Yeast cells do not convert ethanol to acetic acid during diauxic shift. They merely reverse the acetaldehyde to ethanol conversion process.
Ethanol → Acetaldehyde → Acetate → Acetyl-CoA
Acetyl-CoA is fed into the Krebs cycle (a.k.a. TCA cycle).
From the article entitled “How did Saccharomyces evolve to become a good brewer?” (http://www.ub.lu.se/upload/JureArt1.pdf)
I read that they did in the article below (about halfway down the page under the subheading “The hazards of respiration”), but maybe it is an over-simplification or I read it too quickly.
I’m not a microbiologist and have very little science back ground but my experience in almost 20 years of brewing has told me that you want to do everything you can to limit exposure to oxygen after primary fermentation is complete. Beers that are aged in containers purged with Co2 always have had longer shelf life than those who haven’t. Especially hoppy beers, which will go down hill extremely quickly if exposed to o2 after fermentation is complete. I’m going to stick by this is what I promote to other brewers.
+10. There’s info that is perfectly sound but entirely theoretical, and there’s info that you gather by actual trial and (especially) error. 20+ years on my part, too. I have never (and would never) filter my beers, but I have had beers with PLENTY of suspended yeast (read: hefe, wit, etc.) go oxidized in secondary - cardboardy/wet dog/heavy caramel. As said, hoppy beers are especially susceptible.
Acetate = acetic acid in this case.
I would not encourage the yeast to undergo a diauxic shift. Even if they are able to consume all of the O2 in beer caused by splashing during transfers, it is not like these metabolic pathways are zero access highways with entrance and exits only at the ends. All of these pathways are leaky and you get side reactions going on all over the place. Side reactions lead to unintended compounds and unwanted flavors. Pathways on paper are one thing, but the reality is more of an intricate web that we don’t fully understand.
I too started brewing over two decades ago (early 1993). I also started collecting and managing brewing yeast shortly thereafter. I have never had an oxidized beer that was the result of using a secondary fermentation vessel. I never rack clear beer to a secondary fermentation vessel, as there is little point in doing so. I have encountered more off-flavors from leaving green beer in a primary fermentation vessel for an extended period of time than I have from racking green beer to a secondary fermentation vessel after the end of visibly active fermentation.
I am not arguing that one does not need to be careful when racking. It’s just that the turbulence that is created when racking beer through a length of 3/8" I.D. tubing that runs from a racking cane in a primary fermentation vessel to the bottom of a secondary fermentation vessel is not strong enough, nor does it last long enough to oxygenate a batch green beer to the point where the culture cannot clean it up. Racking a batch green beer to a non-porous secondary fermentation vessel at the end of fermentation is far less likely to cause off-flavors than leaving it on the trub in a food-grade bucket for an extended period of time.
With respect to acetate production, that metabolic pathway is also open during glycolysis. The question here is how much acetate leaks out of the metabolic process during diauxic shift? Is it a major concern?
With the above said, one of the most commonly encountered oxidized flavors in beer is trans-2-nonenal. The oxidation that causes this staling reaction is more often than not introduced during packaging. It’s no secret that bottle-conditioned beer is less susceptible to oxidation than bottles filled from a keg. I have yet to see a brewer purge his/her bottles with CO2 when bottling bottle-conditioned beer. However, we all know that care must be taken when filling bottles from a keg, especially a keg that contains brilliantly clear beer.
Talking to a brewer recently who graduated from a German brewing school and he, too, mentioned the fact that live yeast in unfiltered beer can help protect against oxidation. “Help Protect” were his words. Not a silver bullet, you still run the risk of oxidation. But the yeast can absorb much of the o2. So I stand slightly corrected but the key here is it is still a risk especially after much of the yeast has gone dormant.
My reasoning for purging the secondary isn’t so much the o2 you pick up during racking but the o2 in the head space is what I feel like is a problem. If the beer has sufficient co2 present i suspension it can push some of that o2 out. But I’m not sure it is worth taking that risk.
Also there is the potential of oxidation damaging hop flavors and the reforming of diacetyl - I’m not convinced that live yeast present in the beer eliminates this problem entirely.
Anyway, I do feel like I learned something. And am more convinced now of the benefits of unfiltered beer! So that is all good! 8)
