Starter changed ABV

Discussion in 'General Brewing Discussions' started by AGbrewer, Apr 3, 2021.

  1. AGbrewer

    AGbrewer Well-Known Member

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    So I started out with a beer that was 1.126 SG with about 3.3 gallons. Then I pitched a 2 liter starter (with 200 grams of DME) that was at high krausen into the wort. No idea what the starter gravity was when i pitched it. My hydrometer says it is now an OG of 1.098 rather than the starting 1.126 that I measured earlier. I'm sure that there is a way to calculate the OG based on the SG of the 3.3 gallons of wort and the 2 liters of starter wort, but I'm apparently not smart enough to figure it out...but I'm sure someone on this forum has already ran into this problem and solved it.

    So the questions are:
    1. What is the correct SG?
    2. Roughly how much alcohol was already present in the starter?
    3. How can I calculate this again the next time as this is a batch I do every month?
     
  2. sbaclimber

    sbaclimber Well-Known Member

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    Someone will chime in with the numbers, but how to calculate is fairly straightforward.
    Quick and dirty, forget ABV, it is irrelevant. Important is the starter volume and (pre-fermentation) SG vs the wort volume and SG. If the 2 SGs are the same, then everything remains equal. Think of the starter as just pre-fermenting a portion of the wort.
    In your case, you have a (very) high SG wort with a relatively high volume but low SG starter. Fermented or not, your starter will "thin" your wort significantly.
     
  3. AGbrewer

    AGbrewer Well-Known Member

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    I realize the flavor drift issue, but I'm really trying to figure out what the SG difference should be. I've done this several times before and it doesn't really "thin" the beer or affect the flavor much at all. I think it's because it is such a big beer and it's a stout. If I was doing a pilsner, this probably wouldn't work well.

    Perhaps I could simply add 200 grams of DME and 2 liters of water to the original recipe to figure out what the difference is on the OG? I tried it and it basically dropped the SG by about 10 points. Just don't know if that is the correct thing to do.
     
  4. sbaclimber

    sbaclimber Well-Known Member

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    That will give you the correct OG, yes.
    And yes, the unexpected thinning is because your starter has a large volume in relation to your wort (starter = 16%/vol). I for example you had 7 liters of wort, you would only notice "half" of a difference.
    Normally I would say, if you knew the current SG of the starter when you added it, then you could have checked the difference with SGxVol+SGxVol=SGxVol, but I just punched in the numbers assuming you just added 2 liters of pure h2o and still ended up with an SG of >1.100, so it looks like you might also have a bit of a mesuring error somewhere...
     
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  5. AGbrewer

    AGbrewer Well-Known Member

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    Given that I pitched the yeast starter at high krausen into the the wort, I would expect the SG to be at or below 1.100 because the yeast have already consumed most if not all of the DME that was in the starter, so there is actually a fair bit of alcohol in 2 liters of wort rather than just being water and DME. I would think that this would explain the low SG reading I got after pitching the yeast starter into the wort.

    Do you agree or think there is something else off?
     
  6. Silver_Is_Money

    Silver_Is_Money Active Member

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    There are two ways to solve this problem:

    1) Make up your starter at the same OG as for that projected by your recipe, and pitch all of it.

    2) Settle the starter and decant most of the predominantly clear liquid Wort off of it, and then only pitch the small remaining yeast slurry portion to your fermenter. Dispose of the decanted Wort.
     
  7. Donoroto

    Donoroto Well-Known Member

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    Assume you pitched 2 liters of water into 12 liters of 1.126 wort. Then 126 * 6/7 = 108, meaning pure water would bring it down to 1.108.

    Since you measured 1.098, I suspect a measurement issue, since your starter was more than water.
     
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  8. AGbrewer

    AGbrewer Well-Known Member

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    Thanks everyone for the help.

    There could have been a measurement issue, but I did 3 checks on SG (Hydrometer, refractometer, and Tilt hydrometer), so I don't think that is the issue. Regarding volume, I have a fermenter that is stamped with volume marks. So I don't think it was a volume issue either.

    The more I think about it, the more I think that the alcohol in the Starter is what threw the SG off so much. No big deal, just needed to understand why it was off and I think that really explains why it happened.

    Also, simply decanting the yeast of the 2 Liter Starter and doing a small (e.g. 100 - 200 mL) starter on Brewday or simply pitching straight away would prevent the issue from being that big of a problem in the future.
     
  9. Silver_Is_Money

    Silver_Is_Money Active Member

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    As I think of it, even a starter made to the same OG as the recipe would drive the OG down due to the alcohol vs. sugars issue. But for that case it will all even out at the FG stage. This would call for taking and recording the OG before pitching in the yeast starter.
     
  10. Donoroto

    Donoroto Well-Known Member

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    Ok, I wasn't going to mention it, but it is my understanding that the liquid atop a starter isn't exactly the kind of flavor you might enjoy. Most pour it off before pitching.
     
  11. Silver_Is_Money

    Silver_Is_Money Active Member

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    #11 Silver_Is_Money, Apr 5, 2021
    Last edited: Apr 5, 2021
    Generally it's only DME, and decent beer can be made with DME. That said, most do decant.
     
  12. BarbarianBrewer

    BarbarianBrewer Well-Known Member

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    I use this calculation for combining two volumes of wort at different gravities. From <https://www.homebrewtalk.com/thread...g-two-volumes-of-wort-at-different-sg.216625/>

    upload_2021-4-4_21-5-4.png


    Using the recipe calculator you could determine the starting gravity of your starter. Then use this formula to determine the theoretical SG of your starter (pre-fermentation) and your main wort. Compare that theoretical SG of the whole batch to the actual gravity you measured. Use that gravity drop to determine the alcohol present in the whole batch. Since your starter makes up 16% of the volume, multiply ABV by 6.25 to get the ABV of your starter.
     
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  13. Hawkbox

    Hawkbox Well-Known Member

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    I completely ignore the starter in my calculations, but I'm doing 21L batches with 2L starters so it's a lot less volume addition.
     
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  14. Nosybear

    Nosybear Well-Known Member

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    I decant all of my starters so am adding minimal amounts of diluting "bad beer."
     
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  15. BarbarianBrewer

    BarbarianBrewer Well-Known Member

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    Do you do this for all starters because not doing it impacted flavors in past beers or just as a precaution?
    What is your normal batch and starter size?
     
  16. Nosybear

    Nosybear Well-Known Member

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    I do it as a very simple insurance against off flavors. Example: I'm doing a lager, a 3-liter starter at room temperature. I intend to ferment my 5-gallon (19 liter) batch at 48 degrees F. I'm adding a significant amount of oxygenated beer fermented at far too high a temperature to my beer if I don't decant. My starter size varies depending on the cell counts I need but the rationale is always the same: I have beer made of extract, constantly aerated and fermented at uncontrolled temperature. Sooooo.... Does that sound like something you want in a carefully malted wort to be fermented at closely controlled temperature? Particularly when decanting from an Ehrlenmeyer flask is very easy.
     
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  17. Minbari

    Minbari Active Member

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    I also decant my starters. no sense adding cheap DME to the batch, I just want the yeast.

    also, when calculating your 2L of starter, dont forget that after 48 hours, that starter that was 1.040 or so, is now ALOT lower, so the forumlas have to be adjusted accordingly. in the end, I would say it is just fine, you added 16% more liquid, that was likely around a SG of 1.014 or less.
     

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