I was under the impression that yeast with O2 create more yeast, and yeast without O2 create alcohol. So once my brews get going, I just try to release CO2, not add O2.
I've tried that approach, and it seems to work if the yeast population density is high enough, but if it isn't, then the result seems to be unhappy yeast. I'd be curious if it's true or not, but it
seems as though the yeast are able to sense their own population density. Why do I say that? Well, consider this quotation from the Fermaid K datasheet:
"In all alcohol fermentations yeast growth must occur before the anaerobic or alcohol fermentation can begin. In most wine applications, the usual inoculation rate is 25 grams per hectoliter (250 ppm or 2 pounds per 1,000 gallons) which results in an initial cell concentration of 3 to 4 million viable yeast cells per milliliter of must. Under normal conditions for virtually all types of fermentation media the cell population will increase to 100 to 150 million viable yeast cells per milliliter before growth stops and alcohol fermentation takes over." [emphasis added by me]
https://catalogapp.lallemandwine.co.../1b340d1ae3fc0a693339355555cdfcfa4971a1e4.pdf
So, taken literally, this means the yeast
must increase their population by up to 50x from their typical initial inoculation population before they they will happily make alcohol. On the other hand, if from the get-go you pitch at a rate of 100 to 150 million viable yeast cells per milliliter of must, it seems that then, yes, you can slap on that airlock and the yeast won't complain about making alcohol instead of reproducing.
What does this mean in practice? I think it means that in order to have happy yeast, you need to supply them with oxygen to support their reproduction up until the yeast population is in the range of 100 to 150 million viable yeast cells per milliliter of must. Therefore, the lower the pitch rate, the more oxygen they'll need.
Is this right?