Beer always contains some form of phenols (phenols and polyphenols, etc.), such as tannins (directly derived from hops and malt). Although low levels of polyphenols can enhance the taste of beer, high levels of polyphenols can result in dryness and astringency.
When referring to phenols in beer, it typically means volatile phenols. Volatile phenols have low flavor and aroma thresholds, and most people can detect them at very low concentrations (sometimes less than one part per billion). While volatile phenols are generally undesirable, some of them are sought after in certain types of beer.
What Are Phenols?
Phenols encompass a wide range of compounds, and their flavors and aromas are often described as clove-like, medicinal, smoky, or “band-aid” notes, making them distinct in most beer styles. However, in other beer styles, they are considered iconic flavors. Chemically, phenolic compounds contain hydroxyl (-OH) groups and an aromatic hydrocarbon ring consisting of hydrogen and carbon atoms. Nature has produced numerous types of these compounds, leading to well-known flavors and aromas:
- Capsaicin gives chiles their fiery bite.
- Carvacrol is responsible for oregano’s pungency.
- Eugenol is found in cloves, nutmeg, cinnamon, and vanilla.
- Guaiacol is the smoky essence in whiskey and roasted coffee.
- Methyl salicylate smells and tastes like wintergreen.
- Raspberry ketone smells like raspberries.
- Thymol gives thyme its distinctive aroma.
What Phenols Are Contained In Beer?
Currently, researchers have identified 67 basic phenols and hundreds of phenolic compounds in beer. Most of these phenols fall into the category of polyphenols, complex chains formed by simple phenolic structural units. These macromolecular phenols contribute to the smoky, pungent, and sour tastes of beer, but their primary function is astringency and bitterness (rather than the desired “clean” bitterness provided by phenolic compounds in hop resin). Lignin and tannin are well-known examples of polyphenol compounds. If their content increases inadvertently, they can significantly alter the characteristics of beer, affecting not only its flavor but also its transparency and shelf life.
The remaining 10-20% of phenolic compounds can be categorized as monophenols and flavonoids. Despite their seemingly simple name, monophenols do not exist freely in plants but often bind to specific sugars. For instance, ferulic acid can form compounds that give estery, clove-like notes to Weizens. Monophenols contribute a wider range of flavors and, in small quantities, subtly influence the flavor balance of beer.
Flavonoids, also known as bisphenols, are believed to be derived from a single monophenol through a series of complex reactions. Flavanols and catechins are examples of flavonoids involved in brewing. Like monophenols, these molecules usually bind to sugars, but their flavonoid-sugar bonds (ether bonds) break under different conditions during the brewing process.
While phenolic compounds can directly impact the flavor characteristics of beer in both positive and negative ways, their active participation in oxidation and reduction reactions during the brewing process adds further intrigue.
Where Do The Phenols In Beer Come From?
In most beers, approximately 75% of the phenols come from malt, while the remaining 25% comes from hops. Of all the phenolic compounds in beer, only around 10-20% are monophenols and flavonoids, with the rest being polyphenols.
Malt husks contain lignin and some attached phenols. Phenolic substances are found in the bran, as well as the aleurone layer beneath the husk. The cell wall components contain cellulose and pentose. The husk and the layer beneath it contain most of the phenolic compounds in malt. The barley husk is impermeable to water, so the extraction of phenolic compounds depends on the degree of grinding. Excessive milling of barley malt leads to more phenolic substances entering the beer.
Specialty malts contribute higher levels of phenolics to beer. Malt smoked with hardwood or peat absorbs substantial amounts of burnt lignin aldehydes. Burning peat at certain temperatures can also chemically transform some simple lignophenols in malt into compounds not typically found in plants. Similar phenols are present in highly roasted malt and barley. Specific smoked malts introduce phenolic compounds into the wort, imparting unique flavors depending on the type of smoked malt used.
The phenol content in hops is approximately 50 times that of malt. This is why the concentration of phenols in beer increases significantly when large amounts of hops are used. Due to the smaller quantities used, fewer phenols are produced overall. Phenolics generally contribute bitterness, one of the main reasons why plants produce phenols (bitterness deters herbivores from consuming them). Therefore, adding hops to beer imparts bitterness to the final product.
When it comes to phenolic compounds and brewing water, the main concern is the formation of chlorophenols due to chlorine. Elevated levels of chlorinated phenols are detrimental to beer quality. Chlorine typically enters water supplies in municipal systems as chloramines or chlorine. Brewers often employ potassium metabisulfite, carbon block filtration, or reverse osmosis filtration to remove chlorine from the water. Consequently, using bleach as a disinfectant for brewery equipment is not recommended.
Mashing And Sparging Process
Polyphenols, commonly known as tannins, represent another form of phenolic substances. Tannins contribute to astringency or bitterness in finished beer. They may also cause permanent or cold turbidity (turbidity that occurs when beer is chilled). Tannins can be extracted through over-sparging (continuing to sparge for too long during mashing), sparging at excessively high temperatures (above 170℉ or 77℃), or mashing at a pH level that is too high. pH levels above 5.5 are particularly prone to tannin extraction, with an ideal pH level during mashing being around 5.1 or 5.2. Any of these factors can result in cloudy beer, chill haze, and an astringent or bitter taste.
Yeast can biotransform certain phenolic compounds, converting one phenolic compound into another. Although yeast consumes or absorbs a small number of phenolic compounds, its influence on the phenolic content in beer is minimal. A well-studied example is POF+ (phenolic odor-positive) yeast strains, such as typical German hefeweizen yeast. They can convert ferulic acid (a common phenol in wort) into clove-like 4-vinyl guaiacol (4VG). 4VG is a distinctive characteristic of Bavarian wheat beers (weizens) and many Belgian beers. Brewers carefully manage their brewing processes and select yeasts known to produce this phenol. 4VG imparts aromas and flavors described as clove-like, spicy, or herbal, which are desirable in these beer styles. Unwanted organisms in beer can also produce chlorophenols.
Some minor beer ingredients also increase the phenolic content. Spices used in specialty beers contain phenolic compounds. For example, orange peel (both sweet and curaçao) used in Belgian witbiers contains considerable amounts of bitter flavonoids. Oak fermentation vessels can leach phenolics into beer (particularly lignins and tannins, which are abundant in oak species).