Handbook of Enology, Volume 2. Pascal Ribéreau-Gayon

102 Valeraldehyde 3‐Methylbutanal 92 Isovaleraldehyde CH₃−CH₂−CH₂−CH₂−CH₂−CHO Hexanal 128 Caproaldehyde CH₃−CH₂−CH₂−CH=CH−CHO 2‐Hexenal Only presentin grapes CH₃−(CH₂)₅−CHO Heptanal 155 Enanthaldehyde CH₃−(CH₂)₆−CHO Octanal 167 Caprylaldehyde CH₃−(CH₂)₇−CHO Nonanal 185 Pelargonaldehyde CH₃−(CH₂)₈−CHO Decanal 208 Caprinaldehyde CH₃−(CH₂)₁₀−CHO Dodecanal Lauraldehyde CH₃−CO−CH₃ Propanone 56 Acetone CH₃−CH₂−CO−CH₃ Butanone 80 Methyl ethyl ketone CH₃−CH₂−CH₂−CO−CH₃ 2‐Pentanone 102 CH₃−CHOH−CO−CH₃ Acetyl methyl carbinol 143 Acetoin CH₃−CO−CO−CH₃ Diacetyl 87 4‐Mercapto‐4‐methyl‐2‐pentanone Boxwood aroma of Sauvignon Blanc wines Benzaldehyde 178 Vanillin 285 Cinnamaldehyde 253 Hydroxymethylfurfural Grape juice or wine subjected to heat treatment

      2.6.2 Acetals

Acetal is formed every time an aldehyde comes into contact with an alcohol. The reaction involves two alcohol molecules and one aldehyde molecule, as shown in Figure 2.9.

About 20 compounds of this type have been reported in wine. The most important of these, diethoxyethane, results from a reaction between acetaldehyde and ethanol (Figure 2.10). Acetalization is a slow, reversible reaction, catalyzed by H⁺ ions. The reaction is completed in a few hours at pH 2–3, while it takes several days at pH 4. In 10% vol. alcohol solution, 3% of the acetaldehyde may react, while 6.5% reacts if the alcohol content is 20% vol.

      In view of the very small quantities of free acetaldehyde present in still wines, their acetal content is practically zero. Only wines with a high acetaldehyde content have a significant concentration of acetal. Sherry, with an acetaldehyde concentration on the order of 280 mg/l, contains 45–60 mg/l, while the concentration in Vin Jaune from the Jura region of France may be as high as 150 mg/l. These are all oxidized wines.

FIGURE 2.9 Formation of an acetal.

FIGURE 2.10 Acetalization of acetaldehyde and formation of diethoxyethane.

FIGURE 2.11 Formation of γ‐butyrolactone.

      Acetals have a vegetal odor that may add to the aroma complexity of Sherry. Diethoxyethane is described by Arctander (1969) as having a pleasant, fruity odor.

      2.6.3 Lactones

      Lactones are formed by an internal esterification reaction between an acid function and an alcohol function in the same molecule. This reaction produces an oxygenated heterocycle.

Volatile lactones, produced during fermentation, are likely to contribute to wine aroma. The best known is γ‐butyrolactone, present in wine at concentrations on the order of a mg/l. This compound results from the lactonization of γ‐hydroxybutyric acid, an unstable molecule produced by deamination and decarboxylation of glutamic acid, according to the Ehrlich reaction