Calculating ABV after adding fruit puree to fermenting wort

Discussion in 'Homebrewing' started by TappaKeggaBrew, Sep 1, 2018.

  1. TappaKeggaBrew

    TappaKeggaBrew Zealot (526) Apr 30, 2009 Virginia

    I'm currently fermenting a gluten-free strawberry IPA.
    The wort went into the fermenter with an OG of 1.050 (a little lower than I'd aimed for, but non-traditional grains can be less predictable).
    Three days later, the yeast had been working, and the gravity was down to 1.021. I stirred in a few pounds of strawberries I'd pureed in the blender, and the gravity climbed to 1.033, presumably due to the addition of sugars from the fruit.
    My question is about how to calculate the ABV when I check the final gravity before bottling tomorrow. My thought was to calculate the amount of alcohol created during the fermentation prior to adding the puree -- 1.050 to 1.021 --> ABV=3.81% -- and add it to the calculated alcohol from after the addition of strawberries to the final gravity -- 1.033 to (let's say) 1.015 --> 1.58%). If that approach is correct, my beer will have an ABV = 3.81 + 1.59 = 5.4%.
    So, is that an accurate way to calculate my ABV? Or am I completely off track, having scuttled any hope of accurate calculation when I added the strawberries?
  2. Brewday

    Brewday Initiate (121) Dec 25, 2015 New York

    #2 Brewday, Sep 1, 2018
    Last edited: Sep 1, 2018
  3. skleice

    skleice Aspirant (249) Aug 6, 2015 Connecticut

    Wow, another GF Brewer!? All grain?
    Layerup and GormBrewhouse like this.
  4. dmtaylor

    dmtaylor Aspirant (206) Dec 30, 2003 Wisconsin

    The fruit changed the gravity from 1.021 to 1.033, so it's like it added 0.012 points to the beer total. You can add this difference to the OG of 1.050 to get a new effective OG of 1.062. Then you can use the following formula to figure out ABV like normal:

    (OG - FG) * 131 = ABV

    So for instance if you end up at 1.021 again, you'll have (1.062 - 1.012) * 131 = 6.55% ABV

    Simple as that, really.
    Lukass and GormBrewhouse like this.
  5. pweis909

    pweis909 Poo-Bah (1,762) Aug 13, 2005 Wisconsin

    Fruit is tricky. It can take time for sugars to escape cells and dissolve. Evaluating the true added sugar maybe is best estimated from the amount of sugar contained within the fruit. Or squeeze out some juice drops and use a refractometer. I use nutrition labels/info to estimate sugar and assume all sugar get incorporated into the beer eventually. Fruit is also contributing water. The water becomes part of your final volume. The fibrous particles that settle out do not. Evaluating the true added volume is probably something that gets ignored by most. Anyhow, guessing at sugar and guessing at volume is going to limit the accuracy of your calculation.
  6. TappaKeggaBrew

    TappaKeggaBrew Zealot (526) Apr 30, 2009 Virginia

    Thanks for everyone's input. It sounds like I can get a close, but probably not precise, ABV with what you've provided.

    Grain bill = 7.7-lbs buckwheat (from 10-lbs sprouted, then oven baked brown and loose particles separated/removed), 1.5-lbs steel-cut oats, and 2-lbs of wild rice blend. I also boosted the fermentable sugars with 5-lbs of dextrose (ending up with 1.050 OG, as I noted above). This is similar to a GF Heady clone I recently attempted, but with some adjustments in the process due to lessons learned from that one (it's delicious, but I ended up with way too much gooey sediment and and lost volume because of it).
    I've made a few GF beers now, and have found that the final product is usually good, if not excellent; but they require a lot more work, mainly because the homebrew suppliers don't sell most of the grains necessary, so I have to prepare them all myself.
  7. skleice

    skleice Aspirant (249) Aug 6, 2015 Connecticut

    Nice. It certainly is more work! I've never malted my own grains. That's a big job. I take the easy (but more expensive) road...
  8. TappaKeggaBrew

    TappaKeggaBrew Zealot (526) Apr 30, 2009 Virginia

    That is just what I've been looking for! (many Google searches, but never found that site.)
    Time is money, so I'd rather spend $50 than hours of my time.
    I tasted the raw beer coming out of my fermenter today, and although I might add a little more lactose to boost sweetness next time (it's a little tart, probably from the fermented strawberries), it's promising to be a very refreshing (dare I say sessionable) beverage - a month of bottle conditioning should perfect it.. If you're a GF home brewer, I can share the recipe.
  9. EvenMoreJesus

    EvenMoreJesus Champion (886) Jun 8, 2017 Pennsylvania
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