What are Esters?
Esters represent the largest class of flavor compounds in alcoholic beverages, responsible for contributing to both flavour and aroma of beer. The aromas and flavors attributable to esters include: anise, apple, banana, honey, juicy fruit gum, nail polish remover, pear, pineapple, strawberry. While they are best known for creating desirable fruity flavours, esters can also produce undesirable solvent-like flavours.
Esters v Phenols
Although esters and phenols are often described interchangeably, they represent different types of organic compounds. Sidestepping the subject of differences in chemical composition, esters are generally associated with fruit flavours, while phenols are known to impart clove-like, medicinal or smoky flavours and aromas. Both phenols and esters may be present in the same beer, a hefeweizen for example, gets its distinctive banana flavours from esters, while phenols give it the signature spicy, clove-like characteristics.
How Esters are Produced
Esters are formed in beer by the “esterification,” a reaction between organic acids present in the wort and the developing alcohol during fermentation. During this process, ethanol combines with fatty acids and a molecule called acetyl coenzyme-A (ACOA) forming ethyl acetate, a member of the ester family.
In low concentrations, ethyl acetate has a light pear-like character, but can come across as solvent-like in higher concentrations. Additional esters may be produced with other alcohols. A hefeweizen, for example, gets its banana characteristics from isoamyl acetate, resulting from a combination of isoamyl alcohol and acetic acid.
Esters & Yeast
While fruity esters are a desirable component to many beer styles, including Weizens, Belgian and English ales, they are an unwelcome addition, or off-flavour in most lagers. For this reason, yeast strain selection is crucial to brewers, who will choose to inhibit or encourage ester production in beer.
Yeast is primarily responsible for determining both the type and level of ester production in beer. A yeast enzyme called acetate transferase (AAT) is ultimately responsible for initiating the production of esters. Simply put, rapidly reproducing yeast enhances AAT production, resulting in more esters in finished beer, where a low AAT yeast strain will inhibit ester production.
Additional Factors
Other major factors that affect esterification include the composition of wort (or unfinished beer) and fermentation temperatures. High levels of dissolved oxygen levels in the wort slows the formation of esters, where under-oxygenating will boost ester production in finished beer. Higher fermentation temperatures will also result in greater ester production by increasing AAT through high yeast activity, while lower fermentation temperatures will inhibit ester production.
Though high level of esters can create unwanted off-flavours, lower levels of esters can attribute a wide variety of fruit flavours that complement and elevate many styles of beer. Esters are essential to so many beer styles, often imparting a beer with its signature flavour, or defining it through difference as something truly unique.
(Leah is a Toronto based freelance writer as well as Head Beer Weenie and a server at C’est What)