Phenolic flavors and aromas are often described as clove-like, medicinal, smoky, or “band-aid” and are considered off-flavors in most beer styles.
Beer always contains some form of phenol; polyphenols, such as tannins, are derived directly from hops and malt. Low levels of polyphenol can contribute to mouthfeel, whereas high levels can cause a drying, mouth-puckering astringency.
When a beer is described as phenolic, it is usually with reference to volatile phenols. Volatile phenols have low flavor and aroma thresholds and most people taste and smell them at very low concentrations, sometimes under 10 parts per billion. Although volatile phenols are generally not desirable, certain of these are sought after in particular beer types.
There are three main sources of volatile phenols: ingredients, chemical taints, and yeasts and bacteria.
The two brewing ingredients most likely to contribute volatile phenols are water and smoked malts. Although it should not, mains water may already contain phenols when entering the brewery. (Most countries specify a very low maximum level of phenols, including chlorophenols from added chlorine for mains water.) Breweries relying on their own water supply are typically more at risk of introducing phenols into the brewing process from this source. Once added, phenols will not be removed by the normal brewing process.
Rauchmalt is a type of malt dried over an open fire made with beechwood logs. It is a specialty of the Bavarian region of Franconia in Germany. Rauchmalt adds the phenols guaiacol and syringol. These give a beer a smoky aroma and taste, sometimes described as “campfire” or “barbecue potato chips.” These phenols are a distinguishing characteristic of rauchbier, a Bamberg speciality with powerfully smoky flavors. Craft brewers in the United States and other countries are experimenting with rauchmalt and creating interesting smoky beers that often pair well with food.
Peated malt is produced by smoking malt over burning peat. Although mainly used in the production of whiskey, some brewers are experimenting with peated malt for the resultant smoky, earthy phenols.
The main chemicals giving rise to volatile phenols are chlorine and bromine. These two chemicals combine with phenols (including polyphenols) already in the beer to create chlorophenols and bromophenols, respectively.
Chlorine is typically added to main water supplies and must be removed before the water can be used for brewing. Chlorine-based cleaners and sanitizers are very popular in breweries. Poor rinsing can result in chlorine contaminating the beer. Bromine can be introduced by packaging materials.
Chlorophenols and bromophenols are detectable at much lower concentrations than other phenols. Chlorophenols remind people of antiseptics or mouthwash, bromophenols of old television sets, hot Bakelite, or the smell of an electrical short. None of these characteristics is welcome in beer.
Most frequently, beers derive volatile phenols from yeasts or bacteria. 4-Vinyl guaiacol, known as 4VG, is a signature characteristic of Bavarian wheat beer (weizens) and many Belgian beers. Brewers manage their brewing processes and select yeasts known to produce this phenol. 4VG gives beers aromas and flavors described as being clove-like, spicy, or herbal and is considered desirable in these beers at certain levels.
Yeasts produce 4VG through the decarboxylation of ferulic acid. Brewers can increase the concentration of ferulic acid in wort by having a mash rest at 45°C (113°F) and by raising sparge liquor temperatures. Wheat malt and some barley varieties produce elevated ferulic acid levels. Higher fermentation temperatures also promote the production of 4VG. However, as these beers age, 4VG starts breaking down, giving a vanilla-like character and losing the 4VG signature.
A smaller subset of Belgian beers, including lambics, contains the phenol 4-ethylphenol (4-EP). This phenol is produced by the wild yeast type brettanomyces and it reminds people of farmyards, medicines, and mice. p-Coumaric acid, from malt, is the precursor to 4-vinylphenol, which in turn is the precursor to 4-EP.
Although 4VG, and to a lesser extent 4-EP, is expected in certain beer styles, in most it is not welcome and is seen as a fault. Brettanomyces also produces 4-ethylguaiacol, which gives beer smoked meat or clovey, spicy character. This phenol is largely unwelcome in beer; however, many craft brewers are experimenting with brettanomyces, especially in the production of sour and “wild” beer styles. Just as brettanomyces character in wine is usually considered a fault, but sometimes considered a desired “complexing agent,” so it is in beer.
Wort spoilage bacteria are those which change the flavor and aroma of beer at the start of fermentation, before the yeast establishes itself. Certain Gram-negative, indole-negative, short-rod, wort spoilage bacteria have been reported to produce a medicinal phenolic taste in the resultant beer. This is always unpleasant and can be a difficult problem for a brewery to eradicate once established.