[tschmidlin]

Here's the main reason I don't believe it.  We know that yeast can survive in air, and in a saturated liquid that's supposed to be 8 ppm O2.  We also know that when first pitched yeast will rapidly take up ALL of the available O2, and if brewers are oxygenating to 12 ppm that's 12 ppm they can remove.  So hypothetically, if the yeast can drop the level to 8 ppm they're fine, thus 20 ppm should be no problem at all.  And that is considerably higher than 12 ppm.  Further, a healthy pitch of lager yeast (which is recommended to be twice as much as ale yeast) should be able to handle twice as much O2.  So I call BS on the 12 ppm number without seeing the original data.

I suspect that the primary research will indicate some factor that will make it irrelevant for our purposes - probably that the media was continuously oxygenated, which is not what we do.

[a10t2]

FWIW I don't think I've ever seen 12 ppm referenced as an upper limit. And I know I've read several independent sources that recommend ~15 ppm for high-gravity beers.

OTOH, saturation of pure O2 in water is ~50 ppm even at ale pitching temperatures (~60 ppm for lagers). I would have no trouble believing that could be detrimental.

[tschmidlin]

Maybe.  I'd really have to see the research that indicates it's actually bad at 50 ppm before I'll believe it.  I know it sounds high, but the level will start dropping as the yeast take it up.  And as far as I know, the toxicity will generally come from reactive oxygen species which are generated by respiration, and respiration should be suppressed because of the sugar level in the wort.

[maxieboy]

Is the Mixstir powerful enough to get a whirlpool going in the kettle ( 10 gal ) or would a paint stirrer be more appropriate for this?

The plastic blades don't stand up to the hot wort, they'll soften and wrap right around the shaft. Ask me how I know...
 I did make some replacement blades from stainless steel but never used them. I built a hop strainer that's so effective that I don't need to whirlpool.

Pretty sure you aren't supposed to aerate/oxygenate hot wort. I'm surprised no one else caught this comment earlier?!? Aerating/oxygenating is for cooled wort. Isn't that where the (bad) term hot-side aeration comes from?

I've used the red O2 tanks for years. Recently acquired a cheap decommisioned small medical O2 tank. Holds way more than those disposible red ones. Yes, oxygenating costs more. But I figure the yeast want oxygen not just air. Not that good results can't be had with aerating.

Thread drift. I was responding to theDarkSide's question about whirlpooling. I would never aerate or oxygenate hot wort. Back to your regularly scheduled thread...