Yeast Derived Beer Flavor
Have you ever found yourself drinking a delicious cold one and wondered where all the flavors come from? The variety of flavors you smell and taste from beer are largely a result of the particular yeast used for that brew. While it’s true that the hops and grist play a huge role, many of the flavors you’re tasting are the result of fermentation byproducts the yeast produces. We are going to discuss three major byproducts yeast produces during fermentation that have a massive impact on beer flavor. These byproducts are esters, alcohols (not just ethanol), phenols, and vicinal diketones (VDKs). The relative amounts of each of these depends on the strain of yeast used, wort chemistry, and fermentation conditions such as temperature and dissolved oxygen.
What are esters?
Esters are formed as a byproduct of organic acids and alcohols during fermentation. They are responsible for a huge variety of the different flavors in beer, ranging from fruitiness to solvent-like. Depending on the style of beer, ester production can be favorable or unfavorable for the flavor profile of the beer. In general, esters are more complementary in ales and are often seen as undesirable in lagers. However, under similar fermentation conditions, most ale yeasts produce fewer esters than lager yeast. The lack of ester production in lagers is primarily a function of a lower fermentation temperature. Whereas, most ales ferment at temperatures more conducive to ester production.
While each yeast strain carries its own ester profile, other factors such as aeration, wort composition, pitch rate, temperature, and suspended solids (trub) play a factor in the concentrations of esters in the final product. The most impactful esters are ethyl acetate (solvent, slight fruitiness), isoamyl acetate (very fruity, banana candy), ethyl caproate (fruity, apple, slight anise), ethyl caprylate (fruity, winy), ethyl caprate (fruit, winy), and phenyl ethyl acetate (rose, honey). For example, the ester isoamyl acetate is one of the predominant esters Weizen strains such as BSI-300 produce and are responsible for the classic banana flavor you get from Bavarian wheat beers. Other good examples of yeast that produce ester forward beers are Belgians and Saisons. For example, BSI-500 (Trappist Ale 1) Belgian yeast has a distinct fruity flavor associated with it. Like wise, S-11 French Saison yeast produces clean citrus esters, in addition to peppery and spicy phenols. On the opposite end, we have strains like BSI-8 (American Ale 2), which is more catered to a clean, low ester style ale.
What are Phenols?
Starting as an aromatic ring of carbon molecules with a hydroxyl group attached to it, phenolic compounds are generated in many ways including from yeast during fermentation. Hundreds of phenolic compounds have been found to exist in beer. Phenols are chemically derived from alcohol. Like esters, the yeast strain and brewing conditions drastically affects the production of the phenols. If you ferment at too high of temperatures, you will likely generate higher quantities of fusel alcohols, which may create an obvious off flavor. Some describe them as being kerosene-like. A well-known desirable phenol in beer production is 4-Vinyl Guaiacol (4VG). BSI has a long list of yeast strains that can produce 4VG, under the right conditions, including Saisons, Belgian Trappist ales, and German style wheat ales. Similarly wild yeast strains which are becoming more popular by the day also produce these flavors. BSI’s Drei, Brettanomyces bruxellensis, Brettanomyces clausenii, and Brettanomyces lambicus are a few we carry that are capable of producing these compounds. Phenols can also affect the mouthfeel of a beer. For example, the polyphenols/tannins can create a drying or astringent sensation. The mouthfeel caused by the alcohols in beer is often described as having varying degrees of spiciness or of warmth and astringency.
What are VDKs?
Another potential driving force for the overall beer flavor are the compounds known as Vicinal Diketones (VDKs). Perhaps the most common VDK is 3-butanedione (also known as diacetyl). Diacetyl is often characterized as butter, caramel, or butterscotch and is typically considered to be an off flavor in beer. However, in certain styles or interpretations, brewers may choose to leave more diacetyl in the beer. VDKs are produced naturally by the yeast, through the excretion of alpha-acetolactate and alpha-ketobutyrate, which immediately degrade into VDKs during fermentation. As fermentation comes to an end, the yeast will then start to consume any VDKs in solution as an energy source. If the beer is removed from yeast too soon, the yeast will not have had enough time to reabsorb the residual VDKs, leaving more in the final product.
The yeast strain is where it all begins. Brewers know that to produce a quality product you must know the behavior of the chosen strain of yeast. Whether you are striving to produce a specific region’s traditional flavors for the masses, or simply trying to make the best home brew you can, it’s all about the fermentation.
References:
Iorizzo, Massimo, et al. “Role of Yeasts in the Brewing Process: Tradition and Innovation.” Department of Agricultural, Environmental and Food Sciences (DiAAA), University of Molise (2021)
White, Chris. “Yeast: The Practical Guide to Beer Fermentation. Brewers Publications (2010)
By Kyle McMichael & Austin Snow