Esters and phenols are organic compounds that contribute to the aroma and flavor of many beer styles. You frequently hear the terms “estery” and “phenolic” in sensory descriptions, but what exactly do these terms mean?
For starters, here are the technical definitions (which are probably only meaningful to the chemically curious):
An ester is an organic compound in which the hydrogen (H) in one of the compound’s carboxyl groups (-COOH) is replaced by another hydrocarbon.
A phenol is an organic compound in which a hydroxyl group (-OH) is bonded to an aromatic hydrocarbon ring (also called a benzene ring).
Now this is all very well and good, but I don’t like to take organic chemistry texts to my beer tastings. Therefore, I propose a slightly different definition.
Broadly speaking, esters are fruity and frequently desirable, while phenols are spicy and usually unwelcome.
This is a complete oversimplification and exceptions abound, but adopting this definition has helped me gain more intuition for what’s what. I’ve also found it helpful to imagine esters and phenols occupying two sides of a page. The estery left side is filled with pictures of fruit, while the phenolic right side is a vague cloud of filth, à la the Peanuts character, Pigpen. Again, it’s an oversimplification, but hopefully visual learners will find it useful.
Let’s take a closer look at these two broad classes of characterful compounds.
Esters
Esters are very common in beer and are formed when an alcohol and an acid react in a process with the highly original name esterification. Esters can be desirable in certain amounts in certain ale styles, or they may be considered off flavors, especially in lagers. Aromas and flavors attributable to esters include
- Anise
- Apple
- Banana
- Honey
- Juicy Fruit gum
- Nail polish remover
- Pear
- Pineapple
- Strawberry
Because ester production increases with fermentation temperature, ales generally express greater ester character than lagers. Even within the ale family, ester levels can vary dramatically, from the subtle apples of Kölsch to the generically “fruity” nose of a cask bitter all the way to the powerful banana notes in Bavarian Hefeweizen. Other factors that can affect ester production include fermentor geometry, wort composition, and yeast selection.
Phenols
While style-appropriate esters are usually pleasant, phenols are almost always off flavors. Phenols are associated with the following characteristics:
- Cloves
- Medicine (especially sore throat spray)
- Pepper
- Smoke
- Sweaty horse blankets
- Tannins
- Vanilla
The same Bavarian yeast strains that give Hefeweizen its signature banana character also create clove-like phenols. Tannins, also known as polyphenols, deliver an astringent, puckering sensation to the palate. Smoky flavors can come from smoked malt, of course, but they can also arise from bacterial infection. And sweaty horse blankets are simply normal by-products of certain (but not all!) _Brettanomyces _fermentations. Belgian Trappist ales and saisons are excellent illustrations of phenols positively contributing to beer character.
While some phenols arise naturally from specific yeast strains, unwanted phenols can be the result of an infection by bacteria or wild yeast, sparging at too high a temperature (above 168°F/76°C), sparging after the runoff drops below about 1.010 SG, or using chlorine-based sanitizers.
Speaking of chlorine, the chlorine in treated municipal water can react with phenols to create chlorophenols, which have a flavor often described as adhesive bandage-like. The human palate can detect chlorophenols in very small concentrations, so it’s worth the effort to dechlorinate your water or use distilled or reverse osmosis water.
As with most beer characteristics, the best way to learn about esters and phenols is to sample examples that are notably estery and phenolic. Then you can populate your own mental ledger with the images that make the most intuitive sense to you.