Hidden compounds: the secret to aromatic complexity
written by deborahparkerwong
Of the more than 1,000 compounds that make up wine’s aromatic profile, desirable aromas are represented by a combination of 82 different volatile compounds, only some of which are detectable when present above threshold.
These molecules, which act alone or synergistically, are responsible for the majority of our olfactory perceptions of wine; 28 of them are considered major aroma compounds and 54 are trace compounds. Of the latter, methional (which has a scent often likened to that of boiled potatoes) is among the most difficult to detect, as it’s highly volatile and synergistic.
Using sophisticated detection methods, researchers at the University of Zaragoza in Spain grouped the 82 organic compounds into ten different classes, of which esters, phenols, terpenes, lactones, alcohols, acids, and carbonyls are the most significant.
Aromatic classes
Esters in wine are formed during fermentation through the reaction between an alcohol and an acid in a process called esterification. Warmer fermentations, like those common in red winemaking, pro duce more esters, while cooler temperatures help retain esters during and after fermentation.
The primary esters of ethyl acetate (which smells like pear drops), ethyl 3-methylbutyrate (whose odor is vinous and reminiscent of pineapple), and ethyl 2-methylpropanoate (which has an apple peel-like scent) contribute fruity aromas, while ethyl cinnamate adds com plexing notes of balsamic and cinnamon.
Phenolic compounds originate in grapes and develop during fermentation and aging. The most common phenols in wine are anthocyanins and tannins that originate in the skins, seeds, and stems of grapes. As they are not volatile, you can’t smell them in isolation, but they act as precursors that contribute to and synergistically enhance the perception of volatile aromas.
As with esters, higher fermentation temperatures result in the extraction of more phenolic compounds. At low levels, 4-ethylguaiacol (woody, vanilla, barbecue), 4-ethylphenol (spicy, smoky), and 4-vinylphenol (clove, medicinal) are complexing phenols, though they quickly become off-aromas when present above threshold.
Terpenes are the origins of the floral and citrus as well as some herbal aromas found in wines. They vary greatly among grape varieties but are found in significant amounts in Muscat, Riesling, and Gewurz traminer. In the white Spanish wine analyzed by the Zaragoza researchers, geraniol (sweet, floral, specifically rose-like with a hint of citrus) was the only terpene found above threshold. Linalool and citronellol were present below threshold and contributed synergistically to the wine’s floral aromas.
Lactones are byproducts of malolactic fermentation and thus naturally present in most red wines like the young, unoaked Spanish red used in the aforementioned study. Of the eight lactones identified by researchers, none are detectable. However; whiskey lactones that result from oak aging are readily identifiable.
Volatile acids in wine are produced during winemaking. All seven identified are detectable, with the most abundant being acetic acid. At low levels, acetic acid adds brightness and complexity to aromas, becoming problematic only at higher concentrations, when it smells like vinegar.
Acetaldehyde (sharp, fruity), which is also formed during fermentation and aging, is the most abundant carbonyl in wine, followed by benzaldehyde (bitter almond) and phenylacetaldehyde (honey, sweet, floral), both of which can be de tected at the odor threshold. Some aldehydes like hexanal and hexenal and terpenes contribute to the aromas of freshly cut grass and tomato leaf found in Sauvignon Blanc, and can evoke olfactory nostalgia.
Most volatile compounds in wine are not detectable because they are present in miniscule, sub-threshold amounts, yet they work in conjunction with dominant aromas to further complexity. By identifying and quantifying these hidden aroma compounds, researchers are better able to gauge the contributions they make to our perception of wine aroma.