Do yeast really scavenge oxygen post-lag phase?

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palmtrees

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Pretty frequently on here, I see people reference methods of getting their yeast to scavenge oxygen when dry hopping or bottling. It usually involves dry hopping when fermentation is still active or adding a bit of extra sugar, if fermentation is finished. The theory is that the yeast will consume any dissolved oxygen as they finish fermenting/reactivate.

I've always been pretty skeptical of this idea of oxygen scrubbing. With the caveat that I haven't read Yeast, my understanding of the function of oxygen is that the yeast need it during the lag phase while they're dividing like crazy because they break it down into compounds used to build cell walls. It seems to me that once the lag phase is over, and the yeast cells go from dividing to eating sugar, that the need for oxygen would drop precipitously. Are yeast really taking in more oxygen once fermentation has kicked off? The fact that you can oxidize a beer if you use an O2 wand too long pre-pitch suggests that there is a point at which yeast stop picking up dissolved oxygen. And I believe you can also oxidize your beer if you use an O2 wand too late into active fermentation (ie when trying to do a double dose for a very big beer). Given what I know, I just find it hard to believe that the yeast would really scavenge oxygen to the point of avoiding oxidation, if the yeast have already passed through the lag phase.

I could see that in a bottle, they might have some impact, since the yeast colony is presumably much smaller in the bottled sample than in the full batch of beer sitting in primary and may go through a second growth phase. But even there, the amount of cell growth needed to handle carbonation doesn't seem that big.

So, for people with more knowledge of yeast than me, I guess my question is whether yeast scavenging oxygen after active fermentation has begun is evidence-based or just one of those homebrew things that we pass down because it sounds cool.
 

Miraculix

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It's not black and white. If yeast would stop reproducing at one point completely, and wouldn't take in any more oxygen, it would be the last generation.

It's also not like "hey there's oxygen, I don't eat sugar" or "hey there's no oxygen anymore, let's eat all the sugar!!!".

The yeast is capable of switching. But it cannot access the oxygen that might be trapped in the headspace. If there's oxygen, it will slowly dissolve into the liquid and it will continue doing this, even when the yeast has no food left and cannot scavenge the oxygen anymore. That is the real problem.
 
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