348 M. Ugliano and P.A. Henschke
with Cu^2 +salts that can be used in some countries to remove H 2 S and mercaptans.
As is the case with other acetate esters, they undergo slow acid-catalysed hydrolysis
to release the constituent mercaptan, which having a lower sensory threshold, can
lead to off-flavour formation during storage in bottled wine.
8D.5 Biotransformation of Grape and Oak Flavour Compounds
8D.5.1 Monoterpenes, Norisoprenoids, Aliphatics,
and Volatile Phenols
8D.5.1.1 Significance
It is well known that the secondary metabolites of grapes provide the basis of
varietal character in wine. The importantsecondary metabolites are represented
by several groups of compounds that contribute to the distinctive aroma profile
of wines made from particular varieties ofVitis vinifera. They include terpenes,
C 13 -norisoprenoids, aliphatics, benzene-derivatives, volatile phenols and long-chain
polyfuntional thiols. While constituent aroma compounds within these groups occur
in most grape varieties, it is only when one or more of these compounds occur at
concentrations well above their odour threshold that a distinctive varietal aroma
emerges.
Monoterpenes are regarded as key odorants in aromatic grape varieties ofVitis
vinifera, such as Muscat of Alexandria, Riesling and Gew ̈urztraminer, to which they
impart their characteristic floral aromas. The monoterpenes linalool, geraniol, nerol,
citronellol, and -terpineol are the more important aroma active compounds of this
group, and contribute floral, fruity and citrus attributes (Strauss et al. 1986). In the
case of non-aromatic varieties, the contribution of monoterpenes to the aroma char-
acter of wines is more subtle, although itis generally accepted that linalool, the
most powerful odorant in this group of compounds, is an important component in
the aroma of many white wines.
Several C 13 -norisoprenoids, including -damascenone, -ionone, and 1,1,6-
trimethyl-1,2-dihydronaphtalene (TDN), are powerful odorant in wine.
-Damascenone, which can be present in young wine at up to 180 times its
odour threshold of 0.05 g/L, has a stewed apple, rose, honey aroma (Francis and
Newton 2005). -Ionone, which has violet, flower, and raspberry aroma, is typically
present at higher concentration in red wines. TDN develops a kerosene-like odour
in aged whites, particularly Riesling (Francis and Newton 2005).
A variety of benzene-derivatives are found in many grape varieties, including
vinyl phenols, benzyl alcohol, 2-phenyl ethanol and raspberry ketone. Vinyl phe-
nols are characterised by spice and clove-like, 2-phenyl ethanol by rose and lilac,
and raspberry ketone by a raspberry attribute (Francis and Newton 2005). It should
be noted that, although a portion of 2-phenyl ethanol can derive from glycoside
hydrolysis, a greater proportion of this compound is formed in the metabolism of
the amino acid phenylalanine (Ugliano et al. 2006).