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===Phenolic Off Flavor Strains===
Some strains of ''S. cerevisiae'' can convert the ferulic acid, which is found in malted barley and also in higher amounts in wheat, into the phenol 4-vinyl guaiacol (To do4VG). 4VG is less toxic to microorganisms than the ferulic acid and other hydroxycinnamic acids. It gives a clove-like flavor to 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>.
Yeast produced phenolic/clove aromas tend to dominate over hop aromas in dry hopped beer <ref name="Sharp_Presentation">"Recent Advances in Controlling Hoppy Aroma in Beer." Daniel C. Sharp. OSU Brewing Science Presentation.</ref>.
See also:
* [http://beer.suregork.com/?p=3877 Kristoffer Krogerus's blog article on removing POF gene from Saccharomyces hybrids.]