What Causes Phenolic Off Flavor in Beer?

If you’ve been wondering about the phenolic off flavor in beer, either because you hope to integrate it into your brew or you’re hoping to avoid it, fear not. The steps to take are typically simple and involve more knowledge of product, ingredients, and cleaning processes.

Though phenols are in pretty much everything, they don’t have to ruin a beer. Indeed, they can create a delightful experience.

Phenols in Beer

First, it is important to note that you are not going to avoid phenols in beer entirely.

Phenols exist in the grains you use, in the yeast, and even in the water.

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The trick is to understand which phenols you’re working with and how to manipulate them to your will.

Phenols from Grains and Hops

The first type of phenols worth discussing are the desirable ones. The polyphenols you get from malted grains and hops added to beer are derived from the tannins in the plant matter.

They lend texture to your beer, enhancing mouthfeel and making the brew feel rich when it hits your palate. In too great concentrations, however, the polyphenols from tannins can cause the drinker’s mouth to pucker. We see this same effect in wine when the tannins are too high and the wine leaves your mouth feeling dry, absorbing all the saliva in your mouth and making you long for a glass of water.

The trick with tannins is to strike the right balance, which is not usually something a brewer has to worry about as long as you keep an eye on the measurements of your ingredients. Too much toasted barley with too little water can result in your tannins being thrown off and your drinkers with an astringent feeling in their mouths.

Phenols in the Water

The next, and much less sought after phenol in beer can come from the water. Though it is unusual, if a brewer is working with water from a city supply, there may be too much chlorine in the water, which can lead to an abundance of chlorophenols in the beer. Chlorophenols are detectable at much lower concentrations than other phenols, so it only takes a little to be considered an abundance.

Once chlorophenols are in the water that is used to make the wort and beer, they cannot be removed through the natural brewing process. And these phenolic off flavors can come across as tasting like antiseptic or mouthwash, both of which are unwanted in beer.

Phenols in Packing Materials

Bromine is another phenol that is detectable at low concentrations and can sneak into beer via packaging materials. Excessive bromophenols have been described as tasting like old television sets or a burnt electrical current. Definitely not sought after in beer.

Phenols from other Ingredients

Some brewers have been experimenting with various forms of smoking or toasting their malt and other grain, including using peat moss to smoke the grains or beechwood logs. Both practices lead to the creation of phenols guaiacol and syringol. These phenolic off flavors taste like smoke, barbecue chips, campfire, and earth.

They can be a beautiful addition to a strong beer in small quantities, which requires a tremendous amount of experimentation, which should be undergone with caution and a light hand. Again, these phenols will be easily detected, especially in a lighter, crisper beer, and if you hope to add a smoky flavor to your beer, you want hints and subtle notes, not overwhelming flavors and aromas.

Phenols in Yeast and Bacteria

The most common phenolic off flavors in beer come directly from either yeast or bacteria or both.

Certain yeast strains produce a phenol known as 4-vinyl guaiacol, which results in flavors and aromas resembling cloves, spices, and herbs and are not only tolerated but highly sought after in the right beer. 4VG is a classic characteristics of Bavarian wheat beers and Belgian beers, so much so that brewers specifically select yeast strains known to produce 4VG.

Once brewers choose the preferred yeast strain, they can then encourage the production, at higher or lower levels, to come into the beer through temperature control. A process known as decarboxylation of ferulic acid occurs at high mash resting temperatures. So allowing mash to rest at a minimum of 113 degrees Fahrenheit and then raising the temperature during sparging as well can bring out the 4VG in beer to get the desired flavors and aromas.

Finally, the bacteria that produces phenolic off flavors in beer can be both good and bad. Brettanomyces, a wild bacteria, is most often avoided and even feared. It creates a souring effect most brewers do not want in their beer. At the same time, some brewers require Brett specifically for this souring effect; beers like Lambics would not be the same without it.

The problem for most brewers is that Brett is unpredictable and is also known to produce the phenols 4-ethylphenol and 4-ethylguaiacol, which result in flavors of farmyards and medicine and smoked meat, respectively, all of which are not traditionally craved in beer.

Then there are the wort spoilage bacteria which can enter beer through cracked equipment or unclean surfaces. These bacteria produce phenolic off flavors resembling medicine, definitely not a flavor or aroma wanted in beer.

Brewing Quality Control

In the end, the brewer must ensure a clean and sanitized, aseptic, environment in which the yeast and any bacteria intentionally introduced can survive and thrive in the beer but in which nothing unwanted is allowed in.

Thus, it is critical to keep all equipment, surfaces, and even clothing and shoes clean so as not to introduce undesired bacteria or toxins that can produce phenolic off flavors.

And if the goal is to experiment with phenolic off flavors, in the interest of crafting new flavors and aromas at subtle levels, the key will be to work with a light hand and in small batches, and of course to take detailed notes so you can repeat what works and not worry too much about what doesn’t.


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  1. https://beerandbrewing.com/dictionary/9Z95BHQHMb


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