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Fruit addition and figuring out abv after

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  • Fruit addition and figuring out abv after

    So we are adding fruit during fermentation, and itís already sitting at 8.1%. We are adding fruit tomorrow and am wondering how we can calculate abv after the fruit addition, since itís going to bump the gravity back up, what we need help with is calculating once we hit final gravity after fruit addition? how do we figure out the abv of the combination of already fermented and then fruit fermented? Any help is appreciated.

  • #2
    The only way to know for sure is to test it in a lab, but here's a way to get a ballpark estimation:

    -Beer mass in g = 0.9982 x specific gravity x beer volume in mL
    -Ethanol volume in mL = (8.1/100) x beer volume in mL
    -Ethanol mass in g = 0.789 x ethanol volume in mL
    -Figure out total mass of sugar in fruit in g
    -Assume all fruit sugar will ferment -> ethanol mass from fruit in g = (48.956/100) x sugar mass in fruit
    -New ethanol mass = old ethanol mass + ethanol mass from fruit
    -New beer mass = old beer mass + sugar mass in fruit + water mass in fruit
    -Ethanol by weight = 100 x new ethanol mass / new beer mass
    -Measure specific gravity of beer after fruit is fermented
    -Ethanol by volume = 0.9982 x specific gravity of beer x alcohol by weight / 0.789

    For more detailed information on estimating ABV in general, check out the Alcohol_Content file at sites.google.com/site/republicbrewpub (the resulting equations are in the Analysis tab). I hope this helps!

    Joe

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    • #3
      Jean deClerck textbook gives method for distillation test

      If you need to know for sure, the method for distilling off the alcohol and measuring it is detailed in Jean deClerck's Textbook on Brewing. It includes what to do if the beer is sour (volatile acidity).

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      • #4
        Originally posted by MP-Greg View Post
        So we are adding fruit during fermentation, and itís already sitting at 8.1%. We are adding fruit tomorrow and am wondering how we can calculate abv after the fruit addition, since itís going to bump the gravity back up, what we need help with is calculating once we hit final gravity after fruit addition? how do we figure out the abv of the combination of already fermented and then fruit fermented? Any help is appreciated.
        Have you considered using potassium sorbate to prevent re-fermentation of the added fruit? The amount needed is far less than the amount in any of your household condiments or baked goods you buy. You won't get a lot of the fruit character if you ferment it. You will get the color. Beer yeast is far more aggressive than wine yeast, so you are not likely to get the floral/fruity/spicy flavors and aromas you would get using a wine yeast.

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