Appreciate the explanation. If we only have 6x to work with, aren't we just taking the first 1 or 2 multiplications in the starter? Would a yeast created from a starter end up with a higher end count of yeast? Why?
I also don't understand why yeast would be less stressed when coming from a starter. It's basically the same environment (wort), does the aeration of a stirplate help them develop that quickly to this environment?
I'm really not trying to be obtuse, and I apologize if it's coming across this way, I'm just trying to better understand why I bother making starters.
There's a number of factors into how much growth is obtained in a starter. Wort gravity, starter size, aeration, and initial pitch are the big ones. Generally less cells pitched into a larger, well aerated, higher gravity starter will equate to more. However, there's a tradeoff with starter gravity, as higher gravity equates to more stress on the yeast and less yeast health overall, but lower gravity less growth but easier on the yeast. Generally accepted is 1.030-1.040 for yeast propagation under normal circumstances, with higher gravity never being recommended, but lower gravity (down to 1.015 or so) useful for highly fragile yeast (say if you're culturing from a bottle of European beer that's old and been through who knows how much shipping abuse).
So generally if you want more cells, you increase the starer size and/or reduce the initial number of cells. Up to the point of that 6x multiplication factor- above that all you're doing is hurting your yeast.
As far as why yeast get stressed, imagine having a massive amount of work to do, and it's only you to do it. Now imagine you have the same workload, but someone else to help. Very much an analogy, but you get the idea. Point is, there's a difference with the pitching rates in a starter versus the pitching rates in a full batch, with the way the yeast behave. In a starter, the comparative pitching rate is much higher, and you're not concerned with the flavor of the starter, just the growth and health of the yeast.
I
highly recomment picking up the book "Yeast: The Practical Guide to Fermentation" by Chris White (of White Labs) and Jamil Zainasheff (of Brew Strong, the Mr. Malty yeast pitching rate calculator, a pro brewer, and one of the most award winning homebrewers of all time). It will answer all these questions better than I ever could.
Actually I would disagree that underpitching results in an incomplete fermentation. There's no reason it should. All pitch rates will eventually reach the same saturation level of cells/mL and the only difference should be the flavors created along the way. Longer time = more flavors and visa versa. An underpitched beer WILL stress the yeast, so if it's grossly underpitched it could result in the yeast simply dying. But underpitching by say 20%-50% should result in an equally complete fermentation as any other pitch rate.
http://sciencebrewer.com/2012/03/02/pitching-rate-experiment-part-deux-results/
This experiment seems to back this up, though I realize that it's only 1 data point.
As far as this, I routinely underpitch (slightly) with Belgian strains to bring out the esters, and with yeast-forward beers overpitching is known to be detrimental. I'd be curious to see the same experiment done with a lager, better yet a high gravity lager like a Doppelbock, and I would expect very much different results.
