I have a T-shirt from 7venth Sun Brewing that reads, Brettanomyces & Lactobacillus & Pediococcus & Saccharomyces, and every time I wear it, I get puzzled looks and questions from strangers about what these strange words mean. The fact is, many people consume some or all of these items regularly, as they comprise one of the four main building blocks of beer.
Most people know that hops add bitterness and aroma to beers, while malted barley adds body, color, and flavor. And of course, beer is mostly water. But yeast — the most important part of the equation (it's what turns sweet liquid into beer) — is also the least understood.
To put it crudely, yeast eats sugar and poops out alcohol and carbon dioxide. It sounds gross, but centuries of rigorous field testing has proven that the result is actually delicious. In addition to making the magic of fermentation happen, yeast also contributes a variety of flavors to the beer.
I present you with this crash course on beer yeast, which should take you from complete novice to talking yeast like a microbiologist in just a few short paragraphs.
Saccharomyces: Traditional brewer's yeast. It exists in two varieties: top-fermenting (it creates a thick foam at the top of fermenting beer known as krausen) and bottom-fermenting (most of the activity occurs at the bottom of the fermenting vessel). This is what makes ales (top-fermenting) and lagers (bottom-fermenting) different. Saccharomyces comes in a variety of strains, and different strains contribute different flavors to the beer. An American lager strain might be relatively neutral, while a German wheat beer strain might produce a lot of esters — compounds that provide beers with fruity flavors and aromas, such as the banana/clove notes found in hefeweizens.
Try: Saccharomyces is the basic yeast used to ferment nearly all beers. All of the traditional styles, from pale ale and pilsner, to stout and barleywine, are fermented with saccharomyces. Try a Belgian Trappist ale like Westmalle for an example of a fruit, estery strain; or try a clean lager like Pilsner urquell to see what a very neutral strain tastes (or doesn't taste) like.
Brettanomyces: The classic "wild yeast," known as such due to its ubiquitous presence outdoors, as well as its tendency to sneak into saccharomyces-fermented beers and produce undesirable off-flavors — strange esters and phenolic flavors, which can make a beer smell or taste medicinal. However, some styles, such as lambic and some saisons, rely on this yeast to produce complex flavors described with colorful terms like "barnyard" and "horse blanket." These occur because brettanomyces can eat sugars that saccharomyces cannot, producing unusual flavors in the process.
Try: Brettanomyces has become a popular yeast of choice with American brewers. Orval is a classic Belgian ale that is fermented with saccharomyces and re-fermented in the bottle with brettanomyces; Green Flash's Rayon Vert is an excellent domestic take on this style. Some breweries, like Alaska's Anchorage Brewing Company and Colorado's Crooked Stave Artisan Ales, use brettanomyces in all of their beers.
Lactobacillus and Pediococcus: These two are not yeasts at all, but rather wild bacteria found in lambic and other wild ales. Like brettanomyces, these are considered contaminants in most beers, but if you've ever enjoyed a sour, funky gueuze, Flanders red ale, or oud bruin, you have lactobacillus and pediococcus to thank, along with another bacteria called acetobacter that is occasionally present in these beers. While saccharomyces produces alcohol, these produce lactic acid, acetic acid, and other compounds that make beer sour, musty, and even vinegary.
Try: Lindemans Cuvée Rene is a traditional gueuze (blended, carbonated lambic) that contains saccharomyces, brettanomyces, lactobacillus and pediococcus. Duchesse de Bourgogne and Rodenbach hail from a nearby region of Belgium and contain all of the above, as well as acetobacter, which gives them notes of sweet balsamic vinegar.