The 'Gazette'
Whisky’s fruity aromas can be declined into several categories: fresh fruits, candied fruits, jammy fruits, exotic fruits, dry fruits… That’s how complex it is! Esters are the most common and most studied fruit molecules in whisky. Formed during fermentation through biochemical reactions, esters have different chemical structures displaying a large range of fresh fruit aromas.
In chemistry, esters are the result of the reaction between an alcohol molecule and an acid molecule. This reaction is reversible, meaning that the ester produced can be hydrolysed to give the initial alcohol and acid.
Ester’s chemical formation from an acid and an alcohol
Esters are odorous compounds found in both wines and spirits. Their formation mainly occurs during alcoholic fermentation, under the influence of the yeasts. A number of factors come into play in this process:
– Alcohols: particularly ethanol, but also higher alcohols, which are alcohols with a number of carbons higher than ethanol.
– Fatty acids: linear or branched.
– Enzymes.
The esters’ formation by the yeast during fermentation may act as a protective mechanism. Indeed, the esters’ biological synthesis would therefore allow them to eliminate the substances that are toxic for their growth.
In whisky, there are several categories of esters:
Ethyl esters from linear chained fatty acids: provide a variety of fruity aromas, with heavier, soapy notes when the carbon chain is long.
Ethyl esters from branched acids: contribute aromas of strawberry and kiwi, with hints of solvent.
Ethyl esters from hydroxy acids: evoke fruity descriptors reminiscent of strawberry, blackberry or apple.
Acetate esters from higher alcohols: contribute to a wide range of fruity aromas, from banana to pear, as well as floral notes.
Each of the existing esters has its own smell and perception threshold. But the fruity character of an alcohol can also be considered as a whole. In fact, esters can be present in a mixture, as is the case with whisky. So a molecule does not have the same aromatic power, or the same smell, if it is considered on its own or in interaction with other odorous compounds.
Nine of the molecules considered most important in the whisky aroma are esters. By removing these esters, a significant difference was revealed in the perceived odour, illustrating that esters actively contribute to the overall aroma of whisky. This does not mean that these compounds can be the only players responsible for a harmonious aroma, but rather that their contribution to aromatic intensity is important.
Depending on the whisky’s complexity, certain molecules responsible for the fruitiness will express themselves more than others, depending on their concentration, perception threshold, affinity with wood and ethanol, and their ability to interact with other aromatic molecules. It is precisely this balance that truly reflects the fruity aromas of fine whiskies.
Poisson, L., & Schieberle, P. (2008). Characterization of the most odor-active compounds in an American Bourbon whisky by application of the aroma extract dilution analysis. Journal of agricultural and food chemistry, 56(14), 5813-5819
Suomalainen, H. (1981). Yeast esterases and aroma esters in alcoholic beverages. Journal of the Institute of Brewing, 87(5), 296-300
Jackson, R. S. (2008). Wine science: principles and applications. Academic press
Aroma | November 2024 | Dr. Magali Picard