How Yeast Affects Flavor

For centuries, fermentation was an unknowable force that magically turned a soupy concoction of water and grain into a bubbling, intoxicating beverage. Grain selection became more refined and hops found their way into the process, helping to stave off spoilage and certain illness. All the while, the transformation mechanism remained a mystery — was it the magical mash paddle that gave us this gift? Or the kettle, or even the vats? Eventually, we were able to sort out that there was indeed a specific organism responsible for creating alcohol, and from there we sought to domesticate it. What we have now is brewing yeast: a single-celled beast that we choose for its ability to make the type of beer we want to make. 

When setting out to make a beer, brewers have to build a recipe that makes use of certain ingredients to get the desired flavors. A beer — let’s say a blonde ale — with bready malt flavor and bright, citrusy hops will likely incorporate pale malts and American hops to get that result. But when it comes to yeast’s contributions, brewers have a wide range of options, from little flavor contribution to a yeast-dominant flavor profile; we might opt for a lightly fruit-forward yeast in that blonde ale, rather than one that gives off strong phenolics. 

In order to choose a strain, brewers need to understand what flavors yeast can potentially contribute, like esters, phenols and sulfur compounds, to start. 

Esters are a group of compounds formed during fermentation that often resemble fruity or even floral aromas. Yeast combines certain alcohols that it produces with organic acids present in the wort. Specific combinations determine the flavors created as a result. Put simply, to create an ester-driven beer, choose the right strain that will produce esters during fermentation…or manipulate processes to enhance or reduce the presence of esters in the finished product through pitch rate, oxygen levels, and temperature control throughout fermentation. 

Under-pitching yeast, fermenting at warm temperatures, and under-oxygenating wort are a few tricks that some brewers rely on to get beers with strong ester profiles. However, these tricks don’t necessarily work the same from strain to strain. In the case of a German Hefeweizen strain, under-pitching can often encourage ester production while raising temperature doesn’t to the same degree. We’ve also seen in our trials that under-pitching Belgian strains doesn’t necessarily result in a successful ester boost, but fermenting at higher temperatures can. This is where experimentation comes into play: not only can brewers drive ester production through strain choice, but also by manipulating ester synthesis pathways — adjusting processes to see which approach tunes the ester dial to the right level. 

Esters are present in every beer, even those that would seem to have none at all on a sensory level. While hundreds of known esters exist, only a handful are known to be impactful and perceptible in beer. We can measure ester levels using gas chromatography, but for most brewers (and consumers) the sensory impact is what matters most. The difference between some beer styles comes down to the yeast strain selected and what esters it will produce. For example, while both witbiers and English ales will exhibit a generally estery profile, we wouldn’t expect to see a strong presence of isoamyl acetate in a classic, malt-forward bitter. 

Phenols are a bit more complicated, as their origins aren’t necessarily limited to yeast production (malt and chemicals in water can also contribute phenolics). For our purposes, we’ll focus primarily on phenols that yeast can lend to beer. 

The phenolic compound most beer fans are familiar with is 4‑vinyl-guaiacol (4VG), the pleasant, clove-like aroma typically found in Hefeweizens and many Belgian ales. 4VG is a result of a yeast enzyme decarboxylating, or removing a CO₂ molecule from ferulic acid present in the wort, either from malt or hops. POF+ Saccharomyces cerevisiae strains are capable of (and limited to) this transformation, and if a brewer has selected a POF- yeast strain, the presence of 4VG is usually a good indicator that some type of wild yeast has contaminated the batch. 

Speaking of contamination, Brettanomyces is often considered a contaminant in the brewhouse, and the presence of 4‑ethylphenol (4EP), a plastic, bandage-like aroma or 4‑ethylguaiacol (4EG), a smoky, burnt aroma, are often a sure sign that Brett is present. 4‑vinylphenol (4VP) is another flavor that Brett can contribute, which can sometimes lend a barnyard-like or medicinal character. 

Like all flavors, there’s a time and place where these compounds are welcome in varying degrees of intensity, like in a complex lambic or gueuze, but in most beer styles Brett-derived phenols should not be present. Mixed fermentations are a popular way to experiment with phenolics in various combinations with each other, while a straightforward fermentation like a Hefeweizen would bring about a simple 4VG clove note. 

Generally, all yeast strains produce sulfur during fermentation, but sulfur is a pretty volatile compound that tends to be driven off along with CO₂ production. Lager fermentations tend to retain more sulfur due to the generally slower movement of CO₂ in decreased tank temperatures, but they generally shouldn’t be off-putting to the consumer. Typically a healthy, vigorous fermentation is the best way to prevent the less pleasant sulfur flavors from making their way into the finished beer. However, some sulfur compounds can be pretty nice, and even desirable for a full flavor profile — consider thiols, or even a slight whiff of sulfur off of a particularly fresh German lager. 

Sulfur dioxide (SO₂) is a natural product of fermentation that can help fight oxidation in the beer, but at high levels it can be reminiscent of a freshly struck match. SO₂ reduces to another sulfur compound called hydrogen sulfide (H₂S), which gives off an aroma of rotten eggs. Some sulfur compounds can be produced during normal fermentation, but they can also crop up in later fermentation if the yeast are nutrient deprived. Even though both of these compounds will occur during virtually every beer fermentation, healthy yeast will carry it out of the beer along with CO₂ production. Another good strategy to avoid excess sulfur buildup is to ferment a little warmer and include a bit more nutrient to ensure happy fermentation. 

On a more pleasant note, thiols are one of our favorite classes of sulfur compounds because of their tropical fruit-like aromas. 3‑sulfanyl-1-hexanol (3SH or 3MH) is one of the most impactful thiols, with its intense grapefruit and passion fruit aromas. Omega Yeast’s extensive research into thiols found that traditional yeast do not activate the gene required for unlocking these flavors in beer fermentations. You can apply a similar approach to these fruity, inviting sulfur compounds as you would with the less-desirable sulfurs: lower fermentation temperatures will help trap thiols in your beer and let them shine in the final product, while a bit higher of a temp would encourage them to volatilize and reduce the overall thiol expression. Don’t neglect your hop routines, either — many citrusy and fruit-forward hops complement thiol aromas beautifully, creating a complex mix of tropical flavors in your finished beer. 

There’s a lot of focus on ​“off flavors” in beer, but we like to look at yeast contributions as a positive. Take diacetyl, for example: the buttery slickness diacetyl brings is considered to be an off flavor in most situations, but it plays an important role when discussing some classic styles, like a Czech pilsner or English ESB. We recognize these styles with diacetyl as an attribute appropriate to the tradition. The yeast strains used to create more ​“traditional” versions of these styles tend to produce more diacetyl — a different strain would not yield the same result. 

Hefeweizen is a classic that always brings its characteristic isoamyl acetate and 4VG, the banana-clove aroma combination that is a hallmark of the style, derived entirely from fermentation. Hefeweizens are brewed worldwide, and brewers will typically opt for a strain that will give them those components if they want to communicate to the consumer that this beer will provide a classic German wheat beer experience. 

Tradition is one thing, but in modern times we see a trend of brewers opting for yeast that doesn’t necessarily contribute much flavor when making west coast IPAs. In fact, it’s the absence of yeast-derived flavor that makes it attractive— pilsners are often revered for their prominent hop aroma and bitterness, so brewers have taken these lessons and applied them to brewing hop-forward IPAs with a dry finish and little flavor contribution from the yeast so as to let their prized hops shine. Some call it IPL or maybe cold IPA while others have been quietly undertaking this practice for years while consumers have been none the wiser. 

Hazy IPA is another modern addition to the stylebook that has been shaped by the yeast strain most often selected for it. Something like Omega Yeast’s British V strain is a popular choice not just because of its haze stability, but also because of its specific ester profile that complements the strong hop flavors typical of the style. With the advent of strain modification, there may be a future where any strain can contribute haze (or not!), and the doors to experimentation will be blown wide open. As a brewer, you can use your knowledge of classically yeast-driven styles to guide your recipe design. Experiment and see how the style can influence the yeast you choose, and vice-versa. 

As discussed above, some ​“off flavors” like diacetyl come along with the good flavors and provide nuance in certain beer styles. Other flavors we may typically consider to be undesirable could be pretty well tolerated by consumers — there are numerous examples of major beer brands out there with flavor profiles that have come to be defined by ​“off flavors.” Even desirable flavors can be considered ​“off” when experienced out of context or in high concentrations.