Folks in the beer industry like to say that brewers don't really make beer. Brewers make wort—which is the stuff that yeast makes into beer. Folks in the brewery can do a whole lot to ensure the yeast does what they want through temperature control and careful selection of isolated strains of yeast, but in the end, they bow to the whims of the billions of tiny organisms they seek to feed with delicious, delicious grain-derived sugar. Yeast and its performance has a huge impact on a brewer's final product. But what does that taste like?
The most used yeast in craft brewing is Saccharomyces cerevisiae, which is used in the beer world to produce ales. Unlike its clean lager-producing cousin Saccharomyces pastorianus, cerevisiae is known for yielding beers with easily-discernable character—most clearly in the form of esters, phenols, and alcohols.
Esters provide a huge portion of yeast-derived beer flavor. They tend to come off as fruity in flavor, but each ester tastes a little different. There's Isoamyl acetate, which tastes like banana Runts. There's ethyl acetate, which tastes like nail polish remover. There's ethyl caprylate and caproate, which taste like apples, pears, or anise.
Generally speaking, ester production is directly related to fermentation, and warmer fermentation temps increase their presence. There's many other factors at play here, especially the strain of yeast being used, but the lower fermentation temperature is why you don't encounter esters as overtly in lagers as you do in ales.
Phenols, which also occur in every beer, are produced not just from yeast, but also from other beer ingredients or chemicals like chlorine found in brewing water. (For example, tannins are a type of phenol present in both hops and malt). But we're talking about yeast here, and there are a couple well-known and easily-identifiable yeast-derived phenols that you'll encounter in your pint.
If you've ever had a real hefeweizen, you've tasted 4-vinyl guaiacol. Most commonly compared to a clove-like spiciness, it is produced by yeast's interaction with ferulic acid, which is found in barley.
You may have also tasted 4-ethyl phenol—produced by the wild yeast Brettanomyces. This is the phenol people are talking about when they refer to "horse blanket" or "barnyard" character in a beer.
Aside from the above, yeast is most famous for producing CO2 and ethanol. While bubbles are great and all, it's the ethanol that gets most people going (and going in a swervy, uninhibited manner). Detectable at thresholds dependent on other characteristics of the beer, (sweetness and bitterness, for example) ethanol imparts a warming, drying, or astringent effect on the palate.
Other alcohols can form during beer fermentation as well—fusel (German for "hooch" or "bad booze") alcohols are easily generated by beer yeast, especially at elevated fermentation temperatures and with higher-ABV beers that may stress yeast. These can have a harsh effect on the palate, but they are also responsible for some pleasant floral and wine-like aromatics.
So, yeast produces a ton of flavors (the specific esters, phenols and alcohols listed are just the most commonly discussed. There are many, many more out there!)—how can you identify them? Thankfully, there are established expectations for yeast character in most beer styles that will point you in the right direction. Here's what to look for.
American-style beers tend to express minimal yeast character, instead highlighting the flavors and aromas associated malt and hops. The typical American ale yeast strain is very "clean," producing few esters or phenols. As with all yeasts, fusel alcohols will be produced if yeast is worked too hard or allowed to ferment at high temperatures. Grab a classic American Pale Ale like Sierra Nevada Pale Ale or Stone Pale Ale to use as a control—the flavors in these beers can mostly be attributed to hops and malt.
Generally speaking, English ale yeasts tend to express more estery character than their American cousins. They also can ferment a bit less thoroughly than American ale yeast, leaving a bit more sweetness that will emphasize the malty flavors in beer. Find yourself a Fuller's ESB, or better yet, a locally-brewed cask bitter, and look for apple, pear, or generic "fruity" esters, as well as low concentrations of diacetyl, a flavor compound that tastes a bit like butter.
Belgian ales are where yeast character really gets to shine. Traditional yeast strains produce very high levels of esters, phenols, and fusel alcohols, and leave the beer very dry, accentuating these flavors. Look for complex spice and fruit character all over these beers. Grab a yeasty tripel like Westmalle's example, a wit like St. Bernardus, a dry Belgian strong pale ale like Duvel, and a rustic saison like the classic Saison Dupont to get a full feel of what Belgian ale yeast strains can do.
German wheat beers are another great way to taste what yeast can do. Packed with isoamyl acetate and 4-vinyl guaiacol, any traditional German wheat beer will showcase banana and clove-like ester and phenol flavors in bounds. Grab a Weihenstephaner Hefeweissbier for a particularly solid example that showcases both flavors in a big way.
Interested in what wild yeasts and bacteria will do to beers like Flanders reds or lambics? Stay tuned...