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Rogue and Papazian have their own yeast strains (PacMan and Cry Havoc, respectively). Do you know how they got there?
Lots of trial and error? Pitching multiple strains?
Let's say I pitch a bunch of different strains into a batch and stumble on something awesome. I'm still bound by not repitching past the 6-8 generation, and the core culture only lasts 2-3 months.
If I discover the next amazing strain through experimentation, what do I do then?
Yeast mutations are a product of environment. You can apply selective pressure yourself to encourage particular traits, or you can simply re-pitch multiple generations in a very clean environment while varying the fermentation schedule to effect a change. To then store the yeast long term, you would isolate a single colony from a culture plate, grow it up and verify it performs as you like. Then you can store that culture long term on slants or plates, and continually refresh it over the years.
Simplified example: I start with a commercial strain from one of the "banks" (White Labs, Wyeast, ... ). Let's say it's already a very flocculent strain and I want to maintain that, and I want to mutate it to create a more completely fermented beer. I want to select for the strain with the most "staying power" that is still going to floc well. So, I brew a beer, and I harvest yeast only from the very top of the yeast cake in the fermenter, after the beer has started to clear but still close to the end of fermentation. This should represent the yeast that stayed in solution the longest (still fermenting), while also retaining it's flocculent characteristics (it flocculated, and fell to the cake rather than staying in solution). I then isolate a single colony, and repeat.
If I'm lucky, I can thus develop a strain that does even more of what I like, while retaining the flavor profile of the original. If I'm not so lucky, I get the characteristics I was hoping for, but also get some unwanted tag-alongs like off flavors etc.
There are many other variables to consider, but this is the gist of it.
Classically, new strains were developed "by accident". A brewery would repeat the same process over and over again, while maintaining it's own cultures. Over years, that brewery's strain would become uniquely suited to that environment. This is still happening today, although more and more breweries now re-pitch only a few times, and then start over with fresh yeast from a bank to avoid the cost of maintaining their own cultures. In those environments, any mutations are discarded after a few generations.
I have my own yeast also (WY1450 Denny's Favorite) and it came about the same way as Pacman or Cry Havoc. We all found a yeast we liked and kept it banked for a number of years, then released it to a company to market it. There is not necessarily anything different about any of these yeasts than was originally there. None of us combined strains or tried to mutate a strain into something else.
It's mainly time (many many many generations) and environment. You could theoretically isolate yeast from the fermenter that have different properties like flocculation, attenuation, flavor profile etc. and then continue to focus on the desired traits until they behave the way you want.
Yeast reproduce asexually under normal fermentation so pitching 2 strains won't create a 3rd new one.
I recall reading that Cry Havoc was cultured up from a Budweiser keg. Maybe if you keep using the same yeast for 20 years one of the labs will put out a special edition for you :)
The reason why people say that you shouldnt reuse a purchased yeast strain past a few generations is because it can mutate into a new strain which no longer gives you the same properties as the one you wanted. So if you want to create a new strain, keep reusing it.
Better still, split a strain over a number of fermentors, keep them isolated from each other for a number of generations. Then pick the one you like best, split this up and repeat.
That's evolution by selection, the thing that Christians don't believe in.
From my very limited understanding on yeast, I believe you create your own new strain. While you may only repitch the same yeast 6-8 generations, the initial starter can be grown through doubling techniques. I need to learn more about how this works, but I believe if you were to collect some yeast from, say, your 4th generation of repitching, and expand this, and pitch it into a new batch, and repitch a few times, and harvest, etc.... Then you'll have your own unique yeast.
Or, I could be a bit off.
