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Wort DO and Yeast O2 Uptake

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  • Wort DO and Yeast O2 Uptake

    Hi all...

    We are trying to dial in our wort dissolved oxygen at the brewery to get a DO in the range of 8-10 ppm (our desired levels vary slightly from beer to beer). Our current process is to brew 3 80 bbl batches into a 240 bbl fermenter. We generally pitch our yeast during our first knockout and only oxygenate during our first knockout. We have found that the yeast take up most of the O2 within 2 hours or so. My question is: if we oxygenate to ~24 ppm during the first knockout, would that be equivalent to ~8 ppm after 3 knockouts pre yeast pitch? I know that your suggestions may be to pitch the yeast at the end of the 3rd knockout, but there are quite a few reasons why we won't be able to do that. Any info on this would be greatly appreciated!

  • #2
    Maybe oxygenate a bit more but basically carry on as you were

    Originally posted by jcross View Post
    Hi all...

    We are trying to dial in our wort dissolved oxygen at the brewery to get a DO in the range of 8-10 ppm (our desired levels vary slightly from beer to beer). Our current process is to brew 3 80 bbl batches into a 240 bbl fermenter. We generally pitch our yeast during our first knockout and only oxygenate during our first knockout. We have found that the yeast take up most of the O2 within 2 hours or so. My question is: if we oxygenate to ~24 ppm during the first knockout, would that be equivalent to ~8 ppm after 3 knockouts pre yeast pitch? I know that your suggestions may be to pitch the yeast at the end of the 3rd knockout, but there are quite a few reasons why we won't be able to do that. Any info on this would be greatly appreciated!
    Oxygenation is only necessary for yeast growth, otherwise it is to be avoided. So, the question is does your current practice give you adequate oxygenation for yeast growth, which means are your beers fermenting out in 4-5 days. If you don't have enough yeast growth, you will need to repitch more slurry containing older less healthy yeast, but equally you don't want too much growth either (turn that extract into alcohol not yeast biomass). The way to judge if your 4-5 day fermentation is reliant on too much older yeast is if your fermentation performance starts to drop off after a few generations/cycles. You probably don't keep repitching endlessly anyway but cycles 2-8 or 9 should all be indistinguishable if you've got the balance right.

    So in short, what you're doing (pitching and aerating the first of three brewlengths into a common tank) looks pretty ideal and the only question is are you pitching enough slurry/ aerating long enough to give you consistent timely fermentations. I don't think you would need to aerate to 24ppm to get the balance right, but you have to find that balance empirically.

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    • #3
      Originally posted by UpsidedownA View Post
      Oxygenation is only necessary for yeast growth, otherwise it is to be avoided. So, the question is does your current practice give you adequate oxygenation for yeast growth, which means are your beers fermenting out in 4-5 days. If you don't have enough yeast growth, you will need to repitch more slurry containing older less healthy yeast, but equally you don't want too much growth either (turn that extract into alcohol not yeast biomass). The way to judge if your 4-5 day fermentation is reliant on too much older yeast is if your fermentation performance starts to drop off after a few generations/cycles. You probably don't keep repitching endlessly anyway but cycles 2-8 or 9 should all be indistinguishable if you've got the balance right.

      So in short, what you're doing (pitching and aerating the first of three brewlengths into a common tank) looks pretty ideal and the only question is are you pitching enough slurry/ aerating long enough to give you consistent timely fermentations. I don't think you would need to aerate to 24ppm to get the balance right, but you have to find that balance empirically.
      Thanks UpsidedownA. We are generally getting consistent fermentations (we produce mainly lagers, so generally a bit longer than 4-5 days), we have just seen some decline in our yeast growth lately. We have increased the amount of O2 and it seems to help, but I would like to dial in an optimal range in order to maximize consistency and to have a target number with regards to O2 flow rate and time. I understand the effects of over-oxygenating and under-oxygenating, I am just looking to find the sweet spot where we are producing enough biomass and getting good, consistent fermentations repeatably.

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      • #4
        Are you measuring your wort DO with an accurate meter? Or doing calculations? I would assume measuring if you are seeing your DO drop to 0 after 2 hours from KO.

        If your second knockout is only a few hours after your first knockout, I would think 12 ppm injected into both of the first two KO's would be a great solution.

        We do double batches with knockouts a few hours from each other and also see the yeast absorb everything in the first few hours, then during the second knockout we also aerate, the yeast absorb nearly all the oxygen continuously as the wort is coming in, leaving next to nothing after the second KO is complete.
        Last edited by Junkyard; 02-04-2019, 07:09 PM.

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        • #5
          To answer the question - No. Adding 24ppm on brew one is not the same as adding 8ppm on three consecutive KOÂ’s. 8ppm consecutively will produce much more yeast growth (depending on time between KOÂ’s).

          As you observed, the ppm of O2 drops significantly after a couple hours. This is not only due to yeast uptake, but mainly due to dissolution. Yeast will continue to use oxygen if it is available and you will see an increased log phase (cell production) when more O2 is present.

          Bench test - Aerate two different batches, one at 8ppm and one at 24ppm. You will find they both reach “0” at almost the same time. Measure your cell counts. You will likely find little difference. This would be more exaggerated in ales, but the slower metabolism of lagers gives a better test. Ale yeasts will take up more O2 faster, resulting in a more skewed but similar result.

          Point is the yeast can only use so much of the 24ppm before it dissapates. It is more beneficial to add oxygen again during the log phase to keep yeast in aerobic mode. This is common practice in high gravity brewing as well. O2 is often added up to 48 hours after final knock out.

          There are other issues with 24ppm of dissolved O2 as well. You must use bottled O2 and compressed air is out of the question. You can achieve upwards of 14ppm on compressed air using a pressurized Venturi system (ask Bit Burger, itÂ’s true). O2 is toxic to yeast at high concentrations and 24ppm is approaching that limit. I think you would see best results with a staggered 8ppm target. Just my $0.02

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          • #6
            Oh yea, and always pitch yeast as soon as possible. Don’t give other organisms a chance to feed off your perfect mediim. Yeast will protect itself (and our wort) by starting to drop the pH very quickly.

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            • #7
              Thanks for all of the input. Tons of useful information! I will take your advice and start a few trials...cheers!

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