Question from Karen: I have a question for you… If you crush wine grapes and the juice has 20% sugar, is the alcohol 5%??? Looking forward to your answer. Thank you.
Reply: Hi, Karen! Thanks for writing!
Here’s a brief reply from our winemaker, Geoff Gorsuch: Depending on all the parameters, yeast, temperature, nutrients, and how the crush and pre-fermentation treatment is handled you should end up around 9-12% alcohol if fermentation is finished.” By finished, he means if you let the wine ferment dry rather than stopping it in order to retain some sweetness.
The most common conversion rate of sugar to alcohol is 50-60% and Geoff allowed for a little more than that. The choice of yeast is one of the biggest factors and late-model, “cleaned up” yeasts are playing a role in the whole “hang-time” discussion regarding the alcohol in the finished wine.
Here’s how Wikipedia explains fermentation. Hope that helps! Cheers!