much on their direct impact as on their ability to regulate the fruitiness of red wines, both in terms of its complexity and its intensity, in particular via perceptive interactions with other compounds (Pineau et al., 2009; Lytra et al., 2012, 2013, 2014, 2015; Cameleyre et al., 2017).
Alkyl substituted short‐chain fatty acid ethyl esters
The overall contribution of these ethyl esters, which are present at concentrations below their aroma perception threshold, has been well known for several years (Pineau et al., 2009). Ethyl 2‐methylpropanoate and ethyl 2‐methylbutanoate contribute to the blackberry notes of red wines. Ethyl 2‐methylbuanoate plays a role in enhancing fruitiness (Lytra et al., 2014). The S enantiomer, whose aroma is reminiscent of green apples (Granny Smith) and strawberries, is almost exclusively found in red wines, at average concentrations of 50 μg/l. Its presence particularly intensifies the blackberry notes of wines.
Hydroxycarboxylic acid ethyl esters
In wines, ethyl 3‐hydroxybutanoate is present in its two enantiomeric forms (Lytra et al., 2015). In red wines, its average S/R enantiomeric ratio is approximately 75/25 (±13), with an average total concentration of ~450 (±150) μg/l. Contents of the R form progressively increase during bottle aging, but there are no variations in concentration of the S form. Ethyl (3S)‐3‐hydroxybutanoate is mainly described by solvent and alcohol notes, whereas ethyl (3R)‐3‐hydroxybutanoate has a fruitier and butyric aroma. The individual perception thresholds of the (3S) and (3R) enantiomeric forms of ethyl 3‐hydroxybutanoate as well as that of their mixture (85/15, m/m) are, respectively, 21, 63, and 14 mg/l (Table 2.6), which confirms the absence of any direct impact of this ester on fruity aroma perception in wine, since the concentrations found (on the order of microgram per liter) are considerably lower than the thresholds. Nevertheless, even under these conditions, this compound contributes to the red fruit and fresh fruit aromas of red wines, thanks to specific perceptive interactions (Lytra et al., 2015).
Alkyl substituted hydrocarboxylic acid ethyl esters
Ethyl 2‐hydroxy‐4‐methylpentanoate adds a blackberry note to wine (Falcao et al., 2012). Its content is generally higher in red wines than in white wines of the same age (Lytra et al., 2012). In general, white wines contain only the R form, while red wines have both enantiomers at ratios that depend on age. The highest concentrations of the S isomer are found in the oldest wines. The average R/S ratio for ethyl 2‐hydroxy‐4‐methylpentanoate in red wines is 95/5 (m/m). The perception threshold of ethyl (2R)‐2‐hydroxy‐4‐methylpentanoate in a dilute alcohol solution is 126 μg/l or almost twice that of the S enantiomer (55 μl). This clearly shows that these thresholds are dependent on stereochemistry (Table 2.6). The perception threshold for the mixture of the two enantiomers (R/S, 95/5 m/m) is 51 μg/l in a dilute alcohol solution. The two enantiomers have similar aromas, reminiscent of blackberries. In red wines, the presence of these compounds leads to a general intensification of aroma as well as increased fresh black fruit notes.
TABLE 2.6 Main Substituted Esters and Their Impact on Red Wines
| Compounds | Descriptors | Average content in red wines (μg/l) | Perception threshold in dilute alcohol solution (μg/l) | Sensory impact |
|---|---|---|---|---|
| 2‐Methylbutyl (2S)‐acetate | Banana | 70 | 313 | Enhancer of black fruit, fresh fruit, and jammy aromas |
| Ethyl (2S)‐2‐methylbutanoate | Green apple(Granny Smith), strawberry | 50 | 1.53 | Enhancer of black fruit aromas |
| Ethyl (2R)‐2‐hydroxy‐4‐methylpentanoate | Blackberry | 400 | 126 | Enhancer of black fruit and fresh fruit aromas |
| Ethyl (2S)‐2‐hydroxy‐4‐methylpentanoate | Blackberry | 20 | 55 | |
| Ethyl (3R)‐3‐hydroxybutanoate | Fruity, butyric (cheese) | 80 | 63,000 | Enhancer of black fruit and fresh fruit aromas |
| Ethyl (3S)‐3‐hydroxybutanoate | Solvent andalcohol | 350 | 21,000 |
Ethyl 2‐hydroxy‐3‐methylbutanoate, whose content increases during bottle aging, is present at the end of alcoholic fermentation in the R form only. The S enantiomer only appears later on (Gammacurta et al., 2015; Lytra et al., 2017).
2.5.4 Esters of Chemical Origin
The formation of esters continues throughout the aging process, thanks to the presence of nonvolatile acids in wine together with large quantities of ethanol. Research into esterification mechanisms in wine (Ribéreau‐Gayon et al., 1982) showed that, under normal cellar conditions, none of the acids ever reach the equilibrium predicted in theory. The ester content represents approximately 30% of the theoretical limit after 1 year, 50% after 2 or 3 years, and 80% after 50 years. The total ester concentration (regardless of its origins) is governed by the wine's composition and age. It varies from 2 or 3 mEq/l in young wines up to 9 or 10 mEq/l in old wines, in which approximately 10% of the acids are esterified.
Mono‐acids react with ethanol to form only neutral esters, whereas di‐acids may produce one neutral and one acidic ester (e.g. ethyl tartrate and ethyl hydrogen tartrate). On average, wine contains approximately the same quantity of neutral and acidic esters. The latter contribute to wine acidity.
Ethyl lactate is a special case. Its formation is linked to malolactic fermentation, with the possible involvement of an esterase of bacterial origin. However, concentrations of ethyl lactate also increase throughout aging via chemical reactions. In Champagne wines that have completed malolactic fermentation, the ethyl lactate concentration has been observed to increase to a maximum of 2 g/l after two years and then decrease during further aging on the lees. According to Arctander (1969), ethyl lactate has an odor reminiscent of butter or even sour milk. Other authors think that the odor of ethyl lactate has been confused with that of other aroma compounds.
2.6 Miscellaneous Compounds
Among the other volatile products likely to contribute to wine aroma are volatile phenols and sulfur derivatives. The latter are responsible for off‐odors whose causes and consequences are now well known and are described elsewhere in this volume (Sections 8.4 and 8.6). There are also several compounds that contribute to the varietal aromas of different grape varieties, e.g. terpenes in Muscats (Section 7.2.1). These compounds are also discussed at length in Chapter 7.
