With apologies, Dadux, but your account does not help me understand. I am also quite aware of the difference between aertion and degassing. That was not in fact my question. My question was what is the SCIENTIFIC basis for assuming that a) stirring will incorporate any needed O2 that the yeast may need and b) what is the scientific basis for assuming that FERMENTATION needs more O2 once fermentation is under way. Indeed, your post suggests to me that we may be looking at the skies and parroting that the sun orbits the earth.
You refer to sterols , but don't yeast need the sterols during the lag period? If they need oxygen three days into the fermentation to produce sterols does that not mean that we have a less than viable colony of yeast during the first two days? White and Zainasheff seem to agree that added O2 helps produce more yeast cells (through budding) but we are - I assume - pitching a large enough VIABLE colony of yeast from minute 1 . Why am I looking for the yeast to consume sugar in my must to produce daughter cells? For the additional esters the yeast will produce? But my colony was producing all the esters I need/want... If the colony is too small I simply add another packet of yeast..
You use O2 and air as synonyms, but White and Zainasheff state (see page 83) that what they call "high gravity worts" (ie what we might consider as standard musts with gravities of 1.092 and above ) MUST be aerated with O2 and not air as "air will not provide a high enough level of dissolved air"... so are we to stir to provide "insufficient" O2 or are we stirring because stirring DOES provide enough O2 (and so we dismiss White and Zainasheff!) or are we stirring because brewers say their yeast needs O2 because they use and reuse their yeast multiple times... (see page 82 for a discussion of the problem of insufficient O2 and the use of the FIFTH generation of the same yeast colony.). so I ask again, what is the basis for STIRRING and what is the basis for assuming that yeast need air to produce alcohol. Nothing that White and Zainasheff or that Dadux has written provides me with a convincing case. Of course, I may be misunderstanding what they and Dadux mean... But what W & Z say seems to suggest something very different.
I guess what I am looking for is an expert on yeast who can provide me with published peer reviewed material on the rationale (if there is ) for aeration by stirring once the lag phase has ended and throughout the active phase of fermentation. once
Ok, i'll try again.
Leaving stirring for later, about O2, yeast and fermentations (mid fermentat oxigenation)
If an expert writes a paper and says "we did trials when we produced wine adding O2 and not adding O2 at 1/3 sugar break, and a blind sensory panel determined that the first group was better" would you not do it? plus, after you do it, meads seem to turn out better. I think you already got this far, im writing it because its actually what the thread is about. So yes, we should stir and aereate.
Now, to the "why O2 helps". Dont have time to actually support this with scientific material, maybe tomorrow, if you dont mind, but i'll try to explain it basing it in what i wrote earlier.
-You want the yeast to reproduce. A lot. or else, the ferment will take forever and/or stall. This (budding) happens even if you dont aereate. Can you get away with adding more yeast? probably, if you rehidrate with goferm or is a low gravity must (for mead). In fact, this is what is recommended in sack meads and show meads. Its not a coincidence. In normal ferments you dont exaclty want MORE cells, you want HEALTHIER cells.
-O2 is a nutrient. you decide to add Fermaid or whatever is your nutrient brand pick to your mead and this has many things to your mead. Same way with O2. You can just not add the fermaid and add more eyast (show mead), but why would you when its healthier for the yeast to be fed? its stupid. Cells need nitrogen for many things. They also need O2. Yeast is NOT anaerobic only. There is a reason for it being also aerobic. When deprived of oxigen they can live, just not as well
- Lets do some math. Follow my argument for a second. You pitch your yeast and add oxigen at pitch. Then the yeast use it to form compounds that will improve their lives, and so, the final ferment will be better because they will be less stresed. If you understand that, then imagine yeast reproduce... say 5 times in the ferment (total random number, its probably more). That means that when there were two cells, now you got 2x2x2x2x2 = 32. 16 times more. And so, the sterols are 1/16 what they need. So this cells dont have all they need. When you aereate again, when the yeast is growing, they can build enough sterols, and so, continue a happy life. Anyway, if we disregard the math, there are a series of studies that have been made and many places call for aereation for this exact reasons, and also because it diminishes the concentration of H2S. I also want to point out that its harder to find articles about wine aereating for spoilage reasons, but super easy to find articles and posts about starter aereation on beer. And there is no difference. In a starter, yeast is growing. Same in mead fermentations early on.
About aereating by hand (shaking, stirring and whatnot) it has been published in that book you quote that it does add oxigen. Enough? no. For them. However, as i said, i made some arguments about why it could be enough in another thread, and that is, that if you aereate once with a stone and add 15 ppm of O2, and you aereate one with stirring and add 8 ppm of O2, what prevents you from just stirring 2 times/day? Also, lalvin has in its information sheets, a recommended level of O2 in each yeast (unfortunately, they just state if the O2 need is "normal, low or high", same as with nitrogen, but as you can see it might not be the same in all strains). I will say, however, that this comment about if stirring is enough is complete speculation. Im 99% sure its better toi do it than not to do it. Is it enough? I am not sure.
About how oxigen helps the yeast and how yeast that has been cultivated or grown in oxigen-rich medium ferments better, i do have a couple of papers (Its not exactly what you are looking for because it does not talk about stirring mid ferment exactly, just yeast growing when previously provided with O2 or not, sorry its not more but maybe its a start, as i said maybe tomorrow i'll do an extensive search). If you (or anyone) want them PM me with your email, cause they are not for free and i cant upload PDFs.
You are right taht we do not have all the facts (i think we never do in science-related topics anyway) but i think we do have an understanding about why its good to aereate. I've tried my best to make it understandable and i know its not exactly the answer you want, but its all i got.