There's more. As it was explained to me by a biologist, when you underpitch you get only a few generations before you run out of oxygen, and w/o enough oxygen, the yeast start cannibalizing each other.
With a gross overpitch, the yeast use up the sugar but aren't converting enough to alcohol.
So--a starter gets the yeast going, and increases the number of cells available.
****
Below is a bunch of "bullet points" partly from my experience, partly from notes I took at Chris White's (White Labs!) yeast workshop I attended in March.
****
I did an interesting batch last weekend--a kolsch using WLP029. It's a variation of a recipe my son did--he was home, brought a growler of that kolsch he'd brewed. Excellent beer. I had seconds.
So we decided to brew a variation while using most of his methods. He did a rest at 125 degrees which, because he has a grainfather, he can do. Give a kid a hammer and the whole world becomes a nail....
Anyway, he ramped up from there to 149, so I did something similar w/ my RIMS system--started at 132 (underestimated how much the strike water would cool, first time doing this particular approach), held it there for 10 minutes, then ramped it up to 149.
Adjusted the recipe slightly as I was doing LODO techniques--went from my son's 12# of pils malt to 6# pils, 6# 2-row, 12 oz of white wheat, and 12 oz of munich.
Brew day went off without a hitch, pumped the wort into the fermenter, then oxygenated. And then did what Chris White said he'd do (when we were at his yeast workshop in March) which is to just pitch the yeast directly into the fermenter, no starter.
Well...it went against my best instincts so I did it. That's what my son did, trying to approximate his methods. Let it sit at 70 degrees for 8 hours than ramped down to 60 for fermentation. It took a day for there to be evidence of bubbling, and then it occurred to me why I might not want to do that.
****
Why might I not want to just pitch with no starter? One reason I like starters to take off like gangbusters when pitched is to get them going so as to outcompete any nasties that might be in there. The other reason, which I hadn't considered, is that I spent all this effort on using LODO techniques to create the wort, only OXYGENATE the wort and have it sit for 24 hours before any sign of activity. During which time, I'm sure I was oxidizing the wort. Now I know during some of that time the yeast is taking up that oxygen during the lag phase, but still....
[This is actually also an argument for not oxygenating at all and pitching dry yeast.....I have to think on that one.]
Despite that, I pitched on Saturday at 4pm; no evidence of activity until Sunday 4pm. It started slow, then moved ahead with...alacrity, I suppose. This morning, Thursday, at 6:30am, TILT tells me gravity has fallen to 1.010. I bumped up the temp to 69 yesterday afternoon and evening once gravity hit 1.020, and sealed up the fermenter to allow it to self-carbonate.
We'll see how it turns out. I'm sure I muted some of the LODO effect but then again, my son didn't do any of that stuff.
*******
One reason I pitch at a higher temp is that, according to White, optimal temperature during propagation (when yeast multiplies) is 5-10 degrees higher than during fermentation (when yeast are making alcohol). So I pitched the above yeast at 70, let it sit there for 8 hours, then started to drop the temp.
*******
From my notes from the workshop: Chris said to let starters go 24 hours or even 48 hours. He said you want the yeast to starve a bit to build up a reserve. I'm aiming for 18 hours, which isn't what he said. And yet, fermentation takes off when I do it that way, and the beer tastes great (not just me). So....jury still out on this, afaic. Still learning, I am.
*******
White said when he pitched starters, he'd pitch the whole thing, as I do. I'm not sure the reasoning behind that if he let them go up to 48 hours and the yeast had gone dormant at that point, starving a bit, except maybe he eliminated the extra step many go through of crashing and decanting.
******
He also said that overpitching is really only a problem with repitching entire yeast cakes.
******
Effect on flavor byproducts: if you add more yeast, they grow less, and thus make fewer flavor compounds.
******
I don't claim to be a yeast expert. In fact, the more I learn about this, the less I feel I "know." I seem to have stumbled onto an approach that works well, but I'm not saying it's the only approach that can work well.
