First off let me start by sayingim not freaking out....I think everything will be ok. That being said, I made a 1.5 L starter yesterday for a Bavarian Hefe that I'm brewing later this afternoon. Everything went according to plan. I made the starter in a 1 gallon glass jug because I'm too cheap (read: resourceful) to go out and buy a 2 liter erlenmeyer because glass is glass is glass. Guy at my lbs said sanitized foil would do just fine for a "lid/airlock" on my 1 gallon jug. I have been swirling it everytime I walk past it to oxygenate it. Last swirl was 830 this morning before leaving for work. I just got home and swirled it again and I immediately hear pressure building, foam starts foaming, and off pops (pretty quickly actually) my foil "airlock". Now I doubt anything has contaminated my starter, but my question lies in the fact that if it built up a ton of pressure in my glass jug by swirling it just now, logic would dictate that it pretty much had a closed system and wasn't getting any air out throughout this process. I guess I'm curious to know if me swirling and it foaming up something vicious just now could have any adverse effects on my starter. If it was a closed system (which I think it was because of how far the foil shot off upon building pressure inside), did I even make any more yeast cells? I know yeast need oxygen to reproduce.....have I deprived my starter of o2 is whole time? thanks in advance!!