Oxygen will keep yeast in aerobic activity as opposed to anaerobic (where ethyl alcohol is produced) which beer brewers clearly prefer. In beer brewing, you want to oxygenate to allow the yeast proper health and viability to take on the job of converting sugars into ethyl alcohol in anaerobic activity, but not to continue aerating after the start of fermentation (or at least once it's kicked into high gear).
First of all, oxygen/air do not keep the yeast in aerobic activity. Fermentation is an anaerobic process, and the presence of high concentrations of sugar turn off the enzymes that allow aerobic metabolism of sugar in yeast (known as the Crabtree Effect)
Oxygen is needed by yeast to produce sterols and longer fatty acids that are needed to maintain the health of the cell membrane, so that you can have the maximal number of divisions while maintaining maximal tolerance to ethanol. It may also help them reduce the number of shorter-chain fatty acids which happen to be yeast toxins. There is much good data on wine fermentations that show that without aeration (or supplementation of sterols and fatty acids) fermentation tends to be incomplete, with sugar left over and less alcohol produced.
Oxygen requirements for complete fermentation are not terribly high (only about 10 mg/L). It works best if added toward the end of exponential growth - I rationalize this by figuring that after dividing multiple times, the yeast need to replenish those sterols. SO, giving the O2/air around day 2 seems to be optimal.
We often give much more air than this and I have begun to question that practice. While mead (especially traditionals) may be less prone to oxidative damage than beer (and probably wine), they can still be harmed by excessive oxygen. Although I have not seen studies that compare it side by side, I have wondered if doing less aeration might be better in many cases; certainly for melomels, I think it probably is.
Why not use yeast starters and higher cell counts like brewing?
You can use starters or large biomass pitches and have successful fermentations, and building a starter essentially allows you to oxygenate a smaller volume of your must (allowing that sterol production) so that the remainder of the batch is protected. However, there is data from wine studies that show you can have stuck fermentations even after large biomass pitches. That may possibly be due to insufficient alcohol tolerance from lack of adequate sterol production, and/or lack of adaption to the increasing alcohol.
Also doing a little aeration is much easier and takes less time than building starters.
If it's really about revitalizing the yeast (which can be inferred from the staggered additions), why not pump pure oxygen into the partially fermented must until fermentation is mostly completed? Would this not be more effective?
You can use pure O2, but this can supersaturate the must (up to around 26 mg/L at least for a short time) where the normal O2 saturation using air is about 8 mg/L. Using O2 may then saturate the must with more O2 than the yeast need, and anytime that more O2 is given than the yeast can use, the potential for oxidative damage exists. In practice, the active yeast tend to maintain a reductive Redox balance in an active fermentation which is one reason why open fermentations of wine and mead can be done while producing great results and why multiple aerations up through the 1/2 fermentation point (and beyond) can be done and still have really good results. Nevertheless, I am tending toward less aeration these days and so far so good.
Medsen
