Leah’s Beer School: Lesson 16

What are Phenols? 

Phenols are a broad class of organic compounds that add both desirable and undesirable flavours to beer.  While phenolics are generally known for producing off-flavours, they can contribute positive flavouring compounds too. 

Approximately 75% of beer phenolics come from malt, while the other 25% from hops. In barley, the husk and tissue layer just beneath the husk contains the majority of the malt phenolics. Hops actually have significantly higher levels of phenolics than malt, but they are used in much smaller quantities, so contribute fewer phenolics overall.

Low levels of phenolics add flavour and contribute to a beer’s overall balance, where some styles of beer actually target certain phenolic compounds to achieve their signature flavour. Hefeweizens, for example, use ferulic acid to add a spicy, clove-like aroma and Rauchbier use coumaric to achieve a smoky flavour and aroma. In young Lambics and other sour beers, wild yeasts like brettanomyces can interact with phenols to produce complex and acidic flavours. 

While phenolic flavours are desirable for a few specific beer styles, medicinal and astringent flavours are generally perceived as being undesirable in other brews. Bacterial infections and chlorine can produce medicinal off-flavours, like plastic band-aid and barnyard flavours. Some phenols will also oxidize beer, significantly reducing shelf life and may produce astringent or harsh flavours as a by-product of oxidation. Unoxidized phenols, on the other hand, are prone to forming larger polyphenols with protein-bonding abilities, resulting in a chill haze or lasting haze in beer. 

Fortunately, brewers can minimize the damaging effects of phenols in beer. As most phenols come from the grain husks in the malt, proper milling of the grain will help reduce phenolic compounds in beer. Chlorine-related bacterial infections can be avoided by properly rinsing brew equipment and by using dechlorinated water. In addition, brewers can also minimize phenolic interaction by avoiding unnecessary aeration, limiting the quantity of mash water, as well as controlling the pH and temperature of the mash and sparge.

Although they often get a bad rap for producing off-flavours, certain beer styles celebrate phenols for their signature flavour additions. At low levels, phenolic flavours are difficult to discern, but beer tastes dull, bland and somehow less “beery” without them. Phenols provide an important contribution to all beer, working in the background to subtly enhance the balance and flavour of every brew.