 that’s the critical variable. If the beer has been warmed, then cooled, and you base your calculations on the cooler temperature, you may under-carbonate your beer.
More importantly, knowing how the calculators work, I let my lagers warm to room temperature, then calculate priming sugar additions and bottle. I don’t get overcarbonated lagers any more.