Esters are the aromatic, fruity compounds in wine, formed during fermentation, malolactic fermentation (MLF) and ageing. There are many factors that influence the types and quantities of esters present and although all of them are not positive contributors, as a group, they are a major constituent of wine.
Research has identified 11 different esters in grape juice, but wine can have up to 83; this affects the wine’s flavour and complexity in proportion
to their presence. Esters are formed when an acid reacts with alcohol, eliminating a water molecule in the process.
With the many different acids and alcohols found in wine, there is considerable potential for the formation of a wide range of esters. The alcohol in the reaction can be ethanol, or any other alcohol produced by yeast cells, especially from the degradation of amino acids.
In wine, esters can be classified into two groups: those formed enzymatically and those formed during wine ageing, through a chemical esterification between alcohol and acids at a low pH.
Esters can furthermore be grouped into two classes, namely ethyl esters and acetate esters. The ethyl esters comprise a short-chain alcohol group (ethanol) and a longer-chain acid group (such as medium- to long-chain fatty acids). Ethyl esters have a strong influence on a wine’s aroma and examples of ethyl esters are ethyl hexanoate, with reported aroma characteristics that include fruity, strawberry, green apple and anise; ethyl octanoate, with sweet, fruity, ripe fruit, burned and beer characteristics; and ethyl decanoate, which imparts an oily, fruity and floral character.
The acetate esters comprise an acid group (short-chain acetic acid) and a longer-chain alcohol group (fusel alcohols), mostly complex alcohols derived from amino acid metabolism. This group includes esters such as isoamyl acetate, with aromas of banana and fruits; and isobutyl acetate, with fruity and apple flavours.
Ethyl acetate is quantitatively the most prominent ester in wine, due to its spontaneous or enzymatic formation from ethanol and acetic acid ? it is therefore both an acetate and/or ethyl ester.
Ethanol and acetate are present in wine in relatively large concentrations; ethyl acetate is therefore often an important contributor to wine aroma. At low concentrations it may give desirable and fruity character to the wine; however, at higher concentrations it can impart a solvent or nail varnish aroma, and contribute to the perception of volatile acidity (VA).