1. I have been boiling my starter flask in a hot water bath for 30 minutes. There is no vigorous boiling in the flask, but there is plenty of hot break. I have not noticed any signs of infection either. Should I be concerned that my starter never comes to a rolling boil? 2. Should I add O2 to my starter befor I start the stir plate? Does the yeast get all of the O2 it needs from the whirlpool even with an aluminum foil lid? Still experimenting with my stir plate and wondering if anyone else has these concerns.
30 minutes of near boiling should be plenty to pasteurize your wort. I usually give it a good shake first, but it's probably not necessary. As long as the foil allows for gas exchange, you'll get lots of O2 through the whirlpool agitaion.
i've been using loose foil with an occasional shake, but I suspect the shake isn't really necessary given that I have a stir plate. I just make sure the original sanitation is good, cooling is covered, transfer technique is good, and don't worry about it. Honestly I don't have a pyrex flask so I've been boiling and cooling separately, then adding to a sanitized flask, and haven't had any issues. Your pasteurization sounds like it would work fine. Although I'm not a super experienced brewer, I'm pretty familiar with microbiology and I don't see a reason why you'd have to hit a ROLLING boil if you're boiling your flask for 30 minutes. It should be fine. I'm sure anyone who disagrees will speak up!
The foil is on fairly tight to keep out contaminants. Once yeast growth begins there will be a steady stream of CO2 leaving the flask. My concern is that enough O2 will not be let in. I usually give it a good shake first, but it's probably not necessary. As long as the foil allows for gas exchange, you'll get lots of O2 through the whirlpool agitaion.[/quote]
no rolling boiling required for starter, that is needed for iso-alpha conversion... correct, no O2 required or even a shake...more than adequate O2 will enter the flask & wort with the stir plate
The partial pressure of the O2 is the driving potential that gets the O2 into the flask when the CO2 is going out. No need to shake a flask if it is on a stir plate.
During exponential growth, the yeast are consuming sugars. That's why they are growing, and this is exactly when yeast are produced in 'significant numbers.'
