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  • CO2 Toxicity?

    I was chatting the other day to a fellow brewer who was using peracetic acid for his blow-off buckets when I mentioned that I was using spunding valves on me fermenters to naturally carbonate my beers. He asked me if I was worried about CO2 toxicity on my yeast. What is CO2 toxicity and should I be worried? I am brewing mostly session beers of the English, Belgian, and German variety. No lagers, but a kolsch fermented at 55F. Advice and past experiences welcomed!

  • #2
    Harvest your yeast before you cap it off.

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    • #3
      Originally posted by alchemy
      Harvest your yeast before you cap it off.
      Ok, care to explain why?

      I have had concerns about pressure and the yeast viability. CO2 toxicity Re: yeast and repitching is a new one to me, i have worked for several breweries that cap and harvest and have not seen any ill effects compared to the ones who don't cap before harvest.
      Jeff Byrne

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      • #4
        Guess I said that too quickly. What I meant was "if there is an issue with toxicity, harvest yeast before you cap it off." CO2 can certainly be toxic to yeast, though how much has to do with time and concentration. I can't seem to find the reference with the exact numbers right now, but I will post them when I come across them. A simple cell count with viability testing should tell you if you are having any issues, as should attenuation, fermentation time, taste, etc...

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        • #5
          spunding

          I've never had problems with harvesting yeast after spunding. In fact, I've even stored the yeast in the finished and full fermenters until needed. It may not be textbook but it works if you're careful.

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          • #6
            The 'toxicity' referred to is more of a feedback control mechanism for yeast metabolism. I've never heard of it being detrimental to their overall health though.

            One of the best lines I've ever heard is that yeast eat sugar, fart CO2 and piss alcohol. If I was in a room saturated with farts, I'd probably stop eating as well.

            Same thing happens in your fermentor. Also, different yeast strains have differing sensitivities to dissolved CO2. Some seem almost unaffected by high levels of dissolved CO2 while others seem quite sensitive, in my experience.

            Have never looked at the literature but basically, my understanding is that the yeast will stop fermenting, even in the presence of fermentables when the dissoved CO2 hits a point where it triggers a feedback suppression of metabolism.

            Pax.

            Liam
            Liam McKenna
            www.yellowbellybrewery.com

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            • #7
              CO2 Toxicity

              So, with my spunding valves, I can dial in my "pressure" so there will be a certain head pressure during fermentation for natural carbonation and everything else will be expelled through pressure relief. Any recommendation for pressure setting and when to set it?

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              • #8
                For ales i typically put it on the 3rd day and set it at 15PSI. It's usually just enough to build to 15 PSI. It won't totally carbonate at fermentation temp but when you crash the tank it will soak up more. I still have to touch up in the BBT via the stone but it definately saves co2 in the long run and gives good head (retention).
                Jeff Byrne

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                • #9
                  Originally posted by Jephro View Post
                  Ok, care to explain why?

                  I have had concerns about pressure and the yeast viability. CO2 toxicity Re: yeast and repitching is a new one to me, i have worked for several breweries that cap and harvest and have not seen any ill effects compared to the ones who don't cap before harvest.
                  If a tank is kept under pressure will cross the cell walls and kill yeast cells.

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                  • #10
                    Originally posted by liammckenna View Post
                    The 'toxicity' referred to is more of a feedback control mechanism for yeast metabolism. I've never heard of it being detrimental to their overall health though.

                    One of the best lines I've ever heard is that yeast eat sugar, fart CO2 and piss alcohol. If I was in a room saturated with farts, I'd probably stop eating as well.

                    Same thing happens in your fermentor. Also, different yeast strains have differing sensitivities to dissolved CO2. Some seem almost unaffected by high levels of dissolved CO2 while others seem quite sensitive, in my experience.

                    Have never looked at the literature but basically, my understanding is that the yeast will stop fermenting, even in the presence of fermentables when the dissoved CO2 hits a point where it triggers a feedback suppression of metabolism.

                    Pax.

                    Liam
                    LOL, you can attribute that quote to me. I've been using it during brewery tours for decades.
                    Todd G Hicks
                    BeerDenizen Brewing Services

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                    • #11
                      Mostly a non-issue...

                      My SOP for dozens of beers at many breweries is to spund at about 1P to build pressure to 15psi. Your results will vary according to FG, headspace, and fermenter temperature. We harvest yeast after FG and cold crashing. Of dozens of yeast strains, I've never had any issue with CO2 toxicity. It is an overrated problem IMO. Very tall fermenters, higher spunding pressures, particular yeast strains, and yeast stress factors may make CO2 toxicity more of an issue. I would simply monitor viability for every fermentation and ignore worries of toxicity until you see a reason to give it some credence. Just my 2 bits.
                      Phillip Kelm--Palau Brewing Company Manager--

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                      • #12
                        Although CO2 in high concentrations might kill the yeast as it would most living organisms (obligate anaerobes perhaps not) I don't think that is your problem. Think about it. How may bottle condition or keg condition, reaching CO2 concentrations of perhaps 6 or 7 grammes per litre for some Belgian beers I have worked with. So the 5 grammes per litre you are likely to get when spunding is not going to kill it.

                        I suspect the yeast is autolysing due to getting too warm if you don't take it off early enough. Yeast is not a good conductor of heat (or "coolth" from the glycol jackets) so the centre of even a small mass of yeast will warm up, and in anaerobic conditions will quickly die off and start to autolyse.
                        dick

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                        • #13
                          The misleading part of this thread is that despite having your blowoff pipe open, you still have head pressure and dissolved CO2.

                          Atmospheric pressure is 14.7 psi at sea level, meaning that if your pressure gauge reads 0 psig you actually have 14.7 psi absolute pressure from the atmosphere as "head pressure" on the tank.

                          Also depending on the height of the column of liquid in the tank you have hydrostatic pressure of ~.5 psi per foot of height, so if you have a 10' height, you will have an extra 5psi at the bottom of the tank due to the weight of the column of liquid above. (you can take specific gravity into account if you want to really get precise)

                          This would mean that at the bottom of the 10' tank you have close to 20 psi absolute pressure. Once fermentation is underway, CO2 evolution will scrub most of the other gasses out of the tank, so this head pressure "pushes" on the CO2 into dissolving in the beer. Temperature will affect CO2 solubility as well, with lower temp dissolving more CO2 for a given pressure. Typically a beer that has finished fermenting at atmospheric pressure will have at least 1.0 volumes of CO2.

                          so: just because you have not closed your spunding valve, does not mean you will not have any CO2 dissolved. Closing the tank up will mean more, but you are not starting from zero.


                          Fermenting yeast makes CO2 which it has to push/ "fart" across it's membrane into it's surroundings. The concentration difference between the inside and outside will have a bearing on how well it is able to do this, and so will probably at some point will regulate it's ability to fart and thus slow down it's fermentation. Presumably depending on the yeast strain, some may be more or less affected by the amount of CO2 in solution. However It is more likely as Dick has said that yeast health is more of a factor than CO2 top pressure.
                          Last edited by beerme; 03-31-2020, 05:39 AM.

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