Flavor Contribution and Formation of the Intense Roast-Smelling Odorants 2-Propionyl-1-pyrroline and 2-Propionyltetrahydropyridine in Maillard-Type Reactions

Article abstract of DOI:10.1021/jf971101s

Application of aroma extract dilution analysis on two different model mixtures of proline and glucose, reacted under aqueous or dry-heating conditions, revealed 2-propionyl-1-pyrroline (PP) and 2-propionyltetrahydropyridine (PTHP; occurring in two tautomers), besides 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine, as important roast-smelling odorants in both mixtures. A comparison of the isotope distribution in PP and PTHP formed from either ¹³C₆-labeled or unlabeled glucose suggested a formation pathway for both odorants from the same intermediate, 1-pyrroline, when reacted with either 2-oxobutanal (yielding PP) or 1-hydroxy-2-butanone (yielding PTHP). 2-Oxobutanal, a possible precursor of 1-hydroxy-2-butanone, was shown to be formed in high yields (29 mol %) by reacting acetaldehyde and glycolaldehyde, two well-known degradation products of carbohydrates.

Full text of DOI:10.1021/jf971101s

J. Agric. Food Chem. 1998, 46, 2721−2726
2721
F la vor Con tr ibu tion a n d F or m a tion of th e In ten se Roa st-Sm ellin g
Od or a n ts 2-P r op ion yl-1-p yr r olin e a n d
2-P r op ion yltetr a h yd r op yr id in e in Ma illa r d -Typ e Rea ction s
Thomas Hofmann and Peter Schieberle*
Institut fu¨r Lebensmittelchemie and Deutsche Forschungsanstalt fu¨r Lebensmittelchemie,
Lichtenbergstrasse 4, D-85748 Garching, Germany
Application of aroma extract dilution analysis on two different model mixtures of proline and glucose,
reacted under aqueous or dry-heating conditions, revealed 2-propionyl-1-pyrroline (PP) and
2-propionyltetrahydropyridine (PTHP; occurring in two tautomers), besides 2-acetyl-1-pyrroline and
2-acetyltetrahydropyridine, as important roast-smelling odorants in both mixtures. A comparison
of the isotope distribution in PP and PTHP formed from either 13C6-labeled or unlabeled glucose
suggested a formation pathway for both odorants from the same intermediate, 1-pyrroline, when
reacted with either 2-oxobutanal (yielding PP) or 1-hydroxy-2-butanone (yielding PTHP). 2-Oxo-
butanal, a possible precursor of 1-hydroxy-2-butanone, was shown to be formed in high yields (29
mol %) by reacting acetaldehyde and glycolaldehyde, two well-known degradation products of
carbohydrates.
Keyw or d s: Maillard reaction; labeling experiments; 2-acetyl-1-pyrroline; 2-acetyltetrahydropyridine;
2-propionyl-1,4,5,6-tetrahydropyridine; 2-propionyl-3,4,5,6-tetrahydropyridine; 2-propionyl-1-pyrroline
INTRODUCTION
Besides AP and ATHP, 2-propionyl-1-pyrroline (PP)
has been identified as a further key odorant in freshly
popped corn (Schieberle, 1991) and, very recently, in the
roasted skin of fried chicken (Kerscher and Grosch,
The Maillard-type reaction between the amino acid
proline and reducing carbohydrates is well-known to
generate popcorn-like, roasty odors upon thermal treat-
ment (Hunter et al., 1969; Lane and Nursten, 1983).
2-Acetyltetrahydropyridine [ATHP, occurring in an
equilibrium of two tautomers; cf. Tressl et al. (1981) and
Schieberle (1990a)] and 2-acetyl-1-pyrroline (AP) exhibit
such odor notes at the very low odor thresholds of 0.2
and 0.02 ng/L (in air), respectively (Schieberle, 1995a).
Both odorants have earlier been identified in the
volatile fraction of several Maillard-type model reactions
containing proline (Hunter et al., 1969; Tressl et al.,
1981), and it had, therefore, been proposed that ATHP
and AP mainly contribute to the overall roasty odors of
such processed flavors. The application of gas chroma-
tography olfactometry (GCO) techniques, such as Charm
analysis, or aroma extract dilution analysis (AEDA) [cf.
review by Schieberle (1995b)] allows the evaluation of
the relative odor potencies of a single volatile in complex
extracts of volatile compounds by means of the “extract
dilution to odor threshold in air” approach. Using
Charm analysis, Roberts and Acree (1994) recently
proved the two tautomers of ATHP to be the key
odorants (74% of Charm) in a thermally treated (200
°C; 1 min; aqueous conditions) equimolar mixture of
proline and glucose, followed by AP (18% of Charm). 2,3-
Butanedione (4% of Charm) and 4-hydroxy-2,5-di-
methyl-3(2H)-furanone (4% of Charm) were found to
contribute less to the overall odor. Similar results for
ATHP and AP had earlier been reported for a heated
proline/2-oxopropanal mixture (Schieberle, 1990a).
personal communication).
2-Propionyltetrahydro-
pyridines had been tentatively identified in thermally
treated proline/glucose systems (Tressl et al., 1981).
However, to date, these compounds have not been
characterized in foods or reaction flavors based on
sensory and analytical data.
To get insight into the precursors and formation of
the propionyl homologues of AP and ATHP, the key
odorants formed during thermal treatment of proline/
glucose mixtures were reinvestigated by AEDA and
their formation was elucidated using 13C-labeled glu-
cose.
EXPERIMENTAL PROCEDURES
Ch em ica ls. 2,3-Butanedione, 1-hydroxy-2-butanone, ac-
etaldehyde, glycolaldehyde, 1,2-diaminobenzene, and 4-hy-
droxy-2,5-dimethyl-3(2H)-furanone were from Aldrich (Stein-
heim, Germany). [13C6]Glucose was from Sigma (Munich,
Germany).
Syn th eses: P r ep a r a tion of Roa st Od or a n ts. AP and
ATHP were prepared using the new synthetic approaches
described recently (Hofmann and Schieberle, 1998a). PP was
prepared from 1-pyrroline and 2-oxobutanal as previously
reported (Schieberle, 1991). 1-Pyrroline was generated by
oxidation of L-proline as described recently (Schieberle, 1991).
[13C4]-2,3-Butanedione was synthesized as recently described
(Schieberle and Hofmann, 1997a).
2-Propionyltetrahydropyridine (PTHP). On the basis of a
reaction route verified recently for the homologous ATHP
(Hofmann and Schieberle, 1998b), 1-pyrroline (0.1 mmol) and
1-hydroxy-2-butanone (0.1 mmol) were reacted at 100 °C in
phosphate buffer (50 mL; 0.1 mol/L; pH 7.0) for 30 min. The
volatiles formed were isolated by extraction with diethyl ether
followed by sublimation in vacuo (Sen et al., 1991). 1H NMR
* Author to whom correspondence should be addressed
(telephone +49-89-289 141 70; fax +49-89-289 141 83; e-mail
LEBCHEM.Schieberle@lrz.tu-muenchen.de).
S0021-8561(97)01101-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/06/1998

2722 J. Agric. Food Chem., Vol. 46, No. 7, 1998
Hofmann and Schieberle
F igu r e 2. Mass spectra of 2-acetyl-3,4,5,6-tetrahydropyridine
(A) and 2-acetyl-1,4,5,6-tetrahydropyridine (B).
F igu r e 1. Mass spectra (MS/EI) of 2-propionyl-3,4,5,6-tet-
rahydropyridine (I; A) and 2-propionyl-1,4,5,6-tetrahydro-
pyridine (II; B).
and 13C measurements in an aliquot of the distillate gave
signals for 2-propionyl-1,4,5,6- and 2-propionyl-3,4,5,6-tet-
rahydropyridine, which were in agreement with those pub-
lished recently by De Kimpe and Keppens (1996). High-
resolution gas chromatography on an SE-54 column- showed
two main products eluting at retention indices (relative to
n-alkanes) of 1148 (I) and 1239 (II). Their mass spectra are
displayed in Figure 1 [A (I) and B (II)]. By MS/CI, for both a
molecular mass of 139 was established. To correlate the GC
peaks with the structure of the tautomers, compound I was
isolated by preparative gas chromatography using the method
described recently (Schieberle, 1991). The signals obtained by
1H NMR were identical with the data for the synthetic
2-propionyl-3,4,5,6-tetrahydropyridine (De Kimpe and Kep-
pens, 1996). On the basis of this result, compound II was
assigned as 2-propionyl-1,4,5,6-tetrahydropyridine. This order
of elution is further corroborated by the following results: It
was recently shown (Hofmann and Schieberle, 1998b) that on
an SE-54 column the imine tautomer of 2-acetyl-3-methyltet-
rahydropyridine is eluted before the enamine tautomer. Fur-
thermore, the higher intensity of the fragment m/z 57 (CH3-
CH2CtO+) found for I (Figure 1A) is more reasonable for the
imine tautomer (cf. parts A and B of Figure 1). Similar
differences were also found in the mass spectra of the
homologous 2-acetyltetrahydropyridines, also showing a more
intense fragment of the acetyl group m/z 43 (CH3CtO+) in the
imine tautomer (cf. parts A and B of Figure 2). Both propio-
nyltetrahydropyridines showed the same low odor threshold
of 0.2 ng/L in air.
heated for 10 min at 160 °C in closed glass vials (1 cm i.d.;
total volume ) 10 mL). Mixture II was extracted with diethyl
ether (total volume ) 300 mL), and the volatiles were isolated
by sublimation in vacuo (Sen et al., 1991).
The distillates obtained from both mixtures were concen-
trated to 0.2 mL by distilling off the solvent at 35 °C using a
Vigreux column (60 cm × 1 cm i.d.) followed by microdistilla-
tion (Schieberle, 1991). The most odor-active volatiles were
then evaluated by AEDA as described previously (Schieberle,
1991).
La belin g Exp er im en ts. In mixtures I and II, D-glucose
was substituted by [13C6]-D-glucose and, after thermal treat-
ment, the volatile fractions were isolated as described above.
The carbon isotope ratios in the labeled odorants generated
were determined by mass chromatography of the cluster of
molecular ions generated in the electron impact mode. The
relative concentration of each isotopomer was calculated by
using the computer of the mass spectrometer. The data were
compared with results obtained for the respective unlabeled
aroma compounds.
High -Resolu tion Ga s Ch r om a togr a p h y (HRGC)/Ma ss
Sp ectr om etr y (MS). HRGC was performed with a type 5160
gas chromatograph (Fisons Instruments, Mainz, Germany) by
using the following capillaries: FFAP (30 m × 0.32 mm fused
silica capillary, free fatty acid phase, 0.25 µm; J &W Scientific,
Fisons Instruments, Mainz, Germany) and SE-54 (30 m × 0.32
mm fused silica capillary DB-5; 0.25 µm; J &W Scientific,
Fisons Instruments). The samples were applied by the on-
column injection technique at 40 °C. After 2 min, the tem-
perature of the oven was quickly raised at 40 °C/min to 50 °C
(SE-54) or 60 °C (FFAP), respectively, held for 5 min isother-
mally, then raised at 6 °C/min to 230 °C, and held for 15 min.
The flow of the helium carrier gas was 2.5 mL/min. At the
end of the capillary, the eluate was split 1:1 (by volume) into
an FID and a sniffing port using deactivated but uncoated
fused silica capillaries (50 cm × 0.32 mm). The FID and the
sniffing port were held at 180 °C. Linear retention indices
(RI) of the compounds were calculated from the retention times
Mod el Rea ction s: Isola tion of th e Vola tiles. Two
different model systems were studied. In mixture I, a diluted
aqueous mixture of L-proline (2 mmol) and D-glucose (1 mmol)
was boiled in phosphate buffer (200 mL; pH 7.0; 0.1 mol/L)
for 2 h and the volatiles formed were simultaneously steam-
distilled and extracted using the apparatus described by
Nickerson and Likens (1966). In mixture II, L-proline (2 mmol)
and D-glucose (1 mmol) were mixed with silica gel (2.7 g
containing 300 µL of the same phosphate buffer) and then

Formation of 2-Propionyl-1-pyrroline and 2-Propionyltetrahydropyridine
J. Agric. Food Chem., Vol. 46, No. 7, 1998 2723
Ta ble 1. Key Od or a n ts (F D g 16) Gen er a ted in Th er m a lly Tr ea ted P r olin e/Glu cose Rea ction Mixtu r esa
FD factord in
RI on
no.
odorantb
2,3-butanedione
2-acetyl-1-pyrroline
2-propionyl-1-pyrroline
2-acetyl-3,4,5,6-tetrahydropyridine
4-hydroxy-2,5-dimethyl-3(2H)-furanone
2-acetyl-1,4,5,6-tetrahydropyridine
2-propionyl-3,4,5,6-tetrahydropyridine
2-propionyl-1,4,5,6-tetrahydropyridine
unknown
odor qualityc
SE-54
mixture I
mixture II
1
2
3
4
5
6
7
8
9
buttery
592
922
16
64
16
4096
<1
4096
128
128
<1
16
16384
512
2048
8192
2048
128
popcorn-like
popcorn-like
popcorn-like
caramel-like
popcorn-like
popcorn-like
popcorn-like
roasty
1024
1049
1068
1145
1148
1239
1352
128
16
a
Mixtures of L-proline (2 mmol) and D-glucose (1 mmol) were reacted under different conditions. Mixture I, simultaneous steam
b
distillation/extraction for 2 h; mixture II, dry heating for 10 min at 160 °C. The compound was identified by comparing it with the
reference substance on the basis of the following criteria: retention index (RI) on two capillary columns given in the table, mass spectra
obtained by MS(EI) and MS (CI), and odor quality and odor threshold perceived at the sniffing port. c Odor quality perceived at the
d
sniffing port. Flavor dilution (FD) factor determined in distillates containing the complete set of the volatiles. Analyses were performed
by two assessors in duplicates. The data differed by not more than two FD factors. RI: Linear retention index.
of n-alkanes by using a computer program. MS analysis was
performed with an MS 95 S (Finnigan, Bremen, Germany) in
tandem with the capillaries described above. Mass spectra
in the electron impact mode (MS/EI) were generated at 70 eV
and in the chemical ionization mode (MS/CI) at 115 eV with
isobutane as reactant gas.
Qu a n tifica tion of 2-Oxobu ta n a l a n d 2,3-Bu ta n ed ion e.
Glycolaldehyde (1 mmol) and acetaldehyde (1 mmol) were
reacted for 20 min at 145 °C in phosphate buffer (50 mL; 0.5
mol/L; pH 7.0). After cooling, the solution was made up to a
defined volume (100 mL) and 1 mL of this mixture was spiked
with [13C4]-2,3-butanedione (20 µg) as the internal standard.
The aqueous solution was three times extracted with diethyl
ether (total volume ) 10 mL), 1,2-diaminobenzene (2 mg) was
added, and the mixture was stored overnight at 8 °C. The
quinoxaline derivatives formed were separated by HRGC on
the SE-54 column, and the amount of 2,3-butanedione and
2-oxobutanal was calculated by using the 13C4-labeled quin-
oxaline formed from the labeled 2,3-butanedione as the
internal standard (Schieberle and Hofmann, 1997a).
RESULTS AND DISCUSSION
AEDA. A distillate, obtained by boiling and simul-
taneously extracting an aqueous mixture of L-proline
and D-glucose, was judged to elicit an intense roasty,
popcorn-like odor. Application of AEDA to the distillate
revealed seven odor-active volatiles in the flavor dilution
(FD) factor range of 16-4096. Among them, six odor-
ants elicited a popcorn-like, roasty odor. The identifica-
tion experiments, which were based on a comparison of
two sensory criteria (odor quality and odor threshold)
and at least two of four analytical criteria (RI on SE-
54; RI on FFAP; MS/CI; MS/EI) with the reference
odorants revealed that 2-acetyl-3,4,5,6-tetrahydro- (4 in
mixture I, Table 1) and 2-acetyl-1,4,5,6-tetrahydro-
pyridine (6, Table 1) showed by far the highest FD
factors. This result is in good agreement with data
published by Roberts and Acree (1994) using similar
and, also, aqueous reaction conditions in the thermal
treatment of proline/glucose. Next in rank to the two
2-acetyltetrahydropyridines, we identified 2-propionyl-
3,4,5,6-tetrahydro- (7, Table 1) and 2-propionyl-1,4,5,6-
tetrahydropyridine (8), both PTHPs showing the same
popcorn-like odor note and the same low odor threshold
as the ATHP tautomers. The structures of these pro-
pionyl homologues were elucidated for the first time in
a reaction flavor or a food, respectively, based on mass
spectrometric data and synthetic experiments. On the
basis of the FD factors, their contribution to the overall
odor of the mixture was, however, lower than the
contribution of the ATHP isomers. In agreement with
F igu r e 3. Mass spectra of [13C4]-2-propionyl-3,4,5,6-tetrahy-
dropyridine (A) and [13C4]-2-propionyl-1,4,5,6-tetrahydropyri-
dine (B): b, carbon-13 label from U-13C-labeled glucose.
the findings of Roberts and Acree, AP (2, Table 1)
showed a low odor contribution to the overall roasted
odor of mixture I. The sixth popcorn-like-smelling
odorant present in the distillate was identified as PP
(3 in Table 1). Although PP is an established contribu-
tor to food flavors [cf. Schieberle (1991) and Buttery et
al. (1997)], it has, however, not yet been reported as a
flavor constituent in processed flavors.
It is well-known that in the production of Maillard-
type processed flavors, the reaction parameters signifi-
cantly influence the overall odors (Lane and Nursten,
1983; Schieberle and Hofmann, 1997b). To gain insight
into the influence of the reaction conditions on the
formation of the popcorn-like-smelling odorants, another
proline/glucose mixture was thermally treated at 160
°C under dry-heating conditions (mixture II). The

2724 J. Agric. Food Chem., Vol. 46, No. 7, 1998
Hofmann and Schieberle
Ta ble 2. Ca r bon Isotop e Ra tio in P THP Gen er a ted fr om
P r olin e a n d Glu cose or [13C6]Glu cosea
rel distribution (%) in PTHP generated in
m/z
(A) glucose
(B) [13C6]glucose
137
138
139
140
141
142
143
144
145
0.1
20.1
73.7
5.8
<0.1
<0.1
<0.1
<0.1
1.4
26.1
68.5
3.8
0.3
<0.1
<0.1
<0.1
<0.1
0.2
a
L-Proline (2 mmol) and either glucose (1 mmol) or [13C6]glucose
(1 mmol) were dissolved in phosphate buffer (200 mL; 0.1 mol/L:
pH 7.0) and simultaneously distilled and extracted for 2 h.
F igu r e 5. Formation of 2-oxobutanal (III), 2,3-butanedione
(V), 1-hydroxy-2-butanone (IV), and 2,3-dihydroxybutanal (I)
from glycolaldehyde and acetaldehyde.
not allow the conclusion that this odorant had not been
formed in mixture I.
La belin g Exp er im en ts. Formation of 2-Propionyl-
tetrahydropyridines. To get some insight in the forma-
tion of the 2-propionyl compounds, the carbon isotope
ratio in the 2-propionyltetrahydropyridines formed by
heating proline and 13C6-labeled glucose was determined
from the cluster of their molecular ions. As indicated
in Table 2, the main molecular ion was m/z 143 (B in
Table 2). Compared with the main molecular ion m/z
139 in the 2-propionyltetrahydropyridines generated
from unlabeled glucose (A in Table 2), this corresponds
to an upward shift of 4 mass units, indicating the
incorporation of four labeled carbons in the PTHP
formed from [13C6]glucose. The mass spectra of the two
labeled tautomers are displayed in Figure 3. A com-
parison with the spectra of the respective unlabeled
PTHPs (Figure 1) revealed that in both tautomers, three
labeled carbons are present in the propionyl group (m/z
60 versus 57), whereas one labeled carbon is incorpo-
rated in the piperidine ring (m/z 83/84 versus 82/83). A
similar result was found for the two labeled ATHP
tautomers (Figure 4). Compared with the spectra of the
respective unlabeled ATHPs (Figure 2), two labeled
carbon atoms were detected in the acetyl group (m/z 45
versus 43) and one labeled carbon in the piperidine ring
(m/z 83/84 versus 82/83).
F igu r e 4. Mass spectra of [13C3]-2-acetyl-3,4,5,6-tetrahydro-
pyridine (A) and [13C3]-2-acetyl-1,4,5,6-tetrahydropyridine
(B): b, carbon-13 label from U-13C-labeled glucose.
results revealed the same key odorants as in mixture I.
However, as displayed by their FD factors (mixture II
in Table 1), the different reaction parameters signifi-
cantly changed the odor contributions of the respective
odorants. Compared to mixture I (aqueous conditions),
especially, AP was drastically increased under the dry-
heating conditions to become the most important odor-
ant in this mixture (2, Table 1). Besides AP, also the
homologous PP was significantly enhanced. In contrast
to the two pyrrolines, the four tetrahydropyridine
derivatives remained constant in their odor activities
(cf. compounds 4 and 5 and 7 and 8 in mixtures I and
II).
The caramel-like-smelling 4-hydroxy-2,5-dimethyl-
3(2H)-furanone (HDMF) was detected among the key
odorants only under dry-heating conditions. However,
HDMF is not steam volatile and, therefore, the data do

Formation of 2-Propionyl-1-pyrroline and 2-Propionyltetrahydropyridine
J. Agric. Food Chem., Vol. 46, No. 7, 1998 2725
Ta ble 3. Am ou n ts of 2-Oxobu ta n a l a n d 2,3-Bu ta n ed ion e
Gen er a ted fr om Aceta ld eh yd e a n d Glycola ld eh yd ea
proposed a new reaction pathway involving a ring
enlargement of the 1-pyrroline. Application of this
pathway to the homologous reaction of 1-hydroxy-2-
butanone with 1-pyrroline should yield PTHP. On the
basis of a reaction of 13C4-labeled 1-hydroxy-2-butanone
(which might be formed from labeled glucose) and
1-pyrroline, a PTHP isotopomer would arise having
three labeled carbons in the propionyl group and one
in the ring. This assumption is in very good agreement
with the analytical data described above.
reaction product
amount (µg)
yield (%)
2-oxobutanal
2,3-butanedione
25100
755
29.2
0.9
a
Glycolaldehyde (1 mmol) and acetaldehyde (1 mmol) were
reacted for 20 min at 145 °C in phosphate buffer (50 mL; 0.5 mol/
L; pH 7.0).
To confirm both compounds as intermediates in PTHP
formation, 1-pyrroline and 1-hydroxy-2-butanone (HB)
were reacted in an aqueous buffer and the amounts of
PTHP formed were determined by a stable isotope
dilution analysis using deuterium-labeled ATHP as the
internal standard (Schieberle, 1995a). Heating for 1 h
at 100 °C yielded 0.5% of PTHP (sum of both tautomers),
thereby corroborating the key role of 1-pyrroline and
HB in PTHP formation.
The formation of HB from glucose can be explained
by different pathways. We followed the idea that
acetaldehyde and glycolaldehyde might be the precur-
sors of HB (Figure 5). Both compounds are well-known
degradation products of carbohydrates. It is suggested
that glycolaldehyde and acetaldehyde yield, by an Aldol-
type reaction, 2,3-dihydroxybutanal (I in Figure 5),
which after enolization and elimination of water would
result in 2-oxobutanal (III in Figure 5). This R-diketo
compound might be reduced, for example, by a dispro-
portionation with an ene-diol such as II (Figure 5), to
yield HB (IV). To confirm this proposal, a mixture of
acetaldehyde and hydroxyacetaldehyde was reacted in
aqueous solution. The results (Table 3) showed that
2-oxobutanal is formed in very high yields. The comple-
mentary reaction pathway (right side in Figure 5)
leading to 2,3-butanedione is obviously not favored,
because a 30 times lower amount of this dione was
generated (Table 3).
In summary, these results establish the key role of
1-pyrroline also in the formation of the 2-propionyl-
tetrahydropyridines.
F igu r e 6. Mass spectra (MS/EI) of PP generated from proline/
glucose (A) and proline/U-13C-labeled glucose (B).
Formation of PP. In Table 4, the carbon isotope ratios
in PP formed from proline in the presence of either [13C6]-
glucose (B in Table 4) or unlabeled glucose (A) are
contrasted. The main molecular ion in the labeled PP
(B; m/z 128) was shifted up by 3 mass units compared
In a very recent investigation we could show (Hof-
mann and Schieberle, 1998b) that the homologous
ATHP is formed in relatively high yields from a reaction
of 1-pyrroline with hydroxy-2-propanone and have
F igu r e 7. Hypothetical reaction pathway leading from 1-pyrroline and 2-oxobutanal to PP: b, carbon-13 label (from U-13C-
labeled glucose).

2726 J. Agric. Food Chem., Vol. 46, No. 7, 1998
Hofmann and Schieberle
Ta ble 4. Ca r bon Isotop e Ra tio in
pyridine from their corresponding cyclic R-amino acids. J .
Agric. Food Chem. 1998a , 46, 616-619.
Hofmann, T.; Schieberle, P. 2-Oxopropanal, hydroxy-2-pro-
panone, and 1-pyrrolinesImportant intermediates in the
generation of the roast-smelling food flavor compounds
2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine. J . Ag-
ric. Food Chem. 1998b, 46, 2270-2277.
Hunter, I. R.; Walden, M. K.; Scherer, J . R.; Lundin, R. E.
Preparation and properties of 1,4,5,6-tetrahydro-2-aceto-
pyridine, a cracker-odor constituent of bread aroma. Cereal
Chem. 1969, 189-195.
Lane, M. C.; Nursten, H. E. The variety of odors produced in
Maillard model systems and how they are influenced by the
reaction conditions. In The Maillard Reaction in Food and
Nutrition; ACS Symposium Series 215; American Chemical
Society: Washington, DC, 1983; pp 141-158.
2-P r op ion yl-1-p yr r olin e (P P ) Gen er a ted fr om P r olin e a n d
Glu cose or [13C6]Glu cosea
rel distribution (%) in PP generated in
m/z
(A) glucose
(B) [13C6]glucose
123
124
125
126
127
128
129
130
0.9
43.7
51.4
4.0
<0.1
<0.1
<0.1
2.7
35.5
57.3
4.3
0.1
<0.1
<0.1
<0.1
0.2
a
Cf. Table 2.
Nickerson, G. B.; Likens, S. T. Gas chromatographic evidence
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matogr. 1966, 2, 677-678.
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with the unlabeled PP (m/z 125). These data clearly
indicate that three carbon atoms of the labeled glucose
had been incorporated in the flavor compound. From a
comparison of the MS/EI of the unlabeled PP with that
of the [13C]3PP (Figure 6) it can be derived that the three
labeled carbons are present in the propionyl group as
represented by the fragments m/z 60 (labeled) and m/z
57 (unlabeled). There is, however, no labeling in the
pyrroline ring (m/z 69/68 in either isotopomer).
In recent investigations we had established 1-pyrro-
line and 2-oxopropanal as the key intermediates in the
formation of AP (Schieberle, 1995; Hofmann and Schie-
berle, 1998b). The reaction pathway proposed suggests
that in the course of the reaction, one carbon atom of
the 2-oxopropanal is lost as carbon dioxide. Because it
has already been shown that the reaction of 2-oxobu-
tanal and 1-pyrroline yields PP (Schieberle, 1991), a
similar reaction mechanism can be proposed starting
from 1-pyrroline and 2-oxobutanal as the key interme-
diates in PP formation (Figure 7). As indicated in the
figure, the 13C4-labeled 2-oxobutanal (generated from
the labeled glucose) will lose the aldehyde carbon in the
course of the reaction. The oxidation steps assumed in
the reaction pathway have recently been established
based on a new synthesis of AP and ATHP (Hofmann
and Schieberle, 1998a).
Con clu sion s. The results have shown that 2-pro-
pionyl-1-pyrroline and the two tautomers of 2-propion-
yltetrahydropyridine are further significant roast odor-
ants generated upon heating the amino acid proline in
the presence of carbohydrates. On the basis of the
results of the model studies presented, it can be as-
sumed that these roast odorants are formed in foods
from 1-pyrroline, the Strecker degradation product of
proline, by similar mechanisms as recently confirmed
for 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine.
The specific precursor for PP is 2-oxobutanal, and that
for PTHP is hydroxy-2-butanone. In a food, the latter
two intermediates are supplied by cleavage of carbohy-
drate skeletons.
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and maltose and their importance to food flavour. In Flavour
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Received for review December 29, 1997. Revised manuscript
received April 22, 1998. Accepted April 24, 1998.
J F971101S