Yeast, the Modern Day Alchemist: Part 1 --Phenols

Have you ever had a beer that tasted like it had those little Runt bananas in it or a Belgian ale that you knew had to have cloves added. Well in many cases what you tasted was not coming from fruit or spices but are the products of yeast.

Brewing yeast, Phylum Ascomycota, class Saccharomycetes (Latin for sugar fungi), is a humble cousin to one of the most expensive foods on the planet, the truffle. Under the guise of their grunt work consisting of converting sugars to alcohol (Figure 1), they clandestinely spin some of the alcohols and other products into flavorful and aromatic gold, like an army of microscopic Rumplestiltskins. They won’t steal your first born, but they may spin some flavors that you weren't asking for if you’re not careful. Of course we know that alchemy never seems to work so here’s how they are actually making some of these compounds that give different characters to beer.

Figure 1. Simple Ethanol Fermentation.(from Wikipedia.org)

Yeasts can produce a couple different types of products in addition to the well-known alcohol and CO2. These include various esters and phenols. For the sake of keeping this post relatively short I’m just going to talk about phenols and leave the esters for later.

4-vinylguaiacol, perceived as clove can be produced by thermal decompsition of the carboxyl group in ferulic acid. In addition, yeast enzymes called feruloyl esterases (figure 2) can also decarboxylate ferulic acid to form 4-vinylguaiacol. The amount of this phenol can vary greatly depending on the specific yeast strain used to ferment the wort. Grain can also contribute to the initial abundance of ferulic acid available for conversion. While the majority of ferulic acid was released during the brewing process of barley, 60-90% of ferulic acid was hydrolyzed during fermentation in beer containing wheat malt ⁽¹⁾ which leads to a greater amount of that tasty clove-like spicyness found in many Belgian style beers.

Figure 2. Conversion of Ferulic Acid to 4-vinylguaiacol

Another type of Phenol that you may run into is 4-ethyl phenol. This one is certainly for the more adventurous beer drinker. It lends the “barn yard” or “horse blanket” character to beers fermented with the wild yeast Brettanomyces sp. Typical styles include the farm-house ales, not because they taste that way, but because they were historically fermented in open containers during the cooler months to reduce the chance of getting nasty microscopic bugs. Although, it seems like brewers would want to steer clear of this character, it does lend a certain complexity to beers and can be quite enjoyable.

Now for the synthesis! Cinnamate decarboxylase removes the carboxyl group from p-coumaric acid to produce 4-vinylphenol, which is then reduced by vinyl phenol reductase to create the final product 4-ethylphenol. Like ferulic acid, p-coumaric acid can be found naturally occurring in grains. Interestingly, a study found that organically produced wheat had significantly higher concentrations of both ferulic and p-coumaric acid ⁽²⁾.

Figure 3. Synthesis of 4-ethylphenol from p-coumaric Acid (From Wikipedia.org).

Be adventurous, try a farmhouse ale. Maybe you’ll like it, or maybe you will want to scrape your tongue off with a putty knife. Either way, at least you’ll know how it’s synthesized and that should count for something.

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1. Coghe, S., Benoot, K., Delvaux, F., Vanderhaegen, B., Delvaux FR. 2004. Ferulic acid release and 4-vinylguaiacol formation during brewing and fermentation: indications for feruloyl esterase activity in Saccharomyces cerevisiae. J Agric Food Chem 52(3): 602-8 
2. Zuchowski, J., Jonczyk, K., Oleszek, W. 2011. Phenolic acid concentrations in organically and conventionally cultivated spring and winter wheat. J Sci Food Agric 91(6): 1089-95