I read an interesting article a few months ago, suggesting a brewer could produce an exceptionally complex beer by blending different stains of yeast, with each strain of yeast adding its own nuance to the beer. Think of it like baking bread: you have flour, water, salt, and yeast. For brewing beer, you have grain, water, hops, and yeast. For bread we primarily only think of two yeasts, regular dry or sourdough.
With beer there are dozens of yeast varieties. Some work better at cold temps, some at warm, some make the beer fruity, some make the beer sour. I've done a little bit of yeast blending in the past, such as starting a beer with a flavor-producing yeast, then using a neutral, alcohol-tolerant yeast to eat up the rest of the sugars and dry the beer out. But this article suggested keeping the grain and hops dead simple, but mixing in 3-5 different strains of yeast to produce a fruity, complex beer similar to the Trappist Monks.
So I decided to try it. I brewed a high gravity ale and mixed in a commercial abbey yeast from W'yeast, the recultured yeast from a Chimay White and a Rince Cochon bottle, plus Matt gave me a vial of his Rochefort yeast from an earlier homebrew. Now they're all swirling together, fermenting away in a carboy in a dark corner of the house. I'm excited for the results, but I've also decided this is pretty much a crap shoot. With hops or spices, I know how much I'm added to the beer, but in the dog-eat-dog world of microbial yeast, I have no idea if that W'yeast has totally dominated the beer and pushed out the different yeast. Or perhaps the Chimay White was a fresher beer with more viable yeast that's now in control of my homebrew. With the four yeasts I added, I doubt each one with evenly ferment 25% of the available malt sugars. Honestly, this beer is in the yeast's hands now.