alcohol content to be displayed on wine labels. Historically, wines were marketed on the basis of alcohol content.
In temperate climates, the natural alcohol content depends directly on grape ripeness. Wines only have a high alcohol level in years when the weather is particularly good, and if vineyard conditions and exposure to the sun are favorable. Great vintages are often years when wines reach high alcohol contents. It would, however, be ridiculous to suppose that alcohol is the only quality factor. Some excellent Médoc wines have an alcohol content of around 10% vol., while other high‐alcohol wines are heavy, undistinguished, and unattractive.
In chemical terms, ethanol is a primary alcohol (Figure 2.1), i.e. its C1 is tetrahedrally hybridized sp3 and carries two hydrogen atoms twinned with the hydroxyl radical (─OH). Alcohol functions should not be confused with enol functions
or phenol functions (OH bonded to a carbon in a benzene cycle). In both of these cases, carbon, the hydroxyl radical carrier, is hybridized sp2 and the acid character of the hydroxyl proton is much more pronounced. Consequently, the function is more easily salifiable.
In ethanol produced by fermentation, some hydrogen atoms on C1 and C2 in certain molecules are replaced by deuterium (2H or D), an isotope of hydrogen. These molecules are present in very small amounts, and the exact proportion depends on the origin of the sugar that was fermented (grapes, beets, and/or sugarcane), as well as on the growing region. A method for controlling the addition of sugar to must (chaptalization) and detecting fraud has been developed on the basis of this property (Martin and Brun, 1987). This method has been officially recognized by the International Organisation of Wine and Vine (OIV).
FIGURE 2.1 Structure of ethanol and definition of the alcohol function.
Ethanol's affinity for water and its solubility, by forming hydrogen bonds, makes it a powerful dehydrant. This property is useful in flocculating hydrophilic colloids, proteins, and polysaccharides. It also gives ethanol disinfectant properties that are particularly valuable in aging wine. The combination of ethanol and acidity makes it possible to keep wine for a long time without any noticeable spoilage. The addition of ethanol to stabilize certain wines is a long‐standing winemaking tradition (Port, vins doux naturels). However, ethanol is toxic for humans, affecting the nerve cells and liver.
Ethanol's solvent properties are also useful for dissolving phenolic compounds from grape skins during red winemaking. This property is involved in solubilizing certain aroma molecules and certainly contributes to the expression of aromas in wine.
Ethanol has all the chemical properties of an alcohol function. In particular, it esterifies with tartaric, malic, and lactic acids (Section 2.5.3). Ethyl acetate gives wine an unpleasant odor and is a sign of bacterial spoilage (Section 2.5.1).
Ethanol may also react with hydrogen sulfide, produced by fermenting yeast or resulting from the residues of some vineyard treatment products (Section 8.6.3). This substitution reaction generates ethanethiol (Figure 2.2), which has a very unpleasant smell. As this compound is much less volatile than H2S, it is more difficult to eliminate. It is therefore advisable to rack wines as soon as alcoholic fermentation is completed and again immediately after malolactic fermentation, since hydrogen sulfide may also be produced by lactic acid bacteria (Volume 1, Section 13.9.1). Furthermore, the oxidation–reduction equilibrium may also cause ethanethiol to form diethyl disulfide (Figure 2.3). This compound is even less volatile and has a very unpleasant smell, which is negatively perceived during wine tasting.
FIGURE 2.2 Reaction between hydrogen sulfide and ethanol.
FIGURE 2.3 Oxidation–reduction equilibrium of the thiol/disulfide system.
2.2 Other Simple Alcohols
2.2.1 Methanol
Methanol is always present in wine in very small quantities, between 60 and 150 mg/l. It has no organoleptic impact. Methanol is not formed by alcoholic fermentation, but results exclusively from enzymatic hydrolysis of the methoxyl groups of pectins during fermentation to form pectic acids:
As grapes have a relatively low pectin content, wine is the fermented beverage with the lowest methanol concentration.
Methanol content depends on the extent to which the grape solids, especially the pectin‐rich skins, are macerated. Red wines have a higher concentration (152 mg/l) than rosés (91 mg/l), while white wines have even less (63 mg/l) (Ribéreau‐Gayon et al., 1982). Wines made from hybrid grape varieties have a higher methanol content than those made from Vitis vinifera. This is due to the higher pectin content of hybrid grape skins. The use of pectolytic enzymes to facilitate extraction or clarification of the must may cause an increase in methanol as a result of the pectin esterase activity.
Methanol's toxicity is well known. Following ingestion, it oxidizes to produce formaldehyde and formic acid, both toxic to the central nervous system. Formaldehyde damages the optical nerve, causing blindness. Wines made in the normal way never have methanol concentrations anywhere near hazardous levels (Skrzydlewska, 2003).
2.2.2 Higher Alcohols of Fermentation Origin
Alcohols with more than two carbon atoms are known as higher alcohols or fusel oil (Table 2.1). Several of these are produced during fermentation. For reasons of simplicity, they are generally referred to collectively, reaching total concentrations on the order of 150–550 mg/l in wine (Ribéreau‐Gayon et al., 1982; Jackson, 1994). These alcohols and their esters have intense odors that play a role in wine aromas. The main higher alcohols of fermentation origin are isobutyl alcohol (2‐methyl‐1‐propanol) and amyl alcohols (a mixture of 2‐methyl‐1‐butanol and 3‐methyl‐1‐butanol). At low concentrations (less than 300 mg/l), they contribute to a wine's aroma complexity. At higher levels, their odors mask the wine's aroma finesse. Acetates of these alcohols, especially isoamyl acetate, have a banana odor that may play a positive role in the aroma of some young red wines (nouveau‐type).
Higher alcohols are formed by yeast, either directly from sugars or from grape amino acids by the Ehrlich reaction (Figure 2.4). This reaction is caused by the activity of a FAD+ dehydrogenase, which oxidizes amino acids into imino acids. These are hydrolyzed into α‐ketoacids and then subjected to the action of a decarboxylase with thiamine