When making a massive starter I find it needs to go in the fridge to crash the yeast so I can decant the wort and then build up the yeast again with more fresh wort.
I use aluminum foil over the flask while it's boiling to sterilize everything, and keep that same foil on the flask while fermenting and ultimately crashing in the fridge.
BUT, after the starter has fermented and goes in the fridge, the temp cools the wort and tho' there is foil over the mouth of the flask, the change in temp will cause the flask to suck in air. In this case, air in the fridge that has got to be filled with all sorts of nasties you don't want in a starter.
Doesn't this make sense? So wouldn't putting a rubber stopper over the flask with an air filter be a better way to go?