According to Wyeast (sorry for the poor formatting)
Method DO ppm Time
Siphon Spray 4 ppm 0 sec.
Splashing & Shaking 8 ppm 40 sec.
Aquarium Pump w/ stone 8 ppm 5 min
Pure Oxygen w/ stone 0-26ppm 60 sec (12ppm)
It was concluded that pumping compressed air through a stone is not an efficient way to provide adequate levels of DO. Traditional splashing and shaking, although laborious, is fairly efficient at dissolving up to 8 ppm oxygen. To increase levels of oxygen, the carboy headspace can be purged with pure oxygen prior to shaking. The easiest and most effective method remains injecting pure oxygen through a scintered stone.
I have seen this before about shaking for only 40 seconds and, frankly, I just never really believed it. I always wondered if the "40 sec" was a typo and should have said "40 minutes."
I aerate with pure o2 and I am pretty sure that I have over aerated in a yeast starter to toxic levels before (I have no proof, but the yeast should have been healthy and had a loooooong lag. Like over 4 days). But, in my carboys I have aerated with pure o2 from 1-3 minutes per 5 gallons and have never had a problem. A DO meter is a handy tool to have in our hobby - and an expensive one. Kai, do you mind if I ask how much you spent?
Does anyone know what ppm level o2 becomes toxic to the yeast? It seems like I read it before but I cannot remember. Also, I thought that wort could hold only a ppm level that was under the toxicity level? I know that colder wort can absorb more o2 than warmer wort, but can the colder wort absorb enough to toxicity levels?