8D Yeasts and Wine Flavour 333
8D.4.2 Higher Alcohols
8D.4.2.1 Significance
Higher alcohols, sometimes referred to as fusel alcohols, are, from a quantitative
point of view, the most important group of volatile compounds produced by yeast
during alcoholic fermentation of sugars. They are characterised by containing more
than two carbon atoms and include the branched-chain alcohols 2-methylpropanol
(isobutanol), 2-methylbutanol (amyl alcohol), and 3-methylbutanol (isoamyl alco-
hol), and the aromatic alcohols 2-phenylethanol and tyrosol. According to Rapp
and Versini (1996) concentrations of higher alcohols below 300 mg/L add desirable
complexity to wine, whereas higher concentrations can be detrimental to wine qual-
ity. Conversely, 2-phenylethanol is generally a positive contributor to wine aroma,
being characterised by a pleasant rose-like aroma (Swiegers et al. 2005). The con-
centrations of higher alcohols that act positively or negatively on wine aroma are
likely to depend on aroma intensity and style of wine. Correlation of the composition
of wine volatile compounds with data obtained by formal descriptive sensory anal-
ysis of white wines suggests that for Chardonnay wines 2-methyl propanol and 2-
and 3-methylbutanol were important whereas for Riesling wines none of the higher
alcohols measured were important to the aroma profile (Smyth et al. 2005).
8D.4.2.2 Metabolism
Higher alcohols are formed by decarboxylation and subsequent reduction of -
keto-acids produced as intermediates of amino acids biosynthesis and catabolism
(Fig 8D.5). The latter pathway has been named after Ehrlich, its discoverer. Studies
with labelled amino acids have shown that the greater proportion of higher alcohols
is synthesized from the -keto acids derived from glycolysis and destined for amino
acids biosynthesis (Ayr ̈ ̈ ap ̈a ̈a 1971; Chen 1978). Accordingly, amino acid biosyn-
thesis accounts for most of the higher alcohols formed during fermentation. This is
consistent with the observations that consumption of the structurally related amino
acids is not correlated with the final concentration of the corresponding higher
alcohols.
The structurally related amino acids for the branched chain aliphatic alcohols
2-methylpropanol, 2-methylbutanol and 3-methylbutanol are valine, leucine and
isoleucine, respectively. For the aromatic alcohols 2-phenylethanol, tyrosol and
tryptophol, they are phenylalanine, tyrosine and tryptophan, respectively, and for the
thioalcohol methionol, the corresponding amino acid is methionine. Uptake of the
branched-chain amino acids is mediated by several transport proteins: the branched-
chain amino acid permease Bap2p and Bap3p, and the general amino acid permease
Gap1p. The aromatic amino acids are transported by Tat1p and Tat2p, and also
Gap1p and Bap2p, whereas methionine is transported by Mup1p, Mup3p and Gap1
(Regenberg et al. 1999). The first step in the Ehrlich degradation pathway involves
transamination to form the -keto acids, catalysed by branched-chain (Bat1p and
Bat2p) and aromatic (Aro8p and Aro9p)amino acid transferases, (Hazelwood