[Pi]

when rehydrating dry yeast, using cooled boiled water, the fact that the O2 has been cooked off any concern?

[denny]

when rehydrating dry yeast, using cooled boiled water, the fact that the O2 has been cooked off any concern?

I don't think so.  You need O2 for cell growth, and when you rehydrate dry yeast you're not going for cell growth.

[tom]

And it is all set up with lots of sterols.  Dry yeast may need less O2 than liquid.

[CASK1]

Here's a nice article on the topic from Northern Brewer:

http://www.northernbrewer.com/connect/2010/06/the-importance-of-being-hydrated/

[bazowie]

The yeast book I bought from AHA recommends to rehydrate at 105 deg, wait, stir, cool to pitch temp, and pitch.

[dimik]

Actually I don't think you need oxygen for rehydration for two reasons:
1 - Dry yeast should have whatever they need to get started in the preservative.
2 - Oxygen is actually a very toxic thing. Just because we evolved to use it doesn't mean that it's good for life all the time. So I'd imagine high oxygen concentration in water/wort would result in even more cell death while their membranes are still fragile and not rebuilt. Afterwards, pitching into a well oxygenated wort would be good because it would help in growing and forming stronger membranes, which would prolong and improve their activity and viability.

Here is a little experiment I did to follow up on the original topic
http://bkyeast.wordpress.com/2013/03/13/more-on-yeast-rehydration/

[davidgzach]

when rehydrating dry yeast, using cooled boiled water, the fact that the O2 has been cooked off any concern?

I don't think so.  You need O2 for cell growth, and when you rehydrate dry yeast you're not going for cell growth.

+1.  Or in it's absence for cell growth, sterols and fatty acids.  Hence the olive oil experiment.