orfy said:
It's all about the type of yeast, not the wort.
Do you mean "it's all about the
quantity of yeast"? As I understand it, aeration puts the yeast into an aerobic cycle which causes the cells to reproduce. Picture a bunch of yeast riding stationary bicycles. Ok. That has nothing to do with anything. Anyway, the reproduction phase increases the number of yeasties, but during this time, no alcohol is being produced. It's only when the yeast go anaerobic that they start excreting alcohol.
I think of it as being like our muscles. So long as there is stored oxygen in our red blood cells, we can excercise and whatever, but once our muscles run out of oxygen, and we're breathing as hard as we can, and our oxygen demands out-strip the supply in our blood, then our muscles start using up stored energy. This stored energy can be converted to power without the use of oxygen. And the byproduct is lactic acid. That's what makes your muscles sore after a long work out. Your muscles convert to using a different chemical process to keep you active. Yeast kind of do the same thing, but instead of producing lactic acid in their anaerobic stage, they produce alcohol as their byproduct. This is what brewers depend on.
So, adding oxygen makes the yeast multiply, but it doesn't help you produce alcohol. However, once the O2 is used up, then the yeast stop reproducing and, however many there are, set about eating the sugars for energy and begin pissing out alcohol. Yes, alcohol is a secretion/waste product to the yeast. Imagine a bunch of yeast pissing in your beer. Again, that has nothing to do with anything, it's just what's happening.
So, if you have a starter and a ton of yeast, maybe you're completely happy with them going straight to an anaerobic alcohol-producing phase. However, if you think you don't have enough yeast to kick the butt of any infection that may be trying to take hold, then you want the oxygen there to increase the size of your yeast army. Imagine a bunch of zombies stripping the clothes off of lindsay lohan and giving her much sexual pleasure. That has nothing to do with anything, but it is an amusing thought.
Hope that helps.