Esters and phenols are organic compounds that contribute to the aroma and flavour of many beer styles. Terms such as estery and phenolic are used by beer drinkers in sensory description when tasting beer. Esters are fruity and frequently desirable, whilst phenols are spicy and usually unwelcome. Here's a more comprehensive look into what esters and phenols are, what aromas and flavours they omit, and how they are regulated.
Esters
An ester is an organic compound in which a hydrogen molecule (H) in one of the compound's carboxyl (-COOH) is replaced by another hydrocarbon. Esters are common by-products formed in a process called esterification. During fermentation, a reaction between the organic acids in the wort and the developing alcohol cause esters to form. Esters can be preferable in certain quantities with many ale styles, whereas with lagers they are considered 'off flavours'.
Aromas and flavours attributable to esters include: banana, pear drop, rose, apple, orange, bubblegum, pear, and solvent-like. Ester production increases with fermentation temperature so ales generally express greater ester character than lagers. A Kölsch offers subtle notes of apple and pear, a Hefeweizen has powerful banana esters, and a cask bitter has a 'fruity nose'.
While the reaction between the acids and alcohol actually form esters, three variables influence the amount of esters that can potentially develop. By understanding and managing these variables, brewers can have a certain amount of control over the character and level of esters produced.
Wort composition: High concentrations of sugar, zinc, and free amino acids tend to promote higher ester levels. Higher levels of dissolved oxygen and lipid content can inhibit ester formation.
Yeast strain selection: Some yeast strains are more inclined to produce esters. Bavarian yeast strains impart powerful banana notes because of the high content of isoamyl acetate.
Fermentation environment and conditions: The shape of the fermenter can impact the production of esters. Shallower, more open fermenters will produce higher levels of esters than tall, narrower fermenters. This is because high hydrostatic pressure and levels of CO2 in the tall, narrow fermenters inhibit ester formation.
Phenols
A phenol is an organic compound in which a hydroxyl group (-OH) is bonded to an aromatic hydrocarbon ring, better known as a benzene ring. Phenols are usually considered unwanted by-products by brewers however there are exceptions, like with Belgian trappist ales and saisons, where they are essential components of the beer styles.
Phenols are associated with the following aromas and flavours: cloves, pepper, smoke, tannins, nutmeg, cinnamon, all-spice, sweaty horse blankets. The same Bavarian yeast strains that give Hefeweizen its signature banana character also creates clove-like phenols. Saisons often displays a mix of clove and pepper-like phenols. Belgian trappist ales like Westvleteren 12 exhibits phenolic, yeasty notes and hints of dark fruit, including plums, cherries, and figs. Aecht Schlenkerla Rauchbier, an Urbock, has an overwhelming smoked impression.
Smoky flavours can come from smoked malt or peated water, but they can also arise from bacterial infection. Tannins, known as polyphenols, deliver an astringent, puckering sensation to the palate. One particular phenol is often described as smelling like 'sweaty horse blankets'; a common by-product of Brettanomyces bruxellensis fermentations.
Some form of phenols or polyphenols are always present in beer, but when beer drinkers note they are picking up phenols or polyphenols, they tend to be referring to the aromas of volatile phenols. Phenols are detectable, even at very small concentrations and tend to show up as clove-like, medicinal, or smoky aromas. Volatile phenols are generally derived from three different sources.
Water and smoked malts: Water can often times contain phenols that are transferred to beer, which are not eradicated during the boil. Rauchmalt and peated malt, add phenols that impart smoky, earthy notes reminiscent of a camp fire.
Chemicals: Chlorine and bromine tend to encourage high levels of phenols and polyphenols. Chlorine is common in water. Chlorine and bromine derived phenols tend to take on antiseptic and burning characteristics.
Yeast and/or bacteria: The clove-like phenol in Bavarian wheat and Belgian beers is caused by the high content of 4-vinyl guaiacol, which is produced by the decarboxylation of ferulic acid.
So there you have it! A dip into the complex world of esters and phenols. I hope you learnt a lil something more about beer.
By Lucas Barker, a budding craft beer enthusiast of 2-3 years. 26/4/22
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