Characterizing German and Belgian Strains for Phenolic Off Flavor (POF)

Clove, peppery, and spicy are all adjectives used to describe the flavor and aroma of volatile phenols in beer. These flavors are specifically noted in the Beer Judge Certification Program (BJCP) style guidelines as components of certain traditional German and Belgian styles. However, the presence of these flavors in many other ale and lager styles is considered a fault. Generally, this flavor is referred to as phenolic off flavor (POF). Since POF is the result of metabolites released during fermentation, yeast are classified based on their ability to synthesize this flavor as POF+ or POF-.

The primary volatile phenol produced by POF+ yeast during beer fermentations is 4-vinyl-guiacol (4VG).¹ This compound is synthesized by Saccharomyces cerevisiae through decarboxylation of hydroxycinnamic acids, a reaction which requires two enzymes, phenylacrylic acid decarboxylase (PAD1) and ferulic acid decarboxylase (FDC1).  Two major hydroxycinnamic acids are present in wort, ferulic acid and p-coumaric acid, both of which can be converted to 4VG by yeast.

Many commercial brewing yeast strains do not produce clove or peppery aromas, allowing for a “clean” flavor profile that is desired in most ale styles and all lagers. Genetic sequencing by research organizations has shown that POF- yeast strains have loss of function mutations in the FDC1 or PAD1 genes, preventing synthesis of 4VG. ² This phenotype is likely a result of brewers throughout history selecting for yeast strains which produce the desired “clean” flavor profile.

Genetic sequencing is one method to determine if a yeast strain is capable of producing 4VG but other methods are more readily available. The American Society of Brewing Chemists (ASBC) and European Brewing Council (EBC) method for screening yeast for POF involves growing yeast in a medium which is spiked with ferulic acid and using sensory analysis to detect the clove aroma. A similar spectrophotometric assay has been developed which measures the amount of ferulic acid remaining after a yeast strain is grown in a medium spiked with ferulic acid.³ Since the amount of ferulic acid present after fermentation is inversely correlated to the concentration of 4VG, this assay also provides an indication of the intensity of the phenolic aroma that a strain can produce.

Both the spectrophotometric assay and the sensory assay were used to screen strains in Wyeast’s culture collection for their ability to convert ferulic acid to 4VG. A total of 21 S. cerevisiae strains in Wyeast’s culture collection recommended for use in German and Belgian Beer styles were investigated. Of these strains, 17 produced phenolic aroma detected by trained panelists, resulting in their status as POF+ (Table 1).  The results of the absorbance based assay correlated strongly with the intensity of the phenolic aroma reported by panelists. Using the quantitative results of the spectrophotometric assay, the 17 POF+ strains were assigned to groups based on the relative amounts of ferulic acid metabolized during fermentation. A total of 10 strains were characterized as producers of strong phenolic aroma while the remaining 7 produced moderate to weak phenolic aroma (Figure 1).

Table 1. Relative intensity of phenolic aroma produced by Belgian and German yeast strains in Wyeast’s culture collection.

Figure 1. Characterization of POF+ strains by the intensity of the phenolic aroma produced during fermentation of YM broth with 100 mg/L ferulic acid. Results shown are the relative absorbance at 325 nm at the end of fermentation.

Understanding a yeast strain’s ability to produce 4VG is beneficial during recipe development. In certain styles, some phenolic aroma may be desired but not in amounts that dominate the flavor profile. Some of the POF- strains shown above can also be used for producing well-attenuated Belgian beers with fruity ester character but without phenolic aroma. Strain selection is a relatively simple solution to adjusting a beer’s flavor profile compared to other strategies of modifying phenolic aroma such as manipulating the mash schedule to adjust extraction of hydroxycinnamic acids from malt.

For additional assistance with yeast strain selection for modifying phenolic character, reach out to our customer service team custmerservice@wyeastlab.com. We are available to help you choose the correct yeast strain for your next award-winning recipe.

References

1. Lentz, M. (2018) The Impact of Simple Phenolic Compounds on Beer Aroma and Flavor. Fermentation 4(20).
2. Gallone, B.; Steensels, J.; Prahl, T.; Soriaga, L.; Saels, V.; Herrera-Malaver, B.; Merlevede, A.; Roncoroni, M.; Voordeckers, L.; Miraglia, L.; Teiling, C.; Steffy, B.; Taylor, M.; Schwartz, A.; Richardson, T.; White, C.; Baele, G.; Maere, S.; Verstrepen, K. (2016) Domestication and Divergence of Saccharomyces cerevisiae Beer Yeasts. Cell 166:1397–1410.
3. Mertens, S.; Steensels, J.; Gallone, B.; Souffriau, B.; Malcorps, P.; Verstrepen, K. (2017) Rapid Screening Method for Phenolic Off-Flavor (POF) Production in Yeast. Journal of the American Society of Brewing Chemists 75(4): 318-323.