section is thus exclusively devoted to carbonyl compounds, lactones, and acetals.
2.6.1 Carbonyl Compounds (Aldehydes and Ketones)
Acetaldehyde is the most important of these compounds (Table 2.7). The many ways it can be produced, its high reactivity, its rapid binding with sulfur dioxide at low temperatures, and its organoleptic properties make acetaldehyde a very important component of wine. The presence of acetaldehyde, produced by the oxidation of ethanol, is closely linked to oxidation–reduction phenomena. It is involved in the alcoholic fermentation mechanism. Furthermore, acetaldehyde plays a role in the color changes occurring in red wines during aging by facilitating the copolymerization of phenols (anthocyanins and catechins) (Section 6.3.10).
In wine preserved with regular, light sulfiting, the bound form of acetaldehyde and sulfite (CH3−CHOH−SO3H), stable in an acidic medium, is the most prevalent form (Volume 1, Section 8.4.1). When grapes have been heavily sulfited, the acetaldehyde concentration increases and may exceed 100 mg/l in the bound form with sulfite. This sulfite binding of acetaldehyde protects yeast from the antiseptic effects of SO2.
Wines containing excess acetaldehyde as compared with the quantity of SO2, i.e. free (unbound) acetaldehyde, are described as “flat” (Section 8.2.3). A slight trace of free acetaldehyde is sufficient to produce a characteristic odor, reminiscent of oxidized apple. This problem disappears rapidly if a little SO2 is added, as it binds with the free acetaldehyde. This is one of the reasons for sulfiting barrels during racking (Section 10.3.3).
A few other aldehydes are present in wine in trace amounts (Table 2.7). These include Strecker aldehydes, associated with the oxidative aging of white wines, such as methional, formed from methionine and phenylacetaldehyde (from phenylalanine) (Section 8.6). The neutralizing effect of sulfur dioxide on the fruitiness of certain white wines is due to the fact that it binds with the aldehyde fraction.
Aldehydes in the aromatic series are also present in wine. The most significant of these is vanillin, which is associated with barrel aging and has a distinctive vanilla aroma.
Grapes apparently contain few aldehydes. Hexenal and hexenol have, however, been identified as contributing to the vegetal and grassy odors of C6 compounds (Section 2.2.3).
Several molecules with ketone functions have been identified, including propanone, butanone, and pentanone. As previously mentioned, the most important of these are acetoin (acetyl methyl carbinol) and diacetyl (Section 2.3.2).
Compounds with more than one aldehyde or ketone function have been identified: glyoxal and methyl glyoxal (Volume 1, Section 8.4.4).
Finally, a mercaptopentanone (4‐MMP) has been identified, along with various other thiols, as one of the specific components of Sauvignon Blanc aroma (Section 7.5.1).
TABLE 2.7 Aldehydes and Ketones in Wine
| Formula | Name | Boiling point (°C) | Comments |
|---|---|---|---|
| H−CHO | Methanal | 21 | Formaldehyde |
| CH3−CHO | Ethanal | 21 | Acetaldehyde. In bound form with SO2. Only oxidized wines (Rancio, Sherry, etc.) contain free acetaldehyde |
| CH3−CH2−CHO | Propanal | 49 | |
| CH3−CH2−CH2−CHO | Butanal | 76 | Butyraldehyde |
|
|
2‐Methyl‐propanal | 92 | Isobutyraldehyde |
| CH3−CH2−CH2−CH2−CHO | Pentanal | 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 |
