Guide to beer off-flavors: Phenolics

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 increase the taste of beer, high levels of polyphenols can cause dryness and astringency.
When beer glasses are described as phenols, they usually refer to volatile phenols. Volatile phenols have low flavor and aroma thresholds, and most people can taste and smell them at very low concentrations (sometimes less than one part per billion). Although volatile phenols are generally undesirable, some of them are sought after in certain types of beer.

What are phenols?

Phenols represent a large group of compounds, and their flavor and aroma are usually described as clove, medicinal, smoky or “band-aid”, so they are considered peculiar in most beer styles. But in other beer styles, it will be considered an iconic beer flavor. Chemically speaking, phenolic compounds refer to containing hydroxyl (-OH) and a hydrogen and carbon molecular ring (aromatic hydrocarbon ring). There are many types of these compounds in nature, and they produce many 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 just like wintergreen
Raspberry ketone smells like raspberries
Thymol is what gives thyme its distinctive aroma

What phenols are contained in beer?

At present, researchers have identified 67 alkaline phenols and hundreds of phenolic compounds in beer. Most of these phenols can be classified as polyphenols, which are complex chains formed by simple phenol structural units. These macromolecular phenols can bring smoky, pungent and sour taste to beer, but their main function is astringency and bitterness (rather than the ideal “clean” bitterness added by the phenolic compounds in hop resin). Lignin and tannin are well-known examples of polyphenol compounds. If you accidentally increase their content, they will greatly change the characteristics of beer, not only affect the flavor of the beer, but also affect its transparency, and even affect its shelf life.
The remaining 10–20% of phenolic compounds can be grouped as monophenols and flavanoids. Monophenols, despite their simple-sounding name, do not exist freely in plants, but instead often bind to particular sugars. Ferulic acid, for example, can form compounds that give estery, clovey notes to Weizens. Monophenols contribute a larger range of flavors, which in small quantities can contribute subtly to a beer’s flavor balance.
Flavonoids, although also called bisphenols, are actually thought to be formed from a single monophenol through a series of complex reactions. Flavanols and catechins are examples of flavonoids that play a role in brewing. Like the monophenols, these molecules generally bind to sugars, but these flavanoid–sugar bonds (ether bonds) break under very different conditions in the brewing process.
Although phenolic compounds can directly affect the flavor characteristics of beer in both positive and negative ways, it is their role in the brewing process that actively participates in oxidation and reduction reactions that make them more interesting.

Where do the phenols in beer come from?

In most beer, about 75% of beer phenols come from malt, and the remaining 25% comes from hops. Of all the phenolic compounds in beer, only about 10-20% are monophenols and flavonoids, and the rest are polyphenols.

Malt

Malt husk contains lignin and some attached phenols. Phenolic substances are contained in the peel and aleurone under the malt husk. Also, cellulose and pentose are contained in the elements constituting the cell wall. The shell and the tissue layer under the shell together contain most of the phenolic substances in the malt. The hull of barley germ is impermeable to water, so the degree of extraction of phenolic compounds depends on the degree of grinding. Excessive crushing of barley malt will cause more phenolic substances to enter the beer.
Special malt will add more phenolics to beer. Malt smoked with hardwood or peat fire will absorb a lot of burning lignin aldehydes. The temperature of burning peat fire hardwood can also chemically convert some simple lignophenols in malt into compounds that are not normally found in plants. Similar phenols are also found in highly roasted malt and barley. Certain smoked malts also introduce phenolic compounds into the wort, which provide a unique beer flavor according to the type of smoked malt.

Hops

The content of phenols in hops is about 50 times that of malt. This is why the concentration of phenols in beer will rise rapidly when a large amount of hops are used in beer. Because the amount of hops used is small, there are fewer phenols produced overall. Phenolics usually have a bitter taste, which is one of the main reasons why plants produce phenols (bitterness prevents herbivores from continuing to eat them). Therefore, adding some hops to beer will add some bitterness to the beer.

Brewing water

When it comes to phenolic compounds and brewing water, the biggest concern is the relationship between the formation of chlorine and chlorophenols. The increased content of chlorinated phenols is one of the worst things that can happen in beer. Chlorine usually comes from municipal water supply systems in the form of chloramines and chlorine. Brewers usually use potassium metabisulfite, carbon block filtration or reverse osmosis filtration to remove chlorine in the water. For this reason, we do not recommend that brewers use bleach as a disinfectant for brewery equipment.

Mashing and spraying process

Another form of phenolic substances is polyphenols, commonly referred to as tannins. Tannins (polyphenols) will show up as astringency or bitterness in finished beer. It may also cause permanent turbidity or cold turbidity in beer (the turbidity that appears when beer is refrigerated). Tannins can be extracted by over sparging (continuing to sparge too long when mashing), sparging at too high a temperature (greater than 170℉ or 77℃), or mashing at too high a pH level. pH levels above 5.5 are particularly prone to tannin extraction, with 5.1 or 5.2 being an ideal pH level during the mash. Any of these can lead to cloudy beer, chill haze, and an astringent or bitter flavor in the beer.

Yeast

Yeast can biotransform certain phenolic compounds and convert one phenolic compound into other substances. Although yeast consumes or absorbs a small number of phenolic substances, the influence of yeast on the content of phenolic substances in beer is very small. A well-studied example is the POF+ (phenolic odor-positive) yeast strain, such as the typical German hefeweizen yeast. They can convert ferulic acid (a common phenol in the wort) into clove-like 4-vinyl guaiacol (4VG). 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 It is considered desirable in these beers at certain levels. Unwanted organisms in beer can also produce chlorophenols.

Other ingredients

Some minor beer ingredients will also increase the phenolic content of beer. The spices used in some specialty beers contain phenolic compounds. The orange peel (both sweet and curaçao) used in Belgian wit beers, for example, contains considerable amounts of bitter flavonoids. Oak fermentation vessels can also leach phenolics into beer (lignins and tannins are particularly ubiquitous in oak species).

Like many other odors, you may or may not want to find phenols in the beer you make. Know what you need in advance. If you detect unwanted phenols, please consider the sources mentioned above to ensure that your next batch of beer does not contain phenols. Of course, this is also the tenth article of our beer off-flavor guide. You can check our previous articles to learn more about beer off-flavor and know how to deal with off-flavor.

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