Possibly, we observe the yeast begin to react differently over generations because we harvest from the bottom of the fermenter. (At least I have been) We harvest the yeast that has ‘learned’ to do its job then flocculate out. The yeast that doesn’t flocculate ends up in a secondary, keg, or a bottle and in my case is discarded. So, we keep harvesting the yeast that will begin to produce clearer beer because of how we harvest it. …or maybe the Beantown Lager has me reflecting a bit too much in the quiet hours…
I, at least, haven’t experienced such a trend, given consistent practices. I’m inclined to think that after a couple of generations, the yeast has adapted to perform optimally under the conditions we provide it in our breweries rather than the laboratory environment, and we have selected the cells that are so optimized, eliminating outliers, and from that point it will be quite stable and consistent. Thereafter a change in any parameter, including attenuation and flocculation as well as flavor and such, is a sign that it’s no longer behaving normally, and it’s time to replace it with a fresh culture. The first generation is where you’re going to see the most “abnormal” performance for the strain (including in the flavors produced,) and this shouldn’t be taken as a baseline. That’s why when big breweries have to introduce a new pitch, the first few batches get blended off in small portions into batches fermented with established yeast at packaging. The healthiest, most successful cells, I expect, are going to “win the race,” eating all they can and getting set for the next round. Relative underattenuation, lagginess, and the like would seem to me not to be adaptive.
I’ve always heard yeast behaves better after two or three generations. Possibly the yeast harvested from the flocculated yeast does its job and flocculates out of suspension. The non flocculant yeast has been eliminated in the first secondary/keg/bottle so what remains is a yeast that continues to create clear beer. Who knows. I may be full of beans. LOL
When harvesting from a conical, I target the middle of the slurry to exclude exceptionally high- and low- flocculating cells. When scooping out of a bucket, I figure I pretty much get all three, and I suspect that’s at least part of why I start to see fermentation performance drop off sooner when repitching at home. In general, I’ve always found first-generation yeast at any scale to be a little slower and sometimes less attenuative. W206 in particular seems to have a “long tail” relative to other lager strains in my experience.
You can somewhat target that preferred portion of the crop when harvesting at home from bucket, carboy or corny, by swirling up the slurry in the fermenter, letting it settle for a few minutes, and pouring off to your storage vessel some of the thinner part of the slurry from under the frothy top, leaving behind the heavier stuff that has settled out. This at least helps to separate out the lighter proteins and dead cells, and the heavier trub, so it has some benefits anyway, even if it doesn’t nearly as effectively select the medium floccing yeast as does diverting the middle portion of your dump from the cone. This method has regularly gotten me 8-15 generations of lager yeast without any dropoff in performance, at which point I tend to buy a new culture merely out of prudence. (And because I miss playing with my stir plate from time to time. ;D )