fossilsrocks wrote:A couple of thoughts: The flask is a sealed system. No mater how much of a swirl you have in the flask, there is no real mechanism there to draw anything into the flask. In fact, the yeast produce CO2 which will actually cause a positive pressure inside. This has been argued in detail before...
I'm still not entirely sure I believe that, especially before fermentation begins - I think you are picking up constant aeration from the vortex before you see any fermentation. Someone with a DO meter needs to prove that one way or another once and for all. I do agree with aerating before hand - however you should be careful aerating a starter with pure O2 as I seem to have overdone it in the past and prohibited fermentation. o2 can be toxic to yeast and in such a small volume I think you can reach toxic levels. That is from my personal experience, I do not use pure O2 in my starters any longer.
As far as using an airlock on a starter - it probably doesn't really hurt anything (which is why I said "in theory" above) - however it is not necessary - I don't use an airlock even on my fermenters until krausen has subsided (in fact there is evidence that even a small amount of back pressure can increase ester production, whether an air lock can create enough back pressure is another story).
BTW: Dr. M. B. Raines tends to agree that a continual stirring on an un-airlocked starter contributes to constant aeration: http://www.maltosefalcons.com/tech/MB_R ... turing.php