Today most American craft breweries use a single strain of pure-culture yeast to ferment each individual batch of beer, usually acquired through one of the main yeast suppliers (Wyeast Laboratories, White Labs, The Brewing Science Institute, and a small handful of others). They’ll ferment with it and typically reuse it anywhere from a few times to a few dozen times, depending upon their conditions and brewing schedule, careful to harvest it from each fermenting batch in a way that limits the selective pressure they exert on these ever-evolving colonies. When their yeast population’s fermentation characteristics start to diverge from the norm, and unless they have appropriate microbiological facilities on-site, they’ll simply order more.
One of the more illustrative examples of this tendency to evolve comes from Stan Hieronymus’ Brew Like a Monk. In it he writes, “Left on their own, yeast strains change over time. So, while Wyeast may have kept its 1214 [yeast strain] much the same in the twenty years since it was taken from Chimay, Chimay’s itself likely changed.”
That’s for one strain of yeast. Multiple strains will be slightly more complicated. And when one includes bacteria and wild yeasts in the mix (the latter often mutate more rapidly than cultivated brewing yeasts – see sidebar), it gets even harder to maintain those same original characteristics and proportions between batches.
One glance at the fermentation dynamics of authentic Belgian lambic (Raj B. Apte and others’ work on the subject can be found in Jeff Sparrow’s Wild Brews) provides the general gist of how intricate these relationships can be: cresting waves of ascendance and departure developing over many months, as yeasts and bacteria progress through the smorgasbord of available sugars.
Yeast’s ability to evolve over relatively short periods of time, on a human scale at least, can also prove to be rather beneficial. Over the years, breweries have managed (consciously or less so) to select for the yeasty attributes that work best for beer brewing, including favorable flavor and aroma characteristics, tolerance to alcohol, and the ability to fully complete fermentation cycles (often reabsorbing less-than-delicious byproducts in the process).
Raphael Rodrigues, a beer writer based in Brazil, recently reported an example of how some brewers are taking yeast’s adaptive capabilities one step further. Finding inspiration in a lecture by professor Rogelio Brandão (whose research focuses on fermentation studies), Felipe Viegas and pharmacist José Eduardo Marino decided to do an experiment with a strain of cachaça yeast. (Cachaça, Brazil’s most popular spirit, is made from fermented sugarcane.) After introducing the cachaça strain to a mixed solution of sugarcane and malt sugars, successive generations were fed progressively lower and lower concentrations of the sugarcane parts. Eventually, they were able to selectively evolve a population of yeast that would chow down on malt sugars. The result? A 5.3% wheat beer called Carolweiss, fermented entirely from what was once cachaça yeast.
Apparently, rum yeast is slated to be studied next.