As a brewer of mixed fermentation beers, you understand the importance of getting your favorite mix of yeasts and bacteria into your creations. You want diversity, but only the diverse group that you had hand selected to do the job. Once that beer is complete, you probably want to use that same culture of amazing funk and/or sour for future beers, as well. All you need to do is harvest those dregs, and off you go with your prized mix for the next project.
Unfortunately it doesn’t work exactly like that.
You can use a single yeast strain for multiple generations and, slight mutations aside, it will just get better with the first few ferments. But for mixed fermentations, you have often have saccharomyces and brettanomyces competing in some fashion for sugars, as well as lactobacillus and pediococcus bacterias doing the same. It is survival of the fittest until the bodies/cells hit the floor. Many of the players in your vessel are making contributions, but there’s only one or two MVPs being selected to represent after the game.
When he streaks out cultures from finished beers, there are usually only a couple of winners from the yeast and bacteria genera. In addition, most “wild” mixed cultures end up being primarily yeast because of how and when they are harvested and a smaller percentage of cultures that ending up having viable bacteria if there is yeast present. (**What’s important to remember here is that this is my recollection of what he said. If what I’ve typed here is wrong, it is my error not his.**)
That isn’t to say that all of those yeasts and bacteria didn’t have a hand in the final beer, but it is a long shot that you will get the same results with those dregs in your next beer.
Does that mean commercial mixes of twenty brett strains in one vial is just a marketing gimmick? In my opinion, yes. But I have used those cultures with great success, so it is hard to dissuade you from using them, and that is certainly not my goal here. Having said that, if you know only one or two strains will shine through, it seems more logical, albeit maybe less economical, to choose those strains yourself rather than hope they emerge from the hunger games of pitching all the bretts.
The diversity of yeast and bacteria that you add to a beer is a continual endeavor. Your mixed fermentation beer is a living thing, and the cultures that made the beer what it is are not necessarily the ones left to harvest and repitch.