for the formation of acrolein by double dehydration of glycerol."/>
FIGURE 2.5 Mechanism for the formation of acrolein by double dehydration of glycerol.
FIGURE 2.6 Oxidation–reduction equilibria of 2‐3‐butanediol.
The most significant role of 2,3‐butanediol is in maintaining an oxidation–reduction equilibrium with acetoin (or acetyl methyl carbinol) and diacetyl (Figure 2.6). This compound (2,3‐butanediol) is formed following the reduction of acetoin, produced by the condensation of two acetaldehyde molecules.
Acetoin has a slight milky odor and is present at concentrations on the order of 10 mg/l. Diacetyl has a pleasant odor of butter, which may be perceptible at low concentrations (2 mg/l). The diacetyl concentration in wine is generally on the order of 0.3 mg/l.
These two volatile compounds are found in brandy. The concentration in brandy depends on that in the wine, and also on the distillation technique, making it possible to distinguish between Cognac, made by double distillation, and Armagnac, which is distilled only once.
Erythritol (Table 2.3) is also a C4 molecule, but it has four alcohol functions. Small quantities, 30–200 mg/l, are formed by yeast. It is not known to have any special properties.
2.3.3 C5 Polyol: Arabitol
Small quantities (25–350 mg/l) of arabitol are also known to be formed by yeast (Table 2.3). This compound has five alcohol functions and is directly derived from arabinose. Small quantities may also be produced by lactic acid bacteria and larger quantities by B. cinerea.
2.3.4 C6 Polyols: Mannitol, Sorbitol, and meso‐Inositol
These three compounds (Table 2.3) have six alcohol functions. The first two are linear, while the third is cyclic.
Mannitol is derived from the reduction of the aldehyde group on mannose. In wine, it is produced by the reduction of the ketone group on fructose by lactic acid bacteria. Mannitol is usually present in very small quantities. Higher concentrations are due to lactic acid bacteria or possibly B. cinerea. Abnormally high concentrations indicate severe lactic spoilage.
Sorbitol results from the reduction of the aldehyde group on glucose. This diastereoisomer of mannitol is totally absent from healthy grapes. Varying quantities are formed when B. cinerea develops. Alcoholic fermentation produces approximately 30 mg/l. Lactic acid bacteria do not form this compound. Large quantities of sorbitol indicate that wine has been mixed with fruit wines. Besides rowan berries (Sorbus aucuparia, hence its name), apples, pears, and cherries also have a high sorbitol content.
meso‐Inositol is a normal component of grapes and wine. It is a cyclic polyol with six carbon atoms, each carrying a hydroxyl radical. Among the nine inositol stereoisomers, which are all diastereoisomers of each other, meso‐inositol has a plane of symmetry passing through C1 and C4. This polyol is widespread in the animal and plant kingdoms. It is a vital growth factor for many microorganisms, especially certain yeasts.
TABLE 2.4 Aliphatic Fatty Acids Among the Volatile Components in Wine(Ribéreau‐Gayon et al., 1982)
| Formula | Name | Boiling point (°C) | Concentration (g/l) | Comments |
|---|---|---|---|---|
| H−COOH | Formic | 101 | 0.05 | |
| CH 3 −COOH | Acetic | 118 | 0.5 | |
| CH3−CH2−COOH | Propionic | 141 | Traces | |
| CH3−CH2−CH2−COOH | Butyric | 163 | Traces | |
|
|
Isobutyric | 154 | Traces | 2‐Methyl‐propionic acid |
| CH3−CH2−CH2−CH2−COOH | Valerianic | 186 | Traces | |
|
|
Isovalerianic | 177 | ? | 3‐Methyl‐butyric acid |
|
|
2‐Methyl‐butyric | ? | ||
| CH3−(CH2)4−COOH | Caproic | 205 | Traces | Hexanoic acid |
| CH3−(CH2)5−COOH | Enanthic | 223 | Traces | Heptanoic acid |
| CH3−(CH2)6−COOH | Caprylic | Traces | Octanoic acid | |
| CH3−(CH2)7−COOH | Pelargonic | 253 | ? | Nonanoic acid |
| CH3−(CH2)8−COOH | Capric | 270 | Traces | Decanoic acid |
It is difficult to attribute any organoleptic