CO2 Toxicity?

[MaltAlchemist]

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!

[alchemy]

Harvest your yeast before you cap it off.

[Jephro]

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.

[alchemy]

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...

[Sulfur]

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.

[liammckenna]

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

[MaltAlchemist]

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?

[Jephro]

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).