Formation of 4-hydroxy-2,5-dimethyl-3(2H)-furanone and 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone through maillard reaction based on pentose sugars

Article abstract of DOI:10.1021/jf950439o

The caramel-like smelling compounds 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) and 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone (HEMF) were identified by GC-MS and GC-MS/MS in Maillard reaction systems based on pentoses. The reaction was performed in a phosphate buffer by heating xylose, ribose, or arabinose with glycine or L-alanine at 90 °C for 1 h. HEMF was detected in the system pentose/alanine. HDMF was formed in both pentose/glycine and pentose/ alanine systems as well as directly from pentoses. Experiments using ¹³C-labeled glycine and alanine suggest the incorporation of the Strecker degradation products formaldehyde and acetaldehyde into the pentose moiety, forming the furanones HDMF and HEMF, respectively. The presence of ¹²C-HDMF, which was approximately 30% of the total HDMF amount found in xylose/glycine, indicates that HDMF is partly formed by sugar fragmentation. The proposed mechanism for the formation of the furanones is based on decomposition of the Amadori compound via 2,3-enolization, chain elongation by the Strecker aldehydes, and reduction of the resulting acetylformoin-type intermediates to the target molecules.

Full text of DOI:10.1021/jf950439o

J. Agric. Food Chem. 1996, 44, 531
−536
531
F or m a tion of 4-Hyd r oxy-2,5-d im eth yl-3(2H)-fu r a n on e a n d
4-Hyd r oxy-2(or 5)-eth yl-5(or 2)-m eth yl-3(2H)-fu r a n on e th r ou gh
Ma illa r d Rea ction Ba sed on P en tose Su ga r s
Imre Blank* and Laurent B. Fay
Nestle´ Research Center, Nestec Ltd., Vers-chez-les-Blanc, P.O. Box 44, CH-1000 Lausanne 26, Switzerland
The caramel-like smelling compounds 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) and 4-hy-
droxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone (HEMF) were identified by GC-MS and GC-
MS/MS in Maillard reaction systems based on pentoses. The reaction was performed in a phosphate
buffer by heating xylose, ribose, or arabinose with glycine or L-alanine at 90 °C for 1 h. HEMF was
detected in the system pentose/alanine. HDMF was formed in both pentose/glycine and pentose/
alanine systems as well as directly from pentoses. Experiments using 13C-labeled glycine and alanine
suggest the incorporation of the Strecker degradation products formaldehyde and acetaldehyde into
the pentose moiety, forming the furanones HDMF and HEMF, respectively. The presence of 12C-
HDMF, which was approximately 30% of the total HDMF amount found in xylose/glycine, indicates
that HDMF is partly formed by sugar fragmentation. The proposed mechanism for the formation
of the furanones is based on decomposition of the Amadori compound via 2,3-enolization, chain
elongation by the Strecker aldehydes, and reduction of the resulting acetylformoin-type intermediates
to the target molecules.
Keyw or d s: Maillard reaction; pentose model system; 4-hydroxy-2,5-dimethyl-3(2H)-furanone;
4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone; GC-MS/ MS
INTRODUCTION
As shown in model experiments, HDMF can be
formed by thermal degradation of 6-deoxysugars, e.g.
rhamnose (Hodge et al., 1963; Shaw and Berry, 1976),
and of fructose (Mills and Hodge, 1976) in the presence
of amines or amino acids. HDMF can also be generated
directly from hexoses (Shaw et al., 1968; Fagerson,
1969) and hexose-phosphates (Schieberle, 1992). The
latter author showed that acetylformoin [2,4-dihydroxy-
2,5-dimethyl-3(2H)-furanone] is a key intermediate in
the formation of HDMF from hexoses. The formation
of furanones is generally explained to occur via the 2,3-
enolization pathway leading to 1-deoxyosones as inter-
mediates (Hodge et al., 1972).
4-Hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF, Fur-
aneol, a registred trademark of Firmenich S.A., Gene`ve,
Switzerland, 1) and 4-hydroxy-2(or 5)-ethyl-5(or 2)-
methyl-3(2H)-furanone (HEMF, homofuraneol, 2) are
HEMF is produced by yeast during shoyu fermenta-
tion (Sasaki et al., 1991). Intermediates of the pentose-
phosphate cycle are important precursors in the bio-
synthesis of HEMF. Formation of HEMF during heat
processing is not well understood. In analogy to the
formation of HDMF from an Amadori compound of
rhamnose, Huber (1992) speculated on the existence of
6,7-dideoxyheptose as an intermediate.
Similar to the decomposition of hexoses, the degrada-
tion pathway of pentoses proceeds via 2,3-enolization,
forming 4-hydroxy-5-methyl-3(2H)-furanone (HMF, nor-
furaneol, 3) (Feather, 1981). For this type of reaction
the carbon skeleton of the sugar is determinant for the
furanone formed. This means that HDMF is produced
from hexoses and HMF from pentoses. To the best of
our knowledge, no information is available about the
formation of the furanones 1 and 2 from pentoses.
In the present study, we report on the identification
of HDMF and HEMF in Maillard reaction systems
based on pentoses and the amino acids glycine and
alanine. The formation of these furanones was inves-
tigated using 13C-labeled amino acids. GC-MS/MS was
applied to identify the labeled and nonlabeled fur-
anones. On the basis of the results, a formation
pathway is proposed indicating the role of Strecker
aldehydes in generating odor-active furanones.
important flavor compounds contributing to the sensory
properties of many natural products and thermally
processed foods. HDMF has been found in various fruits
such as pineapples (Rodin et al., 1965), strawberries
(Ohloff, 1969), and grapes (Rapp et al., 1980) as well as
in beef broth (Tonsbeek et al., 1968), roasted coffee
(Tressl et al., 1978), bread crust (Schieberle, 1990),
roasted beef (Cerny and Grosch, 1992), roasted sesame
seeds (Schieberle, 1993), and stewed beef (Guth and
Grosch, 1993). HEMF, which exists in the tautomeric
forms 2a and 2b in the ratio of 1:2 (Re et al., 1973), has
so far been reported in soy sauce (Nunomura et al.,
1976), roasted coffee (Tressl et al., 1978), melon (Huber,
1992), lovage (Blank and Schieberle, 1993), and Em-
mentaler cheese (Preininger et al., 1994).
Both HDMF and HEMF smell caramel-like, sweet,
and fruity and have low retronasal odor thresholds, i.e.
160 and 20 µg/kg water, respectively (Huber, 1992).
According to Hodge (1967), the caramel-like flavor is
associated with a planar enol-oxo group of cyclic
dicarbonyl compounds.
* Author to whom correspondence should be ad-
dressed (fax +41/21-785-85-54; e-mail imre.blank@
chlsnr.nestrd.ch).
0021-8561/96/1444-0531$12.00/0
© 1996 American Chemical Society

532 J. Agric. Food Chem., Vol. 44, No. 2, 1996
Blank and Fay
Ta ble 1. Sen sor y Con tr ibu tion of HDMF a n d HEMF
EXPERIMENTAL PROCEDURES
Estim a ted by GC-Olfa ctom etr ya in Ma illa r d System s
Ba sed on P en toses a n d eith er Glycin e (Gly) or L-Ala n in e
(Ala ) a n d in Mod el Rea ction s Con ta in in g On ly P en toseb
Ma ter ia ls a n d Rea gen ts. D-Xylose, D-ribose, D-arabinose,
glycine, and L-alanine of highest purity (>99%) were obtained
from Fluka (Buchs, Switzerland). [2-13C]Glycine and [1-13C]-
D-xylose were from Cambridge Isotope Laboratories (Andover,
MA), and [3-13C]-L-alanine was from Tracer Technologies
(Somerville, MA). The isotopic content of the labeled com-
pounds was 99%. Sodium sulfate, disodium hydrogen phos-
phate dihydrate, and diethyl ether were from Merck (Darm-
stadt, Germany). The reference compounds HDMF (Furaneol,
1) and HEMF (homofuraneol, 2) are commercially available
from Aldrich (Steinheim, Germany) and Givaudan (Du¨bendorf,
Switzerland), respectively. The organic solvents were purified
by slow distillation on a Vigreux column (1 m × 1 cm).
linear retention index
pentose/ pentose/
compd
OV-1701
FFAP
Glyc
Alac
pentosec
HDMF 1
HEMF 2a
HEMF 2b
1235
1310
1325
2040
2090
2190
++
-
-
+
+
-
-
+++
d
+
a
GC-O data are presented in terms of odor intensities per-
b
ceived at the sniffing port (+: weak; +++: intense). The model
mixtures were heated in a phosphate buffer (0.2 mol/L, pH 6) at
90 °C for 1 h. c The pentoses used were ribose, xylose, and
d
arabinose. The tautomer 2b was sensorily detectable only in
highly concentrated samples.
Sa m p le P r ep a r a tion . In a 15 mL Pyrex tube, 5 mmol of
pentose (xylose, ribose, or arabinose) and 5 mmol of amino acid
(glycine or alanine) were dissolved in 5 mL of phosphate buffer
(0.2 mol/L Na2HPO4, pH 6.0). The tube was sealed with a
screw cap and heated at 90 °C for 1 h in an oil bath while
stirring with a magnetic stirrer. The reaction was stopped
by rapid cooling with tap water. During the reaction, the pH
dropped to 5.0 (xylose/glycine) and 5.3 (xylose/alanine). Then,
40 mL of water was added to the dark brown reaction mixture,
which was then saturated with 16 g of NaCl. The pH was
adjusted to 4.0 (aqueous HCl, 2 mol/L), and the neutral
compounds were continuously extracted with 50 mL of diethyl
ether overnight using a rotation perforator (Normag, Wein-
heim, Germany). The organic phase was separated, dried over
sodium sulfate at 4 °C, and concentrated to 0.5 mL using a
Vigreux column (50 cm × 1 cm) and a microdistillation device
according to the procedure of Bemelmans (1979). Experiments
with the labeled compounds were performed in the same way
as described above.
RESULTS AND DISCUSSION
Id en tifica tion of HDMF a n d HEMF . The overall
aroma of Maillard reaction systems mainly depends on
the composition of sugars and amino acids as well as
on the reaction conditions. Simple Maillard systems
consisting of pentoses and glycine or alanine develop
sweet, caramel-like notes under relatively mild reaction
conditions. We produced such aroma qualities by react-
ing the pentoses xylose, ribose, and arabinose with
glycine or alanine in a phosphate buffer at 90 °C for 1
h. Similar notes were also obtained when using pen-
toses alone in the model reaction.
Gas Chromatography-Olfactometry.
A sensory-
directed chemical analysis was performed to identify the
odorants responsible for the caramel-like character. The
odor-active compounds were detected by GC-O. The
potential of the GC-O technique to select odorants from
odorless volatiles has recently been reviewed by Acree
(1993) and Grosch (1993). Caramel-like smelling re-
gions were found in the chromatograms from the model
reactions containing a pentose and glycine or alanine.
They were tentatively identified as 4-hydroxy-2,5-di-
methyl-3(2H)-furanone (HDMF, 1) and 4-hydroxy-2(or
5)-ethyl-5(or 2)-methyl-3(2H)-furanone (HEMF, 2) on
the basis of their characteristic aroma qualities and the
retention indices on two capillaries of different polarity
(Table 1). The sensory and chromatographic properties
were identical to those of the reference compounds.
HDMF was mainly responsible for the caramel-like
note of the reaction mixture containing pentose/glycine
or only a pentose (Table 1). In the system pentose/
alanine, both HDMF (1) and HEMF (2) contributed to
the caramel-like, sweet character. Both furanones were
represented by minor peaks in the gas chromatograms.
Rough estimates of the amounts of HDMF and HEMF
formed from pentoses were in the low milligrams per
kilogram (parts per million) range. The tautomers of
HEMF were base-line separated on both capillaries
used. GC-O indicated that the odor activity of 2a was
significantly higher than that of 2b (Blank, unpublished
results, 1995). As in this paper we are mainly concerned
with the mechanistic aspect, results concerning the
identification of further odorants and volatiles will be
published elsewhere. It should only be mentioned that
HMF (3), which was the major volatile in the pentose/
amino acid systems, did not contribute much to the
caramel-like aroma. This can be explained by its
relatively high retronasal odor threshold of 8300 µg/kg
of water (Huber, 1992).
Ga s Ch r om a togr a p h y-Olfa ctom etr y (GC-O). GC-O
was performed with
a Carlo Erba instrument (Mega 2)
equipped with an automatic cold “on-column” injector. At the
end of the capillaries, the effluent was split 1:1 into the FID
and a sniffing port (Blank and Schieberle, 1993). Fused silica
capillaries of medium (OV-1701) and high polarity (FFAP)
were used, both 30 m × 0.32 mm with a film thickness of 0.25
µm (J&W capillaries, Fisons Instruments, Brechbu¨hler, Schlier-
en, Switzerland). The temperature program was as follows:
50 °C (2 min), 6 °C/min to 180 °C, 10 °C/min to 240 °C (10
min). Linear retention indices were calculated according to
the method of van den Dool and Kratz (1963).
Ga s Ch r om a togr a p h y-Ma ss Sp ectr om etr y (GC-MS).
GC-MS analyses were performed on an HP-5971 mass
spectrometer (Geneva, Switzerland) connected to an HP-5890
gas chromatograph equipped with an HP-7673 autosampler.
A capillary column with Carbowax stationary phase was
employed (30 m × 0.32 mm, 0.25 µm film thickness). Helium
was used as carrier gas at a pressure of 10 psi. The oven
program was 20 °C (0.5 min), 30 °C/min to 100 °C, 4 °C/min
to 145 °C (10 min), 70 °C/min to 220 °C (2.5 min). The samples
were injected using a splitless injector heated at 250 °C, and
the interface was kept at 220 °C. The ion source working in
electron impact mode at 70 eV was held at about 180 °C.
Ga s Ch r om a t ogr a p h y-Ta n d em Ma ss Sp ect r om et r y
(GC-MS/MS). The experiments were carried out using a
Finnigan TSQ-700 mass spectrometer (Bremen, Germany)
connected to an HP-5890 gas chromatograph equipped with
an HP-7673 autosampler. The chromatographic separation
was done using the FFAP capillary column as described above.
The samples were injected in splitless mode (280 °C), and the
oven program was 60 °C (1 min), 10 °C/min to 200, 30 °C/min
to 240 °C (2 min). Helium was used as carrier gas at a
pressure of 10 psi. The ion source working in electron impact
mode at 70 eV was held at 150 °C. The detection was achieved
by tandem mass spectrometry after collision-induced dissocia-
tion (CID) of the molecular ion of the compounds. The
daughter spectra were recorded from 20 to 200 Da. A collision
energy of 10 eV in the laboratory frame was used. The
collision gas argon was set to 1.1 mTorr.
Mass Spectrometric Identification of HDMF and the
HEMF Tautomers. HDMF (1) and the two HEMF
tautomers (2a and 2b) were identified on the basis of
GC-MS data by comparison of their retention indices

Maillard Reaction Based on Pentose Sugars
J. Agric. Food Chem., Vol. 44, No. 2, 1996 533
F igu r e 1. GC-MS/MS spectra of the HEMF tautomer 2b (A)
and of HDMF (B) obtained in the reaction systems D-xylose/
L-alanine and D-xylose/glycine.
F igu r e 2. GC-MS/MS spectra of the tautomer 2-[2-13C]ethyl-
4-hydroxy-5-methyl-3(2H)-furanone 2b (A) and of 4-hydroxy-
2-[13C]methyl-5-methyl-3(2H)-furanone (B) obtained in the
reaction systems D-xylose/[3-13C]-L-alanine and D-xylose/[2-
13C]glycine (the labeled positions are marked with 9).
and mass spectra with the data available in our librar-
ies. However, in such complex matrices, some interfer-
ences can contaminate the mass spectra, particularly
at the low mass ranges (20-100 Da) where character-
istic fragments of HDMF and HEMF are observed. To
achieve unambiguous identification of these compounds,
we used tandem mass spectrometry coupled to gas
chromatography to obtain interference-free spectra. The
molecular ion of each molecule was fragmented by
collision-induced dissociation (CID), and the daughter
spectra were recorded. Comparison of the retention
indices (Table 1) and the daughter spectra obtained from
pure compounds and from the samples allowed identi-
fication of the molecules. The mass spectra of the
furanones 2b and 1, shown in parts A and B of Figure
1, respectively, were identical to those of the reference
compounds and unequivocally demonstrate the presence
of HDMF (1) and HEMF (2). To the best of our
knowledge, this is the first time that the furanones 1
and 2 have been reported in Maillard reaction systems
based on pentoses.
La belin g Exp er im en ts. Model experiments using
labeled precursors have been proven promising in
providing data from which hypotheses can be drawn on
formation mechanisms. Recently, Tressl et al. (1993)
showed the potential of using 1-13C-labeled sugars to
explain the formation of amino acid specific Maillard
reaction products. As shown in Table 1, we found that
glycine and alanine play a key role in generating HDMF
(1) and HEMF (2) from pentoses. On the basis of these
findings, we used [2-13C]glycine and [3-13C]-L-alanine to
obtain insight into the formation mechanisms of HDMF
and HEMF from pentoses using xylose as example.
Reaction of Xylose with [3-13C]-L-Alanine. The
reaction of xylose and [3-13C]-L-alanine gave rise to
unlabeled HDMF and monolabeled HEMF tautomers.
The daughter mass spectrum of the tautomer 2b shown
in Figure 2A was obtained by GC-MS/MS of the parent
molecular ion at m/z 143 using the CID fragmentation
technique. The daughter ions at m/z 127 [M - 13CH3]+
and at m/z 114 [M - CH2
-
13CH3]+ revealed the
presence of the 13C atom in the ethyl group. The
incorporation of the 13C was also indicated by the
fragments at m/z 58, 72, 87, and 100, which correspond
to the ions 57, 71, 86, and 99, respectively, in the mass
spectrum of the unlabeled HEMF (Figure 1A).
As indicated by the fragment [M - CO - CH3]+ at
m/z 100 for the labeled HEMF and at m/z 99 for the
unlabeled HEMF, as well as by the fragment [CO -
CH3]+ at m/z 43 for both compounds, the methyl group
of the labeled HEMF does not bear the 13C atom.
Therefore, the data strongly support the presence of a
13CH3CH2 group attached to C-2 of HEMF, i.e. 2-([2-
13C]ethyl)-4-hydroxy-5-methyl-3(2H)-furanone.
Reaction of Xylose with [2-13C]Glycine. In the
model reaction based on xylose and [13C]glycine, a
mixture of monolabeled and unlabeled HDMF was
detected by GC-MS. The intensity of the molecular
ions at m/z 128 and 129 revealed that about 70% of
HDMF was labeled, thus indicating that HDMF was
preferentially, but not exclusively, produced by incor-
poration of the labeled carbon of glycine. The remaining
30% might be formed by sugar fragmentation.
The daughter mass spectrum of the labeled HDMF
obtained by GC-MS/MS of the parent molecular ion at
m/z 129 using the CID technique is shown in Figure 2B.
The molecular ion and the daughter fragments at m/z
86, 73, 58, and 44 revealed the incorporation of one 13C
atom into the molecule. The ion pairs 43/44, 57/58, and
85/86 of nearly equal intensity correspond to the sym-
metric structure of HDMF which may occur as a 1:1
mixture of two isotopomers. Recently, a similar mass
spectrum was reported for singly 13CH3-labeled HDMF
detected in Maillard reaction systems based on [1-13C]-
hexoses (Tressl et al., 1993), which is in good agreement
with the MS data of the singly labeled HDMF found in
this work (Table 2).
The mass spectrum (Figure 2B) indicates further that
the labeled carbon should not be located at positions C-3

534 J. Agric. Food Chem., Vol. 44, No. 2, 1996
Blank and Fay
Ta ble 2. Ma ss Sp ectr a l Da ta of Non la beled a n d 13C-La beled HDMF
fragmentation pattern (m/z)
compd
technique
ref
HDMF
128
128
129
130
85
---
85/86
86
72
72
73
74
57
---
57/58
58
43
---
43/44
44
GC-MS
Sen et al. (1991)
this work
Tressl et al. (1993)
Sen et al. (1991)
HDMFa
GC-MS/MS
GC-MS
13CH3-HDMFb
[2,5-13CH3]-HDMF
GC-MS
a
b
Parent scan of m/z 72 obtained by GC-MS/MS using commercially available HDMF. MS data reported for a 1:1 mixture of two
singly labeled isotopomers [2-13CH3]HDMF and [5-13CH3]HDMF.
F igu r e 3. Hypothetical formation pathway of HDMF (1) and HEMF (2) from pentoses (explanation in the text): a, early stage
of Maillard reaction including Amadori rearrangement; b, degradation via 2,3-enolization; c, keto/enol or vinylogous keto/enol
tautomerization; d, cyclization; e, dehydration; f, Strecker reaction; g, aldol condensation; h, reduction
and C-4 due to the presence of the singly labeled
fragment [M - 2 × CO]+ at m/z 73. The corresponding
fragment at m/z 72 of the nonlabeled HDMF was formed
exclusively from the molecular ion as verified by a
parent scan (Table 2). The origin of this fragment is
also supported by the data reported by Sen et al. (1991)
showing the presence of the ion at m/z 74 in the
spectrum of [2,5-13CH3]HDMF (Table 2). On the basis
of the fragmentation pattern and literature data, the
13C atom might be located in the methyl group, by
analogy to HEMF preferentially at C-2, i.e. 4-hydroxy-
2-[13C]methyl-5-methyl-3(2H)-furanone.
degradation, i.e. formaldehyde and acetaldehyde. Fur-
ther enolization of the 1-deoxyosone (4) may lead via
compound (5) to the intermediate (6) which reacts with
the Strecker aldehydes via an aldol-type condensation.
The chain elongation by the Strecker aldehydes results
in the intermediates 7a and 7b containing six and seven
carbon atoms, respectively. Enolization and dehydra-
tion give rise to the cyclic intermediates acetylformoin
(8a ) and the corresponding ethyl homologue (8b). The
target molecules HDMF (1) and HEMF (2) are formed
by reduction, enolization, and water elimination of the
intermediates (8a ) and (8b). The reduction may occur
either by a dismutation or by a reaction with further
enoloxo compounds, as recently reported by Schieberle
(1992) for the formation of HDMF from (8a ) in a model
reaction based on hexoses.
Formation of HDMF (1) and HEMF (2). Considering
the results from labeling experiments, the furanones 1
and 2, having six and seven carbon atoms, respectively,
might be formed by the following reaction:
To support the hypothesis shown in Figure 3, [1-13C]-
D-xylose was reacted with [3-13C]-L-alanine to form the
double-labeled tautomers of HEMF (2) which, on the
basis of the hypothesis, should additionally be labeled
in the methyl group. Indeed, the double-labeled HEMF
tautomers were detected by GC-MS/MS. In agreement
with the mass spectrum of the singly labeled HEMF
(Figure 2A), the fragment at m/z 100 in Figure 4A
indicated the incorporation of one 13C into the C-2 ethyl
group. On the other hand, the fragment at m/z 44,
represented by the ion [CO-13CH3]+, suggested the
second labeling position in the C-5 methyl group. In
concordance with the proposed formation pathway
C5 + C1(2) f C6(7)
C5 represents the pentose, indicating that the pentose
carbon chain remains intact. The C1 and C2 units may
be formaldehyde and acetaldehyde, the Strecker deg-
radation products of glycine and alanine, respectively.
The formation mechanism proposed in Figure 3 is
based on the formation of the Amadori compound of the
pentose and its subsequent decomposition via 2,3-
enolization to form a C5 1-deoxydiketose (4). This
compound and other reductones react with the amino
acids forming the corresponding aldehydes by Strecker

Maillard Reaction Based on Pentose Sugars
J. Agric. Food Chem., Vol. 44, No. 2, 1996 535
F igu r e 5. Schematic formation of labeled HDMF and HEMF
as indicated by labeling experiments (the labeled carbons are
marked with 9).
The C1 unit formaldehyde may originate from both
Strecker degradation and sugar fragmentation, i.e. by
retro-aldol cleavage of 1-deoxyosones (Ledl and
Schleicher, 1990). This explains the presence of unla-
beled HDMF in the model reactions containing only
pentoses, xylose/[2-13C]glycine, and xylose/[3-13C]-L-ala-
nine. HDMF can be generated by recombination of
pentose fragmentation products which may also include
the fragments C2, C3, and C4 formed by retro-aldol
reaction.
F igu r e 4. GC-MS/MS spectra of the tautomer 2-[2-13C]ethyl-
4-hydroxy-5-[13C]methyl-3(2H)-furanone 2b (A) and of 4-hy-
droxy-2,5-[13C]dimethyl-3(2H)-furanone (B) obtained in the
reaction systems [1-13C]-D-xylose/[3-13C]-L-alanine and [1-13C]-
D-xylose/[2-13C]glycine (the labeled positions are marked with
9).
The results presented in this paper show the manifold
role of Strecker aldehydes. They may contribute to
flavor on their own (methional, phenylacetaldehyde,
etc.) or participate in the formation of other key odor-
ants. As shown in our laboratory, alanine and glycine
are actively involved in the formation of sensory rel-
evant 3(2H)-furanones (this paper) and alkylpyrazines
(Amrani-Hemaimi et al., 1995). Although the furanones
are formed by a side reaction, these compounds may
significantly contribute to the overall flavor due to their
low threshold values. As a continuation of this study,
the amounts of HDMF and HEMF are being quantified
and parameters affecting this reaction are currently
being studied.
(Figure 3), the data strongly support the presence of
2-[2-13C]et h yl-4-h ydr oxy-5-[13C]m et h yl-3(2H )-fu -
ranone formed in the system [1-13C]-D-xylose/[3-13C]-L-
alanine.
Following the hypothesis, the corresponding reaction
of [1-13C]-D-xylose and [2-13C]glycine should result in
13C2-HDMF labeled in both methyl groups. The mass
spectrum shown in Figure 4B indicated the incorpora-
tion of two 13C atoms, i.e. m/z 130 and 74. The fragment
at m/z 44 ([CO-13CH3]+) suggested that both methyl
groups were labeled, because the fragment at m/z 43
was not at all present in the mass spectrum. The
fragmentation pattern was in good agreement with that
of synthesized [2,5-13CH3]HDMF (Table 2) reported by
Sen et al. (1991), thus suggesting that both methyl
groups were labeled in the 13C2-HDMF produced, i.e.
4-hydroxy-2,5-[13C]dimethyl-3(2H)-furanone.
The total HDMF amount detected in the model
reaction based on [1-13C]-D-xylose and [2-13C]glycine was
composed of unlabeled (4%), singly labeled (12%), and
doubly labeled (84%) HDMF. The corresponding figures
for the system [1-13C]-D-xylose/[3-13C]-L-alanine were
15%, 47%, and 38%, respectively. The data indicate that
HDMF was generated by different types of retro-aldol
fragmentation of the pentose sugar. In the presence of
glycine, HDMF was preferably formed involving the
Strecker reaction.
ACKNOWLEDGMENT
We are grateful to Ms. S. Devaud and Mr. Y. Krebs
for skillful technical assistance. We thank Drs. C.
Cerny, T. Huynh-Ba, and G. Philippossian for valuable
discussions and Dr. E. Prior for critically reading the
manuscript.
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Received for review J uly 14, 1995. Accepted November 22,
1995.X
J F950439O
X Abstract published in Advance ACS Abstracts, Janu-
ary 15, 1996.