Repeated oxygenation of yeast starters

[seafra]

Was just wondering if anyone reintroduces oxygen to their yeast starters?

I oxygenate the wort for my 1 liter starters for about 3 minutes with a .5 micron stainless steel stone, and I use a stir plate. However, I was wondering about the merit of periodically pumping additional oxygen into the flask during the process. My logic is that we are interested in propagation in a yeast starter, not attenuation: hence, a continually oxygen rich environment would further that intention.

After reading the following from Mr Wizard @ BYO, I believe my thoughts my have some merit:
It is important to understand that yeast does not require oxygen to ferment; rather it needs oxygen to multiply - oxygen is required by yeast cells to synthesize sterols and unsaturated fatty acids that are used to build new cell walls. Fresh yeast that comes out of a propagation step contains an excess of these cell-wall-building compounds and is capable of limited multiplication in the absence of oxygen.
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[beerking]

It is my understanding that your are doing precisely that by using a stir plate. I thought the purpose of the stirring action was to keep aerating the wort during propogation.

[seafra]

Originally posted by beerking
It is my understanding that your are doing precisely that by using a stir plate. I thought the purpose of the stirring action was to keep aerating the wort during propogation.

BK,

Yes, the stir plate continues to aerate the wort in a yeast starter, and by keeping the yeast in suspension it allows greater contact with the wort. However, the aerobic phase of propagation will deplete the finite amount of oxygen in the wort. Thereafter, in an oxygen deprived environment, the yeast are unable to produce the sterols required for reproduction. No matter how much you stir a wort, there is simply a limited amount of oxygen in that flask. Sooner or later, the yeast are going to shift from reproduction to anaerobic fermentation: if we can replenish the oxygen, that shift will be later and our cell count will be higher.

I'm sure some have already said to themselves, "this is what a second stage (adding fresh wort) is intended to do." Although this is indeed the case, my contention is still that a higher cell count would result by reintroducing oxygen...with or without a second stage.
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[ClaudiusB]

I aerate my starter 24 hr a day and feed my helpers once a day.

Cheers,
ClaudiusB