I have been brewing fruit wine with wild yeast for some time now and one think that is really hard is determining the ABV of the wine. So there is my question, I have a certain mash with a known concentration of sugar, is there a way to estimate the ABV knowing that there will be lactic acid + alcohol fermentation?

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The short answer to "can I determine the exact concentration of alcohol factoring in the lactic acid produced from only the change in concentration of sugar" is no you can't. That said it might not really matter according to this paper the highest concentration of lactic acid they measured was 7.51 g/L which is 0.75%. Since there shouldn't be any lactic acid at the start of the fermentation, and the conversion from sugar is the same as ethanol. You should be able to subtract that directly from the ABV.

You are using a wild yeast with an unknown species, concentration, and strength of Lactobacillus. Unfortunately there isn't a easy way to measure the amount of lactic acid that I'm aware of. Its probably safe to assume that you could be producing anywhere from 1%-0.4% lactic acid depending on what you wine tastes like.

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