Oxygen & Aeration: Increase or Decrease Lag?

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Oct 6, 2010
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I thought I'd get more discussion here. This is in reference to the article posted on the home page. I think it's a great write up, and I know there are many homebrewers that use this or similar methods for aeration. I'm only curious about one aspect of my understanding of the life cycle of yeast.

Here's the article:

And here's my comment:
"Someone correct me if I'm wrong, but I thought the addition of oxygen actually increases the "lag phase." In the presence of oxygen, yeast cells are focused on reproduction, and while they are multiplying, they are not
expelling ethanol and CO2, at least not at the same rate as during anaerobic respiration. Which means, with proper aeration, while the yeast are multiplying, there won't be any significant krausen.

In my experience, adding
oxygen after krausen has formed has caused the krausen to fall, which means the yeast are switching from anaerobic respiration (producing enthanol and CO2, which causes krausen to form) to aerobic respiration (reproduction.)

that's the case, and you had two separate batches, one aerated, one not, all else equal, the one aerated should have a longer "lag time" or take longer to form krausen because the yeast are focused on reproduction rather
than expelling ethanol and CO2.

Or am I wrong? Have you all experienced shorter lag times with the addition of oxygen (all else equal, of course)?"
The author of the article states that of two batches, one using the Venturi method, one using stirring to aerate, the Venturi method had a shorter lag time. My understanding is that the lag phase is the time during which the yeast are reproducing in the presence of oxygen. My question is, if there is more oxygen, won't the yeast be focused on reproduction longer than if there were less? And if the yeast are reproducing, they aren't expelling as much CO2 which creates the krausen, which we interpret as the lag phase.

Starters and cell count are irrelevant to this discussion, since I'm wanting to compare how the yeast acts relative to dissolved oxygen.

Practically, what have you all experienced? Assuming no other factors were changed, have you experienced longer or shorter lag times with the addition of oxygen? Unfortunately, I started aerating with pure O2 at the same time I began using starters, so I can't comment on that single factor.


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Feb 10, 2008
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Reed City, MI
I think you are correct. Lag is often simply defined as a negative in brewing. It's often caused by a lower-than-recommended pitching rate, or less-than-healthy yeast. In that case the yeast are simply low in number and therefore don't show much signs of fermentation right away.

According to the book Yeast, yeast actually do start making CO2 and alcohol, pretty much right away, but it's the percentage of yeast doing that that causes the visible signs of fermentation. It's not just an on-off kind of thing.

So it makes sense that in the presence of Oxygen, yeast will do what they ought to do in the presence of oxygen: reproduce. Oxygen does more than cause population growth, it's needed for yeast health.

But yes, a large Oxygen has, in my experience, seemed to increase the lag time, or at least not decrease it. But it also has led to a good, strong, clean fermentation.

It's good to remember that the fermentation process contains a lot of variables, like temperature, wort density and makeup, water chemistry, yeast strain and health, pitch rate, etc, etc. Oxygen level is simply one variable out of all of them.

Bottom line is I expected noticeably less lag time when I started adding oxygen to the wort before pitching, but there literally seems to be no difference.