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A.-N. Yu, A.-D. Zhang / Food Chemistry 121 (2010) 1060–1065
Table 2
Mass spectral data of some tentatively identified compounds from L-ascorbic acid and L-cysteine.
Peak
No.a
Compound
LRIb
Major MS data, m/z (relative intensity)
1
2
3-Hexanone
2-Hexanone
<800 57 (100), 43 (96), 71 (77), 100 (58, M+), 41 (27), 39 (14), 42 (9), 72 (8), 44 (7); MW = 100
<800 43 (100), 58 (66), 32 (57), 57 (21), 100 (18, M+), 41 (17), 85 (14), 39 (9), 71 (8), 42 (7); MW = 100
1305 153 (100), 154 (65, M+), 155 (15), 69 (9), 156 (6), 77 (5), 109 (5), 45 (4), 121 (4), 32 (3); MW = 154
1402 153 (100), 168 (43, M+), 32 (15), 154 (10), 155 (10), 69 (7), 167 (6), 169 (6), 109 (5), 170 (5); MW = 168
1085 87 (100), 130 (29, M+), 45 (17), 43 (15), 85 (13), 53 (9), 59 (9), 39 (5); MW = 130
36
41
25
31
34
35
32,33
2-Methylthieno[3,2-b]thiophene
2-Ethylthieno[2,3-b]thiophene
2-Propyltetrahydrothiophene
4-Hydroxybenzothiophene
3-Acetyl-2,5-dimethylthiophene
3-(Vinylthio)thiophene
4,6-Dimethyl-1,2,3-trithiane (cis or
trans)
1213 150 (100, M+), 121 (74), 122 (22), 96 (10), 78 (10), 77 (10), 151 (10), 63 (6), 69 (6); MW = 150
1261 139 (100), 154 (46, M+), 141 (13), 140 (10), 67 (8), 97 (6), 45 (5), 69 (5); MW = 154
1300 142 (100, M+), 141(81), 97 (43), 45 (13), 69 (11), 144 (9), 140 (6), 71 (4), 70 (4); MW = 142
1232 166 (100, M+), 102 (33), 60 (30), 69 (29), 59 (28), 101 (26), 45 (25), 64 (21), 92 (21), 41 (20); MW = 166
40
43
39
1,2,5,6-Tetrathioctane (C4H8S4)
Cyclic octaatomic sulphur (S8)
4-Methyl-1,2-benzenedithiol
1386 59 (100), 184 (79, M+), 60 (50), 124 (48), 64 (19), 45 (18), 119 (17), 186 (14), 58 (12), 61 (10); MW = 184
>1800 32 (100), 64 (68), 256 (62, M+), 160 (40), 128 (39), 192 (26), 258 (23), 96 (17), 162 (9); MW = 256
1358 156 (100, M+), 155 (76), 123 (16), 157 (16), 121 (13), 141 (12), 111 (11), 45 (10), 158 (9); MW = 156
a
Peak nos. correspond to Fig. 1.
LRI calculated for a DB-5 capillary column; mean values.
b
treated with and without supercritical-CO2 (Xu, Liu, Zhao, & Gao,
2008). High concentrations of several derivatives of thienothioph-
ene were determined in these reaction mixtures, especially thie-
no[3,2-b]thiophene and 2-methylthieno[3,2-b]thiophene. The
amounts of thienothiophenes decreased with increasing pH. Cys
was thermally stable when heated alone under dry conditions;
however, heating Cys on its own in the presence of a small amount
of water led to the formation of thienothiophenes. The quantities
were low, since the breakdown of amino acids in the Maillard reac-
tion occurs more readily than thermal degradation in the absence
of a sugar (Mottram & Whitfield, 1995). In this model system, a sig-
nificant number of thienothiophenes, especially thieno[3,2-b]thio-
phene and 2-methylthieno[3,2-b]thiophene was identified. The
results indicated that ASA plays a significant role and can promote
the formation of thienothiophenes in this model system.
ylthiazole and 2-ethylthiazole, possess green, vegetable aromas,
whereas S-compounds such as 4,5-dimethylthiazole, 5-ethyl-2,4-
dimethylthiazole, and 2,4,5-trimethylthiazole have been described
as nutty, roasted, and meaty (Maga, 1975). Like pyrazines, their for-
mation also requires heating at elevated temperatures. Therefore,
they are often detected in fried, roasted, or grilled foods, such as
cooked meats, coffee, roasted peanuts, and potato chips. For in-
stance, 4,5-dimethylthiazole, trimethylthiazole, 2-acetylthiazole,
and 5-ethyl-2,4-dimethylthiazole have been found in grilled meats
(Melton, 1998). In general, thiazoles are mainly formed by non-
enzymatic browning reactions between reducing sugars and amino
acids in the presence of H2S. Therefore, thiazoles can be identified in
nearly all cooked or roasted food aromas. With the exception of 5-
methylthiazole, 4,5-dimethylthiazole, 2-ethyl-4-methylthiazole
and 2-acetylthiazole, all other thiazoles formed in the reaction of
ASA with Cys have a methyl substituent in the 2-position. Accord-
ingly, it is likely that one of the precursors to all of these compounds
is acetaldehyde, derived from the decomposition of ASA (Vernin
et al., 1998). Other compounds likely to be involved in the forma-
tion of such 2-methylthiazoles would be the ASA decomposition
products pyruvaldehyde (for 2,4-dimethylthiazole) and 2,3-
butanedione (for 2,4,5-trimethylthiazole) (Vernin et al., 1998).
The key steps in the reaction sequences to these compounds are
the reaction of ammonia with acetaldehyde to form 1-aminoetha-
3.4. Thiophenones
Two thiophenones were detected in the model system involving
ASA with Cys at five different pHs, i.e. tetrahydrothiophen-3-one
and 2-methyltetrahydrothiophen-3-one. Tetrahydrothiophen-3-
one is a well-known food volatile. Its formation from the amino
acids cystine and cysteine by condensation of two molecules of
mercaptoacetaldehyde (a main Strecker degradation product) is
much easier to explain. The probable precursor of tetrahydrothio-
phene-3-one is 1,4-dimercapto-2-butanone (Güntert et al., 1990).
Tetrahydrothiophen-3-one has been identified in pressure-cooked
beef and yeast extract (Güntert et al., 1990). 2-Methyltetrahydro-
thiophen-3-one is another well-known flavour volatile. Its identifi-
cation as a thermal degradation product of thiamin has been
described previously (Hartman, Carlin, Scheide, & Ho, 1984). A pos-
sible pathway for the formation of 2-methyltetrahydrothiophen-3-
one involves aldol condensation between acetaldehyde, derived
from cysteine or ASA, and pyruvaldehyde, formed from ASA (Ver-
nin et al., 1998). The aldol condensation product reacts with hydro-
gen sulphide to produce 2-methyltetrahydrothiophen-3-one.
nol and the condensation of this compound with appropriate
a-
dicarbonyl compounds. The thiazoles could be formed from these
condensation products by reaction with H2S followed by cyclisation
and dehydration (Vernin & Párkányi, 1982).
3.6. Pyrazines
Six pyrazines were identified in the model reaction systems.
The quantities of pyrazines increased significantly as the pH in-
creased. Some pyrazines were not identified at pH 5–6 except for
methylpyrazine and 2-ethyl-6-methylpyrazine (Table 1). The re-
sults indicated that high pH conditions favoured the formation of
pyrazines. When the pH of the reaction solution was higher than
6, the quantities of pyrazines increased obviously. The thermal
degradation of ASA can produce many carbonyl compounds (Ver-
nin et al., 1998). The base catalysis was probably due both to the
increased reactivity of the amino group of the amino acid toward
the carbonyl and to the increased rearrangement and fragmenta-
tion of ASA (Koehler & Odell, 1970). Pyrazines may contribute to
toasted, roasted, nutty, and burnt notes. There are several precur-
3.5. Thiazoles
Eleven thiazoles were identified in the model reaction systems.
The thiazoles identified in this study indicated that pH could influ-
ence their formation. The formation of thiazoles was favoured un-
der basic conditions more than in an acidic environment, whereas
2-acetylthiazole was more prominent at pH 7–9. Thiazole com-
pounds have been found in various food systems, and they contrib-
sors or pathways for pyrazine compounds. The a-amino carbonyls,
ute to
a
wide variety of characteristic aromas to foods.
which can be formed from the reactions between dicarbonyl com-
Monosubstituted alkylthiazoles, such as 2-methylthiazole, 5-meth-
pounds and amino acids during Strecker degradation, are generally