In this series, we’ve explored hops and malt. Now it’s time to take a closer look at yeast. As a homebrewer, you’re probably already familiar with the two main families of brewing yeasts, knowing that members of Saccharomyces cerevisiae (ale yeast) prefer warmer temperatures and tend to favor ester production, while members of Saccharomyces pastorianus (lager yeast) tolerate cooler fermentation temperatures and are more likely to produce sulfur compounds such as dimethyl sulfide (DMS). You’ve also faced the plethora of choices at your local homebrew shop.
At a finer grain of detail, yeast strains are often characterized based on their attenuation, flocculation, and alcohol tolerance. Attenuation (or “apparent attenuation”) refers to the percentage of wort sugars that the strain can process. Sweet wort is made up of many different kinds of sugar, and some yeasts have trouble with the more complex ones. That affects the final gravity and residual sweetness in your beer.
Flocculation is how well and how quickly the yeast cells settle out of the beer. Strains with high flocculation are likely less attenuative and may yield beers with some level of buttery/butterscotch flavors when the yeast settles out without driving off the naturally produced diacetyl.