Aroma compounds formed from 3-methyl-2,4-nonanedione under photooxidative conditions

Article abstract of DOI:10.1021/jf021110n

The behavior of the prominent aroma compound 3-methyl-2,4-nonanedione under photooxidative conditions was investigated in a model experiment. The four well-known aroma compounds 2,3-butanedione, 2,3-octanedione, acetic acid, and caproic acid were identified. The main oxidation product was 3-hydroxy-3-methyl-2,4-nonanedione, an aroma compound with the odor description of rubbery, earthy, and plasticlike (GC-O). Its structure has been tentatively assigned based on mass (GC-MS) and vapor phase infrared spectra (GC-IR). The formal formation pathways are discussed for these compounds, and other origins described in the literature are presented.

Full text of DOI:10.1021/jf021110n

3426 J. Agric. Food Chem. 2003, 51, 3426−3428
Aroma Compounds Formed from 3-Methyl-2,4-nonanedione
under Photooxidative Conditions
ISABELLE A. SIGRIST, GIUSEPPE G. G. MANZARDO, AND RENATO AMAD OÅ *
Institute of Food Science and Nutrition, Swiss Federal Institute of Technology, Schmelzbergstrasse 9,
ETH-Zentrum, CH-8092 Zurich, Switzerland
The behavior of the prominent aroma compound 3-methyl-2,4-nonanedione under photooxidative
conditions was investigated in a model experiment. The four well-known aroma compounds 2,3-
butanedione, 2,3-octanedione, acetic acid, and caproic acid were identified. The main oxidation product
was 3-hydroxy-3-methyl-2,4-nonanedione, an aroma compound with the odor description of rubbery,
earthy, and plasticlike (GC-O). Its structure has been tentatively assigned based on mass (GC-MS)
and vapor phase infrared spectra (GC-IR). The formal formation pathways are discussed for these
compounds, and other origins described in the literature are presented.
KEYWORDS: Aroma compounds; 3-methyl-2,4-nonanedione; 2,3-butanedione; 2,3-octanedione; 3-hy-
droxy-3-methyl-2,4-nonanedione; gas chromatography-mass spectroscopy; gas chromatography-infrared
spectroscopy
INTRODUCTION
-Methyl-2,4-nonanedione (1) (Figure 1) is the main con-
3
tributor to the off-flavor of reversed soybean oil (1-3), to the
light-induced off-flavor of butter and butter oil (4), and to the
haylike off-flavor that develops in dry parsley (5) and dry
spinach (6). In addition, diketone 1 is reported to contribute
significantly to the flavor of green tea (7, 8) and anchovy (9,
10). Recently, this compound was also detected in dry tarragon,
dry chervil, dry chive, and dry leek after light exposure (11).
Compound 1 is derived from the photosensitized oxidation of
the furan fatty acids 10,13-epoxy-11,12-dimethyloctadeca-10,
Figure 1. Structures of compounds 1−6.
1
1
2-dienoic acid (2) and 12,15-epoxy-13,14-dimethyleicosa-12,
4-dienoic acid (3) (12) (Figure 1). The present investigation
2
0 mg of meso-tetraphenyl porphyrine, and 144 µg of ethyl decanoate
as an internal standard in 200 mL of hexane was exposed to light
20 000 lx) for 24 h under slightly bubbling oxygen. The light source
was undertaken in the context of studies on the development
of 1 in food during light exposure (13) with the aim of
investigating the behavior of pure 1 under photooxidative
conditions.
(
consisted of two 150 W sodium vapor high-pressure lamps with a
spectrum similar to sunlight (VIALOX NAV E 150 de Luxe, Osram
AG, Winterthur, Switzerland) each placed at a distance of 0.45 m from
the reaction vessel. After 24 h, 0.6 mg of BHT was added and the
solution was concentrated to 3 mL at reduced pressure by means of a
Vigreux column and ice cooling. Oxidation in the absence of light was
carried out using the same procedure. The samples were stored at -80
MATERIALS AND METHODS
Chemicals. Oxygen was from Pan Gas (Lucerne, Switzerland), and
2
,6-di-tert-butyl-p-toluene (BHT), etyhl decanoate, hexane, and meso-
tetraphenyl porphyrine were from Fluka (Buchs, Switzerland). Com-
pound 1 was synthesized according to reference 1 and purified according
to reference 14. The reference substances 2,3-butanedione (4) and
hexanal were from Fluka, acetic acid was from Merck (Dietikon,
Switzerland), and 2,3-octanedione (5) was synthesized according to
reference 15.
°
C before gas chromatography/mass spectrometry (GC-MS) analysis.
Capillary GC-MS. GC-MS was performed using the on-column
injection technique on a GC 8065 gas chromatograph (Fisons Instru-
ment, Milano, Italy) directly coupled to a SSQ 710 mass spectrometer
(Finnigan, San Jose, CA). A polar-fused silica SW-10 column (60 m,
320 µm i.d., 0.25 µm film thickness, Supelco, Bellefonte, U.S.A.) with
a deactivated fused silica precolumn (2.8 m, 530 mm i.d., J & W
Scientific, Folsom, U.S.A.) was used with helium as the carrier gas
(100 kPa). The temperature program was as follows: 90 °C for 5 min,
Oxidation of MND. The reaction was carried out according to
reference 16 at 4 °C in a special three neck reaction flask (d
) 10 cm, h ) 14 cm) with a fritted glass bottom (DEMA 13/21,
Hans Mangels, Bornheim, Germany). A solution of 0.17 mmol of 1,
1
) 4 cm,
d
2
9
0-240 °C at 10 °C/min, 240 °C for 10 min. EI mass spectra were
recorded with an ionization energy of 70 eV in a mass range of 40-
420 amu. Data processing was achieved with ICIS 2, version 8 software
*
To whom correspondence should be addressed. Tel: ++41 1 632 32
9
1. Fax: ++41 1 632 11 23. E-mail: renato.amado@ilw.agrl.ethz.ch.
1
0.1021/jf021110n CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/25/2003

Aroma Compounds from 3-Methyl-2,4-Nonanedione
J. Agric. Food Chem., Vol. 51, No. 11, 2003 3427
Figure 2. GC-MS chromatogram of products formed from 1 under
photooxidative conditions in hexane. For peak identification, see Table
1.
Figure 3. GC-MS spectrum of 6.
a
Table 1. Main Products Derived from 1 under Photooxidative
Conditions in Hexane
peak retention
photooxidative
conditions
control
experimentd
b
identificationc
no.
index
compd
e
1
2
3
4
5
6
7
9
996
1301
1449
1640
1727
1751
1861
2270
4
5
RS
RS
RS
2
5
1
nd
f
tr
acetic acid
ethyl decanoate RS
6
1
caproic acid
unknown
nd
1
tr
103
nd
nd
1
MS, IR
RS
RS
(99, 71, 43)
11
46
5
2
a
Data expressed as peak area ratio to the internal standard ethyl decanoate.
b
c
Peak numbers corresponding to those in Figure 2. RS, retention index and
mass spectrum consistent with reference substance; numbers in parentheses,
principal ions observed in decreasing order of intensity. Without light. e Not
detected. Traces.
d
Figure 4. GC-IR spectrum of 6.
f
considered. Among these products, the four well-known aroma
compounds 4, 5 (Figure 1), acetic acid, and caproic acid were
identified. The main oxidation product of 1 was 3-hydroxy-3-
methyl-2,4-nonanedione (6) (Figure 1), an aroma compound
with the odor description of rubbery, earthy, and plasticlike (GC-
O). Its structure has been tentatively assigned based on mass
(GC-MS) and vapor phase infrared spectra (GC-IR). In the mass
spectrum of 6 (Figure 3), the molecular ion was not observed.
The fragment ions at m/z 43, 71, and 99 are formed by
R-cleavage whereas the fragment ions at m/z 88 and 144 are
attributed to McLafferty rearrangements. In the infrared spec-
trum of 6 (Figure 4), the OH stretching band of the tertiary
(
Finnigan). Identification of the products is based either on comparison
of retention indices (according to van den Dool and Kratz (17)) and of
the mass spectra by spiking with reference substances or on mass
spectral and infrared characteristics. The products were determined in
relation to the peak area of the internal standard ethyl decanoate.
Capillary GC/Infrared Spectroscopy (GC-IR). GC-IR was per-
formed using the on-column injection technique on a HP 5890 gas
chromatograph (Hewlett-Packard Co.) with a HP 5965 IR detector
(
Hewlett-Packard Co.). A DB 1701 column (10 m, 180 µm i.d., 0.4
µm film thickness, J & W Scientific) with helium as the carrier gas
100 kPa) was used. The temperature program was as follows: 40 °C
(
for 1 min, 40-300 °C at 10 °C/min, 300 °C for 10 min. Data processing
was achieved with GRAMS/32 software (Portmann Instruments, Biel-
Benken, Switzerland).
Capillary GC/Olfactometry (GC-O). For sniffing experiments, a
GC flame ionization detection system of the type HP GC 5890 series
II (Hewlett-Packard Co.) was equipped with a column end split, leading
to a sniffing port for olfactometry. A polar-fused silica SW-10 column
-
1
hydroxy group of 6 is exhibited at 3478 cm . According to
van der Maas (18), the OH stretching band of a free (tertiary)
-1
OH group can be expected at about 3615 cm for 6. Evidently,
the OH group is hydrogen-bonded to a CdO group; conse-
quently, its stretching frequency is lowered and the peak is
broadened. The formation of a hydroxylated dione as an
oxidation product of 1 is also supported from the literature.
House and Gannon (19) have shown that the oxidation of
enolizable â-diketones with peracids leads to R-hydroxy-â-
diketones. Furthermore, the formation of R-hydroxy-â-diketones
has been reported by Yoshioka et al. (20) in the reaction of
enolizable â-diketones with singlet oxygen.
(60 m, 320 µm i.d., 0.25 µm film thickness, Supelco) with helium as
the carrier gas (100 kPa) was used. The temperature program was as
follows: 90 °C for 7 min, 90-150 °C at 20 °C/min, 150 °C for 10
min, 150-240 °C at 20 °C/min, 240 °C for 10 min. Samples were
injected in the split injection mode (split 1:12). Data processing was
achieved with a HP 3365 Series II integrator (Hewlett-Packard Co.)
and Chem Station Version A.03.34 software (Hewlett-Packard Co.).
In a control experiment, it was shown that the diketones 4-6,
as well as acetic and caproic acid, are only formed under
photooxidative conditions. Without light exposure (Table 1,
control experiment), these compounds were either absent or
present in minute amounts only.
RESULTS AND DISCUSSION
The chromatogram presented in Figure 2 illustrates the
volatile profile of products formed from 1 (Figure 1) under
photooxidative conditions (24 h in hexane). The main products
are reported in Table 1. Only products with a peak area ratio
to the internal standard ethyl decanoate of 1 or more were
The occurrence of a compound with the same mass spectral
characteristics as 6 has already been reported for green tea and

3428 J. Agric. Food Chem., Vol. 51, No. 11, 2003
Sigrist et al.
animal fat. Horita and Hara (21) described an unknown
compound in an investigation on aroma concentrates of green
tea after exposure to light. Braggins (22) reported on an
unknown odor compound of rendered sheep fat, which elutes
in the same region as 6 under comparable chromatographic
conditions. The odor of the compound was described by this
author as plasticlike.
Following Yoshioka et al. (20), the formation of 6 can be
explained through an ene reaction of the enol form with singlet
oxygen to give the R-hydroperoxy-â-diketone. This compound
is subsequently cleaved to the corresponding R-hydroxy-â-
diketone. The formation of 4, 5, acetic acid, and caproic acid
can formally be explained by the oxidation of the enolic double
bonds of the two most favorable enol forms of 1: oxidation of
the 2,3-enol leading to acetic acid and 5 and oxidation of the
(8) Guth, H.; Grosch, W. Identification of potent odourants in static
headspace samples of green and black tea powders on the basis
of aroma extract dilution analysis (AEDA). FlaVour Fragrance
J. 1993, 8, 173-178.
(9) Triqui, R.; Reineccius, G. A. Flavor development in the ripening
of anchovy (Engraulis encrasicholus L.). J. Agric. Food Chem.
1
995, 43, 453-458.
(
10) Triqui, R.; Reineccius, G. A. Changes in flavor profiles with
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(
(
3
,4-enol to caproic acid and 4.
Diketone 5 was reported to be an autoxidative degradation
product of methyl arachidonate (23, 24) and an auto- and
photooxidative degradation product of furan fatty acid 2 (16,
2
5), respectively. Artz et al. (24) identified diketone 4 as an
â-diketones of the type RCOCH
6, 1220-1222.
2
COR. J. Am. Chem. Soc. 1944,
oxidation product of methyl arachidonate, whereas Hollnagel
and Kroh (26) have shown this compound to be a reaction
product from nonenzymatic browning of D-glucose, D-fructose,
maltose, and maltulose, respectively, in model experiments.
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of 2 (16, 25). Compounds 1, 4, 5, and 6 have been shown in
green tea to increase during light exposure (13).
6
(
(
(
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(
(
ACKNOWLEDGMENT
Ms. Brigitte Jegge and Ms. Sandra K u¨ rsteiner-Laube are
acknowledged for technical assistance in the synthesis of 1 and
(
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methyl 1,3-diketones. J. Chem. Soc., Perkin Trans. 1998, 1, 283-
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, and Mr. Simon Kollaart is acknowledged for performing part
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University of Utrecht, The Netherlands) is gratefully acknowl-
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(
(
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(
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