[Robert]

Now that Saccharomyces is back, I can ask this question.  I have (largely) assimilated the lessons previously imparted.  Forgive me if this was back there and I missed it. Where I'm at a loss is this:  Chris White, in his book, states that a stirred starter will grow two to three times more yeast than a non-stirred starter.  Given that providing continuous aeration cannot, due to the Crabtree effect, extend respiratory growth, can remaining effects like driving off CO₂ and keeping yeast in suspension really have that much impact on growth, as I have simply trusted, or is he just wrong?  I have assumed that his assertion is based on experimental data.  (Please ignore the issue of vitality trumping biomass, although White also makes claims for the yeast health in stirred starters. That's another topic.  My question here is strictly about growth.)  TIA.

[Saccharomyces]

Here is the thing. One is not going to exceed maximum cell density for a given volume.  The average White Labs culture has around 100B cells.  Maximum cell density for a 1L starter is approximately 200B cells.   Even if the culture is 50% viable, one is looking at two replication periods to reach maximum cell density.  That is around 180 minutes beyond the lag phase.  If one is pitching a 50% viable White Labs culture into 2L of starter medium, that only extends propagation time by 90 minutes.  Most White Labs 1L  and 2L cultures are ready to pitch in 12 hours or less.  A stir plate buys you nothing.  All it does expose the cells to shear stress.

[JT]

Follow up question, not strictly on topic but along the lines of yeast stress.  Is there any advantage or disadvantage to adding the yeast prior to shaking like it owes you money?  Or is it best to shake the wort first, then pitch? 
Similarly, 5 gallons of wort, any problem or advantage with pure o2 injection while yeast is already in the wort? 

[Saccharomyces]

I used to shake after pitching the culture.  The culture was pitched and then shaken to disperse the cells and aerate the starter medium.  Turning the starter medium into almost all foam was the result of being a much stronger younger man.  I eventually started to shake and then pitch to minimize stress.

When aerating wort with O2, I personally would aerate before pitching.  The wort is ready to go when the culture is pitched.

[Robert]

My SOP with both starters and 6 gal batches is to oxygenate with O₂.  I do it after pitching to help disperse the yeast, but only as many seconds after pitching as it takes to get the stone in.  Any actual harm?  Is dispersing the yeast really necessary?

[Saccharomyces]

I have never run side-by-side tests with O2.   What I did to disperse the cells in a batch is to cap the carboy, place it on its side, and roll it back and forth.

[Robert]

Can't do that with my fermenter, but maybe I'll devise a method of sanitarily mixing a post-oxygenation pitching, and see if I notice a difference in fermentation, or in the harvested yeast.  It won't be scientific, but if there is a noticeable difference, I'll report it.

[JT]

Thanks!  This wort owes my 3 year old money. 

[James K]

I just made a pitched a starter from dregs this past week. I used a stir plate and would leave it cranked for 12 or so hours before decanting and addin fresh wort. On the build ups i would taste the wort and smell it to see if everything was going smoothly. My final starter I did not taste because everything was mixed together, but it did smell much more yeasty than when I was able to decant. Is that smell related to stressed yeast? Or is it because the cells are dispersed and there is no layering of wort and yeast. I used to shake my starters before I got the stir plate, but now most people on here tell me I don’t need the stir plate.

I usually pitch than shake and I haven’t used my o2 wand in like a year. Also, how much o2 is too much o2?

[Robert]

I used to shake after pitching the culture.  The culture was pitched and then shaken to disperse the cells and aerate the starter medium.  Turning the starter medium into almost all foam was the result of being a much stronger younger man.  I eventually started to shake and then pitch to minimize stress.

When aerating wort with O2, I personally would aerate before pitching.  The wort is ready to go when the culture is pitched.

It occurs to me you are likely talking about pitching an active starter from a lab culture.  Since I am usually repitching harvested slurry stored under beer for several days, I can think of no disadvantage, in that case, to oxygenating and mixing after pitching.   Correct me if I'm missing something, please.

[Saccharomyces]

I pitched more harvested slurry than fresh yeast.  I maintained most of the cultures I used for brewing on agar slants, so a new culture was a several day event that started with 20 to 40ml of 5% w/v (1.020) wort that had been pressure cooked.  Many of the cultures I used are not available via the homebrew trade.

[erockrph]

Can't do that with my fermenter, but maybe I'll devise a method of sanitarily mixing a post-oxygenation pitching, and see if I notice a difference in fermentation, or in the harvested yeast.  It won't be scientific, but if there is a noticeable difference, I'll report it.

I can't help but wonder how long it takes for O2 to begin to oxidize the wort that I worked so hard to keep O2 out of. My SOP has become: pitch, seal my fermentation keg, then bubble O2 in via the liquid out post. I'm hoping that the yeast consume the oxygen faster than the wort can oxidize to an appreciable extent.

[Robert]

^^^^
That's where I'm thinking different considerations apply with active starters versus repitched slurry.