One of my biggest surprises as a homebrew beginner was learning just how much influence yeast exerts on the flavor and aroma of beer. Malt, hops, and water are clearly products of the land, but yeast is just a microbe. But, as so often happens, big things come in small packages, and it turns out that often yeast is the defining feature of a brewery’s signature beer.
While commercial yeast labs such as Wyeast, White Labs, GigaYeast, East Coast Yeast, and The Yeast Bay offer more microorganisms than you can shake a stick at, certain strains simply aren’t available commercially. That’s why some homebrewers get a kick out of building up a yeast population from the dregs at the bottom of a bottle-conditioned commercial beer.
Unpasteurized, bottle-conditioned commercial beers may or may not contain the primary yeast strain used for fermentation. Many examples are filtered or centrifuged before packaging and then re-dosed with a neutral ale, lager, or wine strain for packaging. This helps ensure consistent carbonation and can protect a brewery’s proprietary culture. Other brewers, however, are content to ship beer that includes a full complement of their own brewing microbes.