Distribution and organoleptic impact of sotolon enantiomers in dry white wines

Article abstract of DOI:10.1021/jf072337r

The enantiomers of sotolon, a flavor compound typical of oxidized white wines, were separated by preparative HPLC to determine their perception thresholds and distribution in wines. The enantiomeric ratios of chiral sotolon were evaluated in several dry white wines using gas chromatography and a chiral column (β-cyclodextrin) connected to a 2 m precolumn (BP20). The perception threshold of (S)-sotolon (0.8 μg/L) in model wine solution was 100 times lower than that of the (R) form (89 μg/L), indicating that (S)-sotolon contributes to the characteristic aroma of prematurely aged dry white wines. Both enantiomers are detected in white wines. Analysis of commercial dry white wines from various vintages and origins revealed three types of distribution patterns: the racemic form, an excess of R, and an excess of S. The proportions found in these wines may be partially explained by the slow racemization kinetics (20 months) of optically active sotolon.

Full text of DOI:10.1021/jf072337r

1606 J. Agric. Food Chem. 2008, 56, 1606–1610
Distribution and Organoleptic Impact of Sotolon
Enantiomers in Dry White Wines
ALEXANDRE PONS,*,†,§ VALÉRIE LAVIGNE,†,§ YANNICK LANDAIS,#
§
PHILIPPE DARRIET,§ AND DENIS DUBOURDIEU
Seguin Moreau France, Z.I. Merpins, B.P. 94, 16103 Cognac, France; Université Bordeaux 1, Institut des
Sciences Moléculaires, CNRS UMR 5255, 351 cours de la Libération, 33405 Talence Cedex, France; and UMR
1219 Oenologie-Ampelologie, Institut des Sciences de la Vigne et du Vin, Université Victor
Segalen Bordeaux 2, 351 cours de la Libération, 33405 Talence Cedex, France
The enantiomers of sotolon, a flavor compound typical of oxidized white wines, were separated by
preparative HPLC to determine their perception thresholds and distribution in wines. The enantiomeric
ratios of chiral sotolon were evaluated in several dry white wines using gas chromatography and a
chiral column (ꢀ-cyclodextrin) connected to a 2 m precolumn (BP20). The perception threshold of
(S)-sotolon (0.8 µg/L) in model wine solution was 100 times lower than that of the (R) form (89 µg/L),
indicating that (S)-sotolon contributes to the characteristic aroma of prematurely aged dry white wines.
Both enantiomers are detected in white wines. Analysis of commercial dry white wines from various
vintages and origins revealed three types of distribution patterns: the racemic form, an excess of R,
and an excess of S. The proportions found in these wines may be partially explained by the slow
racemization kinetics (20 months) of optically active sotolon.
KEYWORDS: Sotolon; enantiomers; wine; racemization; chiral GC
INTRODUCTION
enantiomers of sotolon and determine the perception thresholds
and distribution of the R and S forms in wine to obtain an
accurate assessment of their organoleptic impact. These data
may also contribute to our understanding of the origins of the
chemical formation of this compound during the oxidative aging
of white wines.
Sotolon [3-hydroxy-4,5-dimethyl-2(5H)-furanone] is a chiral
lactone with an intense curry odor. This volatile compound is
known to impact flavor in many foodstuffs and wines. It
contributes significantly to the burnt note of cane sugar (1) and
aged sake (2), as well as the curry odor of fenugreek seeds (3).
Sotolon contributes to the aromas of “vins jaunes” from the
Jura and sherries (4, 5), as well the “dried fig” and “rancio”
nuances in French fortified wines [Vins doux Naturels (VDN)]
and port (6, 7). Concentrations of sotolon found in these types
of wines are generally >10 µg/L. Sotolon has also been detected
in white wines made from botrytized grapes (8, 9). The
contribution of this powerful flavor compound to the aroma of
prematurely aged dry white wines has now also been clearly
established (10–12). Concentrations in prematurely aged dry
white wines are generally <10 µg/L (10, 13). The perception
threshold is 2 µg/L in model solution and 8 µg/L in dry white
wine. All of these analyses were obtained using a synthetic
racemate molecule.
The stereoisomer distribution of sotolon in sherry and Vin
jaune has already been analyzed by two-dimensional gas-phase
chromatography coupled with an FID detector (16). However,
to our knowledge, their distribution in dry white wines aged
under reducing conditions has not previously been reported.
MATERIALS AND METHODS
Reagents. 4,5-Dimethyl-3-hydroxy-2(5)H-furanone (>99%), dichlo-
romethane (Chromasolv grade), ethyl acetate (HPLC grade), isopropanol
(HPLC grade), and heptane (HPLC grade) were obtained from Sigma-
Aldrich (St Quentin Fallavier, France). Anhydrous sodium sulfate (99%)
was supplied by Prolabo (France). L(+)-Tartaric acid (99.5%) was
supplied by Fluka (France) and sodium hydroxide (99%) by Riedel de
Haen. Ethanol (Lichrosolv grade) was supplied by Merck (France).
Wine Samples. White wines from several vintages and origins were
all analyzed in 2005. The composition of the model wine solution was
as follows: L(+)-tartaric acid (5 g/L), ethanol (12% vol.), adjusted with
NaOH (M) to pH 3.5.
The perception threshold and descriptors of an odoriferous
compound may differ according to the stereoisomer considered
(14, 15). It was, therefore, important to separate the two
Sotolon Extraction from Wine. The extraction procedure was based
on the method described by Cutzach (7). Wine samples (100 mL) were
spiked with 100 µL of 3-octanol (100 mg/L solution in ethanol), as an
internal standard, and 15 g of anhydrous sodium sulfate (to increase
ionic strength and extractability). Wines were extracted three times with
10, 5, and 5 mL of CH2Cl2 (magnetic stirring for 10, 5, and 5 min at
* Author to whom correspondence should be addressed (e-mail
alexandre.pons@oenologie.u-bordeaux2.fr; telephone 33 5 40 00 64
90; fax 33 5 40 00 64 68).
† Seguin Moreau France.
§ Université Victor Segalen Bordeaux 2.
# Université Bordeaux 1.
10.1021/jf072337r CCC: $40.75
2008 American Chemical Society
Published on Web 02/14/2008

Sotolon Enantiomers in White Wines
J. Agric. Food Chem., Vol. 56, No. 5, 2008 1607
750 rpm). The resulting organic phases were mixed and dried over
anhydrous sodium sulfate. The organic extract was then concentrated
to 0.5 mL under a nitrogen stream (100 mL/min). Two microliters of
the extract was injected into a gas chromatograph (GC) with a mass
spectrometer (MS) detector.
Table 1. Impact of Injector Temperature on the Racemization of Sotolon
in GC-MS
injector
(R)-enantiomeric excess (%)
(R - S)/(R + S)
temp (°C)
Racemization Studies in Wine Model Solution. Samples (500 µL)
were extracted twice with 200 µL of ethyl acetate (vortex 2, 2 min).
Organic phases were mixed and dried over anhydrous sodium sulfate.
Two microliters of the extract was injected into a GC with an MS
detector.
150
180
190
200
210
230
99
98
97
92
79
65
Gas Chromatography-Mass Spectrometry (GC-MS) Conditions.
A Star 3400CX gas chromatograph (Varian) fitted to a Saturn 2000
electronic ion trap mass spectrometer from Varian was used to analyze
the wine extracts. The precolumn was a fused silica column coated
with polar phase (BP20; 2 m × 0.25 mm i.d. internal diameter; 0.25
µm film thickness) (SGE, France) followed by a 30 m × 0.25 mm i.d.
fused silica column coated with a 0.25 µm film of a solution of 20%
ꢀ-cyclodextrin in (35% phenyl)-methylpolysiloxane (HPchiral) from
J&W (France). The carrier gas was He (Linde gas, Bordeaux), 5.3 grade,
with a flow rate of 1 mL/min. A Varian 1078 temperature-program-
mable injector was used to inject the 2 µL sample. The injector was
initially set at 180 °C for 0.3 min, and then the temperature was raised
to 230 at 180 °C/min for 30 min. Oven temperature was initially set at
50 °C for 0.1 min, raised to 110 at 1 °C/min, held at that temperature
for 20 min, then raised to 210 at 3 °C/min, and held at that temperature
for a further 20 min.
Odor Thresholds. The sotolon enantiomer perception thresholds
were determined using the method described by Boidron (9). The
sensory panel consisted of 60 persons from 20 to 40 years old, who
received weekly training sessions. Tests were performed at a controlled
room temperature of 20 °C, in individual booths, using covered AFNOR
glasses containing about 40 mL of liquid. Olfactory thresholds were
measured using ranking tests, with five series of triangular tests,
presented with increasing sotoloncontent. The perception threshold
corresponded to the minimum concentration recognized by 50% of the
tasters. The neutral white wine used to determine olfactory thresholds
was previously extracted and analyzed and then supplemented with
sotolon.
Racemization of (R)-Sotolon. A 40 mL sample of a model wine
solution was supplemented with (R)-sotolon (100 mg/L) [enantiomeric
excess (ee) ) 99.5%] and divided among four 10 mL flasks. To study
the influence of the model wine solution’s pH on the racemization of
(R)-sotolon, NaOH was added to obtain pH values of 3.0 and 3.5 (WTW
720 pH-meter; VWR, France). Samples (in duplicate) were kept in the
dark, at room temperature (20 °C). Every 15 days in the first month,
then once per month, an aliquot of each sample (500 µL) was extracted
with ethyl acetate and injected into the GC-MS. The sotolon extraction
procedure and chromatographic conditions were as described above.
The transfer line and manifold were maintained at 210 and 80 °C,
respectively. Trap temperature was maintained at 170 °C. Axial
modulation was 3.5 V. Injection (2 µL) was in splitless mode (closure
time ) 0.75 min). For the sotolon detection segment, 55-80 min after
the beginning of the GC run (Tr ) 55–80 min), current intensity was
20 µamp and the offset multiplier for the voltage value was at 100 V.
The mass spectra were acquired in electron impact mode (ionization
energy ) 70 eV), limiting the mass range between 81 and 130 (SIS
mode). Ion 83 was used to quantify sotolon, and ion 128 confirmed its
presence.
RESULTS AND DISCUSSION
Impact of Temperature Injection by GC on Racemization
of the Sotolon Enantiomer in White Wine Extract. Many
optically active compounds are racemized simply by heating
them (17). For example, Marriot (18) demonstrated partial
racemization of linalool during the distillation (T ) 100 °C) of
a lavender extract. The thermodegradation of sotolon is one of
the known properties applied in this GC-MS on-column assay
mode (13, 19). In a previous work, sotolon degradation was
shown to occur in the gas chromatograph injector at tempera-
tures above 180 °C (20), suggesting that it could be linked with
the racemization phenomenon. However, to our knowledge,
examples of partial or total racemization of volatile compounds
in the chromatograph injector have not been widely reported.
We determined the optimum injector temperature, under our
assay conditions. A model dry white wine solution supplemented
with 10 µg/L sotolon was extracted, as previously described.
The organic extract obtained was analyzed repeatedly by GC-
MS, varying the injector temperature from 150 to 230 °C. The
results obtained are shown in Table 1. Partial racemization was
observed at high temperature (230 °C), with a decrease in the
enantiomeric excess (ee) value from 99 to 65%. Therefore, lower
temperatures (180 °C) would be more appropriate for studying
the distribution of sotolon enantiomers in wine.
Racemization studies were carried out with a GC HP-5890 (Agilent,
France) coupled to MS HP-5972, fitted with the capillary column
previously described. The operating parameters were as follows:
isothermal injection temperature, 180 °C; transfer line temperature, 230
°C. The GC temperature program was the same as for Varian GC.
Impact of Injection Temperature on Sotolon Racemization. A
model solution (12% vol EtOH; 5 g/L tartaric acid, pH 3.5) supple-
mented with (R)-sotolon (10 µg/L) was extracted, as previously
described. The resulting organic extract was analyzed by GC-MS
(Varian), varying the injector temperature from 150 to 230 °C. Injection
was repeated three times at each temperature. The programmable
injector temperature settings and chromatographic conditions were as
described above.
Separation of (R)-Sotolon and (S)-Sotolon from a Commercial
Racemic Mixture. HPLC Separation. HPLC was performed on a
Merck L-7100 pump connected to a Merck variable-wavelength UV
detector. The preparative column was a Chiralpak AS-H model (250
× 20, 5 µm). The eluent consisted of 10% isopropanol in n-heptane
with a flow rate of 20 mL/min at a constant temperature of 25 °C.
When the two compounds were collected after HPLC purification, 50
mg of each sotolon enantiomer was obtained.
Purity Control. The purity of each enantiomer was assessed by HPLC
fitted with a Chiralpak AS-H 5 µm (250 × 4.6 mm). Detection
wavelength was 250 nm. Enantiomeric excesses were 99.5 and 98.5%
for the first and second eluting peaks, respectively.
Distribution of the Sotolon Enantiomers in Dry White
Wine. The optical isomers of sotolon in the dry white wines
Chiral Identity. The absolute configuration of sotolon was elucidated
by measuring the specific rotation values of this optically active
furanone. For the first eluting peak: [R]D20 ) -10.2° (c 0.45; CHCl3)
and for the second eluting peak: [R]D20 ) +14.4° (c 0.5; CHCl3). These
values are close to those obtained by Guichard (16) for the optical
were separated by chiral GC analysis on a ꢀ-cyclodextrin phase
with a polar precolumn. Each enantiomer had previously been
injected separately to identify its retention time (Figure 1). The
commercial dry white wines were tasted by a panel of well-
trained jury prior to analysis. All of the wines were systemati-
cally found to be strongly oxidized with odors reminiscent of
honey.
20
rotation of (R) and (S)-sotolon: [R]D ) -19.3 (c 1.15; CHCl3) and
20
[R]D ) +16.5° (c 1.2; CHCl3), respectively.
Optical Rotation. Optical rotation was determined on a Perkin-
Elmer 341 polarimeter using a 1 dm cell.

1608 J. Agric. Food Chem., Vol. 56, No. 5, 2008
Pons et al.
Figure 1. GC-MS (m/z 83) analysis of (a) a commercial racemic sotolon and (b) a dry white wine organic extract on a ꢀ-cyclodextrin column with a 2 m
polar precolumn. The optical isomers were assigned as described under Materials and Methods.
Table 2. Examples of Enantiomer Distributions and Determination of the
Enantiomeric Excess of Sotolon in Several Dry White Wines
Table 3. Perception Thresholds and Olfactory Descriptors of the Sotolon
Enantiomers
(R - S)/(R + S)
enantiomeric excess (%)
perception
R (%)
S (%)
threshold (µg/L)
descriptorsa
commercial sotolon
52
48
4
(R)-sotolon
(S)-sotolon
racemic
89a–121b
0.8a–5b
2a–8b
walnut, rancid
curry, walnut
curry, walnut
1980 Graves
1981 Graves
52
51
53
52
49
47
54
48
49
47
48
51
53
46
4
2
6
4
2
6
8
a Wine model solution. b White wine.
1981 Pessac Leognan
2003 Entre-Deux-Mers
1975 Pessac Leognan
1992 Pessac Leognan
2000 Entre-Deux-Mers
2000 Entre-Deux-Mers
1973 Pessac Leognan
1997 Pessac Leognan
1999 Bordeaux
45
23
38
22
22
55
77
62
78
78
10 (S)
54 (S)
24 (S)
56 (S)
56 (S)
Figure 2. Sotolon racemization diagram.
enantiomers. Descriptors are given for an odor activity value
(OAV) of 2. Although the aromatic nuances of both enantiomers
were quite similar, the perception threshold of (S)-sotolon in
dilute alcohol solution (12% vol) was >100 times lower than
that of (R)-sotolon (Table 3). These results clearly demonstrated
that the thresholds strongly depend on the stereochemistry of
the odorant.
1999 Bordeaux
1987 Pessac Leognan
1975 Pessac Leognan
2001 Entre-Deux-Mers
71
60
75
29
40
25
42 (R)
20 (R)
50 (R)
The perception threshold of (S)-sotolon was 10 times lower
than the level determined by Guichard (7 µg/L) (14), whereas
that of (R)-sotolon was quite similar (89 µg/L). Thresholds
determined in white wine differed to a lesser extent. Indeed,
the perception threshold of (S)-sotolon in wine was 20 times
lower than that of (R)-sotolon (Table 3).
Chiral GC analysis of these white wines from several vintages
and origins revealed three types of distribution patterns: the
racemic form, an excess of (R), and an excess of (S). The vintage
had no apparent impact on the ratio of the (R) and (S) forms of
sotolon (R2 ) 0.06). The maximum enantiomeric excess was
50% for the (R) form and 56% for the (S) form.
This demonstrated that the S form alone gives sotolon its odor
and organoleptic properties. Only (S)-sotolon contributes to the
characteristic aroma of prematurely aged dry white wines at
the concentrations found, that is, generally <10 µg/L. Therefore,
the aromatic impact of sotolon in dry white wines depends
on the distribution of the (R) and (S) forms.
Racemization of (R)-Sotolon in Wine Model Solution. On
the basis of our previous findings concerning the distribution
of sotolon enantiomers in wine, we hypothesized that racem-
ization (Figure 3) occurred during bottle aging.
The enantiomeric composition of an (R)-sotolon solution was
analyzed over 4 months (Figure 2). We observed very slow
racemization kinetics. This optically pure sotolon enantiomer
was not stable in a model wine solution, resulting in 25%
racemization within 4 months. In the pH range of white wine,
between pH 3.0 and 3.5, racemization was apparently only
slightly affected by variations in pH. Raab (21) found similar
results concerning the racemization of enantiomers of Furaneol
[4-hydroxy-2,5-dimethyl-3(2H)-furanone], which is closely
Commercial sotolon, used as a benchmark, is a racemic
mixture, so it does not correspond to the average distribution
of the R and S forms in wine. The olfactory perception threshold
determined using commercial racemic sotolon (2 µg/L in dilute
alcohol solution) was inadequate for assessing the real olfactory
impact of this compound in wine. Moreover, the presence of
enantio-enriched sotolon in wine was not systematic. This agrees
with Guichard’s findings (16) in a study of the enantiomeric
distribution of sotolon in Vin jaune. The racemization of sotolon
in wine may provide a partial explanation of this enantiomeric
distribution. (Table 2).
Perception Thresholds and Descriptors of the Sotolon
Enantiomers. The sotolon optical isomers were separated by
chiral phase HPLC analysis on a semipreparative Chiralpak
AS-H column. HPLC analysis (using the same analytical
column) of the collected fractions revealed enantiomeric ex-
cesses of up to 98%. The separate sotolon enantiomers were
dissolved in ethanol and stored at low temperature (5 °C) and
then used to determine the perception threshold of sotolon

Sotolon Enantiomers in White Wines
J. Agric. Food Chem., Vol. 56, No. 5, 2008 1609
the (R) and (S) enantiomers in various proportions, as revealed
above in a wide range of white wines, is more difficult to
rationalize. Two possible pathways have, as yet, received only
limited attention. Sotolon may be generated in wine in enan-
tiomerically pure form (ee > 98%), via the transformation of
natural enantiomerically pure chiral sources (possibly including
amino acids, sugars, etc.). This was recently suggested to explain
the formation of sotolon from 4-hydroxy-L-isoleucine (26) in
fenugreek oleoresin. The enantiomerically pure isomer of
sotolon then racemizes slowly over a period of time, depending
on the aging conditions. A second route may also be envisioned,
whereby sotolon is generated by a stereoselective (enantiose-
lective) reaction, resulting in an enantio-enriched form (with
ee varying from >0 to >98%) (27). Sotolon generated via these
processes may indeed present a wide range of enantiomeric
purities, depending on the type of reaction. Sotolon formed in
this way may also racemize over time, as described above.
Whereas the first pathway has already been explored (26), the
second route, involving stereoselective mechanisms, has yet to
be investigated.
Figure 3. Racemization kinetics of sotolon at pH 3.0 (4) and 3.5 (/). A
regression coefficient (first order) was calculated for the racemization
experiment at pH 3.0.
Conclusions. Understanding the mechanisms associated with
the degradation of white wine flavor is an important field of
research in wine chemistry. Several teams have made consider-
able efforts to identify and quantify the key odorants responsible
for deteriorating wine flavor. However, from our point of view,
the organoleptic role played by sotolon was previously under-
estimated, possibly due to the lack of information concerning
its enantiomers in wine.
The separation of sotolon enantiomers in wines using chiral
GC-MS clearly showed the presence of nonracemic forms. The
organoleptic properties of the sotolon enantiomers were also
studied. Compared to the (R) form, (S)-sotolon was found to
possess, by far, the more intense organoleptic properties. Its
perception thresholds in model wine solution and white wine
were 0.8 and 5 µg/L, respectively. Consequently, the aromatic
impact of sotolon in dry white wines depends on the distribution
of the (R) and (S) forms.
Hence, knowledge of the olfactory properties of the sotolon
enantiomers proved to be fundamental for assessing its olfactory
impact on wine, as assaying overall concentrations produced
inadequate results. The presence of nonracemic forms of sotolon
in the wines analyzed suggested that stereoselective mechanisms
were responsible for the excess of one of the enantiomers. It is
now clear that the origin of sotolon is far more complex than
expected, and further studies will be required to elucidate the
formation of enantio-enriched sotolon during white wine aging.
Figure 4. Sotolon racemization via enolization.
related to sotolon. These results are also consistent with data
published for the pH dependency of 3-hydroxyfuran ketone
formation, showing maximum stability for the keto form at pH
3.5–5.5 (22). Our initial results indicated, with a nonlinear first-
order kinetic, that racemization of (R)-sotolon in a model wine
solution at pH 3.0 occurred over 20 months. Further details
concerning the determination of the racemization rate are
published elsewhere (17). This result disagrees with the race-
mization kinetics calculated by Fronza (23). However, the
experiments that led the author to conclude that this furanone
was unstable were carried out under extreme conditions (12 N
HCl, 100 °C), very different from those prevalent during wine
aging.
We suggest that, in a mildly acidic medium (pH 3–3.5),
racemization occurs via a keto–enol tautomerism, catalyzed by
the presence of an acid (21). This may involve the formation
and subsequent protonation of an aromatic furan-type intermedi-
ate, resulting in either enantiomeric form of sotolon, as illustrated
in Figure 4. An alternative pathway, via the acid-mediated
opening of a furanone ring, followed by a 1,4-addition of a
molecule of water onto the corresponding R,ꢀ-unsaturated
system and cyclization, as recently proposed (13), cannot be
completely ruled out. However this mechanism appears less
likely in our case, as that reaction took place in 5 M sulfuric
acid at pH 1, much harsher conditions than those we chose to
mimic natural wine aging.
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Received for review August 3, 2007. Revised manuscript received
December 28, 2007. Accepted January 8, 2008. We thank Seguin
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