Re: Underpichers/Overpichers speak out
"I personally believe in pitching the proper amount of yeast"
Who doesn't! One question: what's the proper amount?
DBear: I assume with over- or underpitching, you mean in relation to the often repeated (in the US) rule of thumb of 1 M cells/ml/P? Naturally there are some other standards used in various places, such as G. Fix's 0.75 M/ml/P for ales and 1.5 M/ml/P for lager, or levels in various brewing papers such as 5, 10, or 15 M cells/mL (for beers of various gravities). Anyway the one's I've seen are in the same range give or take a factor of maybe 2.
Steven Pauwels (Belgian brewer who now heads Boulevard) said in an interview that he knows brewers in Belgium who pitch as little as 0.3 or as much as 3 M cells/mL/P. So, that's a factor of 3 above/below the American rule of thumb.
So hopefully that answers you question about what general range is used for over/underpitching.
With aeration, I agree with you and Sean that aeration has to be part the pitching rate decision. Without getting into detail, the less that yeast have been aerated (in the starter and/or in the wort) the higher the pitch rate has to be, and vice-versa. As Sean says, the Belgians often have very well-aerated yeast--and so do many of us who propagate on a stir plate with continuous aeration (loose cover).
Let's assume your propagation/aeration practices are good (if they are not, then pitch at the very high end!) and you want to choose a pitch rate. There are a lot of books, articles, and internet threads say what'll happen when you change pitch rate--but some of them are incorrect. Here is what I think I know is true for a yeast like WLP530, and can provide good academic (or nearly so) references for if you want them. I've also experienced them myself. So with all other things being equal:
1. lower pitch rate will in higher acetate ester levels (pear, banana, solvent, rose, and others)
2. if pitch rate gets low enough, the yeast will perform end-of-fermentation activities (such as full attenuation, flocculation, diacetyl reduction, etc) more poorly than they otherwise would
There are other effects (perhaps diacetyl, fusels, others)but I'm much less certain about their consistency or size, and anyway I think there are better ways to address those.
I do disagree with Sean about playing with pitch rates--I think they are the best way for me to control acetate esters. In fact, I've been playing with WY3787 pitch rates for a while. I started out "overpitching" at what I think is ~1.4 M cells/ml/P (I don't have a way to count cells so I assume growth of ~100 B new cells/L in my stirred/aerated starter). I have decreasing the pitch rate with each batch to gain more and more acetate esters, and planned to stop when either esters get too high or flocculation/attenuation begin to suffer. My latest beer is about as far as I want to go--it's slightly solventy at room temp but just right when cool. Also the flocculation was a little worse but still fine. FWIW I pitched at what I think is about 0.6 M/ml/L. (Maybe it's more helpful if I say I propagated a WY XL pack in a 1.2 L stirred and loosely covered starter, then pitched into ~5G of 1.070 wort).
Hope that helps,