So I have often wondered this, and a friend who is much more knowledgeable than I actually pointed this out to me. When you make a starter with a Stir Plate the whole point is to exchange CO2 for oxygen, right? So once the active growth phase is done and there is no longer a high amount of CO2 being released, the starter is now pulling in outside air, correct? So now the starter is functionally being infected with wild yeast and bacteria from the air that is being sucked into the starter under the foil cap thus rendering it a possible source of contamination instead of a guard against it, correct? To clarify, stir plate releases CO2 from the starter during the active growth stage keeping unwanted contaminants out of the culture, but once the active growth phase is finished the vortex is now sucking in outside air full of unwanted yeast and bacteria into the culture. If this is true, why would we use them? What is the best way to protect against this and still get maximum growth? Discuss.