1 Biochemistry of Alcoholic Fermentation 21
contribute to a wine’s aroma. Although alcoholic fermentation does not produce
enough acetoin and diacetyl to have a real impact on wine aroma, the subsequent
development of lactic acid bacteria can considerably increaseits concentration.2.Ethanal: Ethanal, also called acetaldehyde, is an intermediary of alcoholic fer-
mentation obtained by thedecarboxylation of pyruvate.Later, ethanal is mainly
reduced to ethanol, but little quantities of it may be released into the wine.
Ethanal gives off a characteristic aroma that contributes to the perception that the
wine is oxidized, although alcoholic fermentation usually produces only small
amounts of this compound. However, ethanal can also be produced from ethanol
by chemical or biological oxidation. Some wines, such as Fino and Manzanilla
from Jerez, or the wine Jaune from Jura, give off a particular aroma which is
characterized by a high concentration of ethanal. These wines are obtained by
aging the wine under a film ofSaccharomyces cerevisiae, which is responsible
for producing ethanal from ethanol.3.Acetic acid: Acetic acid is the main volatile acid of wine. Its presence at high
concentrations gives off a vinegar odour and a disagreeable sensation in the
mouth. For that reason, volatile acidity is one of the most important analyti-
cal parameters in oenology. Acetic acid may be produced by yeast, lactic acid
bacteria and acetic acid bacteria. But normallySaccharomyces cerevisiaeonly
produce small quantities of acetic acid if there are no problems during alcoholic
fermentation (0.1–0.3 g/l). However, stuck and sluggish fermentations can gen-
erate large amounts of this acid. The high production of acetic acid may be due
to the development of lactic disease or because yeasts produce more acetic acid
than normal by hydrolysis of acetyl-coA.4.Higher alcohols: Higher alcohols are produced as a deviation of the metabolism
of amino acids. Higher alcohols are produced when keto acids corresponding
to the carbon skeleton from the different amino acids are decarboxylated and
reduced. Higher alcohols are normally below their limit of detection but they are
the precursors of some esters, which have a large sensory impact.5.Esters: Esters are synthesized from acyl-coA and alcohols by a group of enzymes,
the alcohol-acylcoA transferases (Barre et al. 1998). Basically there are two types
of esters in wine: the acetates of higher alcohols and the esters of fatty acids and
ethanol. The first group are synthesised from acetyl-coA and the different higher
alcohols. These esters give off different odours, such as glue (ethyl acetate),
banana (isoamyl acetate) or rose (phenylethanol acetate). The other group of
esters are synthesised from the different acyl-coA and ethanol. The different
esters of fatty acids and ethanol give off a fruity aroma. All esters, with the
exception of ethyl acetate, give off an agreeable smell and contribute positively
to the wine aroma. Other esters such asethyl lactate and diethyl succinate do not
have any sensory impact at normal concentrations.6.Succinic acid: Succinic acid is quantitatively the third product of alcoholic fer-
mentation. Some authors suggest that succinic acid is synthesised via Krebs cycle
although its functioning is seriously limited (Heerde and Radler 1978). However,
other authors think that this cycle is not operative in conditions of grape juice
fermentation (Salmon et al. 1987). In any case, succinic acid is present in wine