Calculating ABV for Sour Beers

Image of sour beer brands courtesy nytimes.com

 

Yesterday I got into a discussion about what adjustments needed to be made to accurately determine the alcohol content of a sour beer. I’d always assumed that the souring process didn’t have much effect on the standard method, using a formula that takes into account the OG of the wort and the FG of the beer, but I’d never spent any time looking into it.

When we calculate the amount of alcohol in a beer we are judging the relative density of the wort before- and after fermentation. This reduction in density (specific gravity) comes from a couple sources (including yeast biomass growth), but most of it is from the conversion of sugars in the wort to ethanol and carbon dioxide by fermentation. I’d always read that the CO2 escaping into the atmosphere was the main cause of the reduction in density during fermentation.

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