276 D. Dubourdieu and T. Tominaga
8B.2 Volatile Thiols Involved in the Varietal Aromas of Wine
Since the early 1990s, several highly odoriferous volatile thiolshave been identi-
fied in wines made fromVitis viniferaL. cv. Sauvignon blanc. These cover a broad
aromatic palette, including such key descriptors as: green pepper, boxwood, broom,
eucalyptus, blackcurrant buds, rhubarb, tomato leaves, nettles, grapefruit, passion
fruit, white peaches, gooseberries, and asparagus broth, as well as acacia flowers and
wood. After several years’ bottle-aging, some Sauvignon blanc wines also have hints
of smoke, roast meat, or even truffles. For many years, besides methoxypyrazines
(Augustyn et al. 1982; Allen et al. 1991) with their green pepper aroma, the com-
pounds responsible for the other odoriferous characteristics of Sauvignon blanc
wines had not been identified.
The first molecule found to be a characteristic component of Sauvignon blanc
wine aromas was 4-methy-4-sulfanylpentan-2-one (4MSP: III) (Darriet 1993;
Darriet et al. 1995) (Table 8B.1). Assays of this mercaptoketone revealed con-
centrations considerably higher than the perception threshold in certain Sauvignon
blanc wines (Tominaga et al. 1998a), confirming its decisive organoleptic role in
wines characteristic of this grape variety. This compound, with its marked aromas
of “boxwood” and “broom”, has an extremely low perception threshold (0.8 ng/L in
model solution) and concentrations in Sauvignon blanc wines where these aromas
are very strong may be as high as 40 ng/L (Tables 8B.2 and 8B.3). This compound
is found at concentrations ranging from a few to around a 100 ng/L in fresh box
leaves and leafy broom branches (Tominaga and Dubourdieu 1997). Consequently,
the “boxwood” and “broom” descriptors, used for many years to describe Sauvignon
blanc aromas, actually reflect a chemical reality.
Several other odoriferous volatile thiolswere later identified in Sauvignon blanc
wines: 3-sulfanylhexyl acetate (3SHA:XVI) (Tominaga et al. 1996), 4-methyl-4-
sulfanylpentan-2-ol (4MSPOH: VI), 3-sulfanylhexan-1-ol (3SH: XVII), and
3-methyl-3-sulfanylbutan-1-ol (3MSBOH: VIX) (Tominaga et al. 1998b)
(Table 8B.1).
The complex aroma of 3-sulfanylhexyl acetate (3SHA) is reminiscent of box-
wood, but also grapefruit zest and passion fruit. It had already been identified in
that fruit by Engel and Tressl (1991), who described it as: “extremely fruity, rem-
iniscent of passion fruit, with hints of Riesling”. The concentrations of 3SHA in
Sauvignon blanc wines analyzed ranged from 0 to several hundred ng/L. In view of
its perception threshold, 4 ng/L, its olfactory impact is highly variable, but may be
considerable in some young wines. During aging, 3SHA hydrolyzes to form 3SH.
3-Sulfanylhexan-1-ol (3SH) is always present in Sauvignon blanc, at concen-
trations of several hundred ng/L, ranging up to a fewμg/L in some wines. As a
result, this compound, which smells of grapefruit and has a perception threshold
of 60 ng/L, also has considerable olfactory impact. Wines containing the most
3SH also have the highest concentrations of its acetate. The citrus zest aroma of
4-methyl-4-sulfanylpentan-2-ol (4MSOH) plays a more limited organoleptic role.
Concentrations rarely exceed the perception threshold (55ng/L), although it may
reach this value in a few wines (Tables 8B.2 and 8B.3).