Changes

Some strains of ''S. cerevisiae'' can convert the ferulic acid, which is found in malted barley and in wheat (malted wheat more so than raw wheat), into the phenol 4-vinyl guaiacol (4VG). 4VG is less toxic to microorganisms than the ferulic acid and other hydroxycinnamic acids. It gives a clove-like flavor to the beer. While most beer styles consider this an off-flavor, it is an important flavor component for certain styles of beer such as German Hefeweizen, saison, and some Belgian styles. Strains that convert ferulic acid into 4VG during fermentation are known as "phenolic off flavor positive" or "POF+" and are characteristic of wild ''Saccharomyces'', bread yeast, and a small number of brewers yeast. Most strains of brewers yeast, however, were selected for not having this capability, and are known as "phenolic off flavor negative" or "POF-". POF+ strains require two enzymes: phenylacrylic acid decarboxylase (PAD1) and a ferulic acid decarboxylase (FDC1). The PAD1 enzyme creates a precursor required for the FDC1 enzyme to work, which then converts the ferulic acid into 4VG <ref name="lentz_2018">[http://www.mdpi.com/2311-5637/4/1/20/html#B13-fermentation-04-00020 The Impact of Simple Phenolic Compounds on Beer Aroma and Flavor. Michael Lentz. 2018. doi: 10.3390/fermentation4010020.]</ref>.  While most beer strains of ''S. cerevisiae'' are POF-, a survey of over 200 strains of ''S. cerevisiae'' showed that 81-95% of wine yeasts are POF+. It was also shown that 85-97% of wild strains of ''S. cerevisiae'' are POF+. Other genera of yeasts that tend to be POF+ include ''Rhodotorula'', ''Candida'', ''Cryptococcus'', ''Pichia'', ''Hansenula'', and ''Brettanomyces'' <ref>[https://www.ncbi.nlm.nih.gov/m/pubmed/16232824/ Distribution of phenolic yeasts and production of phenolic off-flavors in wine fermentation. Shinohara T, et al. J Biosci Bioeng. 2000.]</ref>.