via a chemical pathway involving esterification of substituted acids, which are themselves produced during alcoholic fermentation (Diaz‐Maroto et al., 2005; Lytra et al., 2017).
TABLE 2.5 Changes in Fatty Acid Ester Concentrations (in μmol/l) Depending on the Aging Time at 25°C and at Two Different pH Values (Garofolo and Piracci, 1994)
| Compounds | pH = 3.00 | pH = 3.50 | ||||||
|---|---|---|---|---|---|---|---|---|
| 0 months | 2 months | 5 months | 29 months | 0 months | 2 months | 5 months | 29 months | |
| Hexyl acetate | 1.90 | 1.20 | 0.00 | 0.00 | 1.70 | 1.50 | 0.40 | 0.00 |
| Isoamyl acetate | 36.60 | 13.30 | 3.10 | 0.40 | 36.50 | 20.60 | 14.00 | 2.50 |
| 2‐Phenylethyl acetate | 11.00 | 2.40 | 0.50 | 0.50 | 4.80 | 3.40 | 2.60 | 0.88 |
| Ethyl hexanoate | 12.20 | 8.70 | 6.40 | 4.30 | 11.00 | 8.80 | 8.40 | 4.60 |
| Ethyl octanoate | 9.30 | 9.00 | 7.40 | 6.40 | 5.70 | 5.50 | 5.50 | 3.69 |
| Ethyl decanoate | 2.70 | 3.40 | 3.10 | 2.00 | 1.20 | 1.20 | 1.40 | 0.79 |
Their sensory importance is based not so 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 |