The word “phenol” often comes up in the beer world, but its use can vary widely. It can be used to describe the clove-like aroma that can be produced from certain brewer’s yeasts. Strains like saison, Belgian, and German hefeweizen are often cited as producers of phenolic compounds. It can be used to describe the off-putting medicinal aromas from ill-wanted microorganisms that can infect a beer. It can be used when talking about astringency, a sensation in your mouth, which can have several sources. It can be used when folks talk about the haze caused when a beer gets heavily dry hopped. It can be used when describing the smoke-flavors of peated malts. So what are phenols?
Hundreds of phenolic compounds have been found to exist in beer. Starting with a basic building block of an aromatic ring of carbon molecules with a hydroxyl group attached to it, phenolic compounds can grow in a multitude of ways or become long chains (polyphenols) — like how sugar chains becomes starch. Many can be bitter, but some can contribute smoky, sour, or harsh flavors as well.
Chemically speaking, phenols are closely related to alcohols. In strong enough levels, both can be used as an antiseptic. In fact the mouthwash Listerine is named after Joseph Lister who started using phenol (long known as carbolic acid) as an antiseptic during surgical amputations and brought the mortality rate from 45% down to 15%. You probably already are familiar with a bunch of phenolic compounds: Aspirin (salicylic acid), vanillin, phenolphthalein (pH indicator), and tannins. Phenolic compounds provide the color to flowers and fruits, for example hydrangea blossoms, red grapes, and blueberries. There are lots more that lie just below the surface of many products we use daily or that are found in plants. If you’ve ever heard of flavonoid compounds, they are contained within the phenol family. Many phenolic compounds have antioxidant characteristics as well, although some can be carcinogenic as well.
So now that we have an idea of the greater world of phenols, let’s focus in on how they relate to our beer.
Malts and Phenols
Let’s begin with malts and phenols because this is really where it all starts. Studies have shown that approximately 75% of the phenols found in beer will come from the malt (hops being the other typical source). Astringent polyphenolic chains, like lignin and tannins, are found in the husks and endosperm of grains. When a brewer mashes in, some of these phenolic chains will begin to disperse into the wort. Brewers will keep the pH of the wort acidic enough so these polyphenolic compounds in the grains have a hard time dispersing. Certain smoked malts can also introduce phenolic compounds into the wort, providing distinctive smoky, bacon, or even peaty-Scotch aromas, depending on which kind of smoked malt you use.
Hops and Phenols
Hops have as much as 50x more phenols per weight compared to malts. This is why when large amounts of hops are used in a beer; the phenolic concentration in a beer can jump quickly. Phenols are often bitter in quality, one of the main reasons plants produce them — they deter herbivory. So adding dry hops will also add some bitterness to the beer. The added polyphenols from dry hopping can also interact with the proteins found in the beer, creating the colloidal haze that typifies the modern hazy IPA. Having the right protein and polyphenol mix and concentration is one key (along with proper yeast selection) to this interaction when planning to brew this style of beer. This protein-polyphenol colloidal haze has been cited to elevate the hop oil aroma and flavor in these beers as well.
Water and Phenols
The biggest concern when it comes to phenolic compounds and water relates to chlorine and the formation of chlorophenols. Elevated levels of chlorinated phenols are one of the worst things that can happen to beer. The chlorine most often comes from municipal water systems in the form of chloramine or chlorine. Steps are often taken by brewers — such as with potassium metabisulfite, carbon block filtration, or reverse osmosis filtering — to remove the chlorine from the water. Brewers are often advised not to use bleach as a sanitizer for this reason as well. Chlorophenols are often noted to have a medicinal, mouthwash, or Band-Aid aroma to them.
Yeast and Phenols
The term biotransformation in beer is often applied to yeast and their interactions with certain hop compounds during active fermentation. But those interactions are not the only time this term can be applied to yeast’s secondary effects during fermentation. Yeast can biotransform certain phenolic compounds as well, changing one phenolic compound into something else. One such well-studied example is the case with POF+ (phenolic off-flavor positive) yeast strains like your typical German hefeweizen yeast. They can change ferulic acid (a common phenol found in wort) into the clove-like 4-vinyl guaiacol. Unwanted organisms in your beer can create chlorophenols as well.