JFS:
Food Chemistry and Toxicology
Production of Volatiles from Amino Acid
Homopolymers by Irradiation
ABSTRACT: Amino acid homopolymers were used to determine production of radiolytic volatiles by irradiation.
Many new volatiles were generated, and the amounts of volatiles in amino acid homopolymers changed after
irradiation. Each amino acid homopolymer group produced different odor characteristics, but the intensities of
odor from all amino acid groups were weak, except for sulfur-containing amino acids. Sulfur-containing amino
acids produced various sulfur compounds; the overall odor intensity of irradiated sulfur amino acids was very high
and the odor characteristics of sulfur amino acids were similar to irradiation odor of meat. Our results indicated
that the contribution of methionine to the irradiation odor would be far greater than that of cysteine.
Keywords: irradiation, volatiles, amino acid homopolymers, sulfur amino acids, irradiation odor
Introduction
O
UR STUDIES SHOWED THAT ALL IRRADIATED MEAT PRODUCED
characteristic irradiation odor regardless of degree of lipid oxidation. Also, irradiated meat produced more volatiles than nonirradiated meat, and the chromatograms of raw and cooked irradiated meat
suggested that lipid oxidation could be responsible for a small part of
the off-odor in irradiated meat (Ahn and others 1997; Ahn and others
1998a, 1998b, 1999). Patterson and Stevenson (1995) found that dimethyl trisulfide is the most potent off-odor compound in irradiated
chicken meat, followed by cis-3- and trans-6-nonenals, oct-1-en-3one and bis(methylthio-)methane. Hashim and others (1995) reported that irradiating uncooked chicken breast and thigh produced a
characteristic “bloody and sweet” aroma that remained after the
thighs were cooked, but was not detectable after the breasts were
cooked. Heath and others (1990) reported that irradiating uncooked
chicken breast and leg at 2 or 3 kGy produced a “hot fat,” “burned oil,”
or “burned feathers” odor that remained after the thighs were
cooked. Chen and others (1999) reported that irradiation before
cooking did not influence lipid oxidation of cooked pork during storage. These studies also supported the concept that the changes that
occur following irradiation are different from those of warmed-over
flavor in oxidized meat, and that other mechanisms such as radiolysis
of proteins play an important role in the production of volatiles in irradiated meat. Irradiation of amino acids produced distinct volatile
compounds via the radiolytic degradation of amino acids (Jo and Ahn
2000). However, radiolytic degradations of amino acids not only occurred at side chains, but also at amino and carboxyl groups. Thus,
the volatile compounds produced from amino acid monomers by irradiation cannot represent the volatiles produced from proteins. To
overcome this problem, we used amino acid homopolymers in this
study. The objective of this study was to determine the volatile compounds produced from each amino acid polymer by irradiation and
the odor characteristics of the irradiated amino acid polymers.
Materials and Methods
Sample preparation
A total of 15 amino acids that included 12 amino acid homopolymers, a dimer, a trimer, and a tetramer were used in this
study and were purchased from Sigma Chemical Co. (Sigma Co.,
© 2002 Institute of Food Technologists
jfsv67n7p2565-2570ms20010619-BW.P65
2565
St. Louis, Mo., U.S.A.). The molecular weights of these amino acid
homopolymers ranged from 2,000 to 40,000 depending upon
amino acid. Each individual amino acid (homopolymer or oligomer) (40 mg) was dissolved in 20 mL of citrate-phosphate buffer
(100 mM, pH 6.0). Two 10-mL portions were transferred to scintillation vials; one was used as a nonirradiated control and the other was irradiated at 5.0-kGy absorbed dose using an electron
beam irradiator (Circe IIIR Thomson CSF Linac, St. Aubin,
France). To confirm the target dose, 2 alanine dosimeters per cart
were attached to the top and bottom surfaces of a sample vial.
Immediately after irradiation, a 2-mL portion of the amino acid
(homopolymer or oligomer) solution was transferred to sample
vials, flushed with helium gas, and analyzed for volatiles using a
purge-and-trap dynamic headspace/gas chromatography-mass
spectrometry (GC-MS), and the rest was also portioned and used
to evaluate odor characteristics using trained sensory panelists.
Four replications were prepared for volatiles.
Dynamic headspace/GC-MS method
A purge-and-trap apparatus (Precept II and Purge & Trap
Concentrator 3000, Tekmar-Dohrmann, Cincinnati, Ohio, U.S.A.)
connected to a GC/MS (Hewlett-Packard Co., Wilmington, Del.,
U.S.A.) was used to analyze volatiles produced (Ahn and others
2001). Sample solution (2 mL) was placed in a 40-mL sample vial,
and the vials were flushed with helium gas (40 psi) for 5 s. The
sample was purged with helium gas (40 mL/min) for 12 min at
40 °C. Volatiles were trapped using a Tenax column (TekmarDohrmann) and desorbed for 2 min at 225 °C, focused in a cryofocusing module (–90 °C), and then thermally desorbed into a
column for 30 s at 225 °C.
An HP-624 column (7.5 m x 0.25 mm internal dia, 1.4 m mm
nominal; Hewlett-Packard Co), and an HP-Wax column (7.5 m x
0.25 mm internal dia, 0.25 mm nominal) were connected using
zero dead-volume column connectors (J & W Scientific, Folsom,
Calif., U.S.A.). Ramped oven temperature was used to improve
volatile separation. The initial oven temperature of 0 °C was held
for 2.50 min. After that, the oven temperature was increased to
15 °C at 2.5 °C per min, increased to 45 °C at 5 °C per min, increased to 110 °C at 20 °C per min, increased to 210 °C at 10 °C
per min, then was held for 2.5 min at that temperature. Constant
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Food Chemistry and Toxicology
D.U. A HN
Volatiles from amino acids by irradiation . . .
Table 1-Production of volatile compounds from acidic
group amino acid homopolymers by irradiation
Volatiles
0 kGy
5 kGy
SEM
Table 3-Production of volatile compounds from aliphatic
hydroxyl group amino acid homopolymers by irradiation
Volatiles
0 kGy
——— Total ion counts x 103 ———
Food Chemistry and Toxicology
Poly-aspartic acid
2-Propanone
Hexane
Methyl cyclopentane
Benzene
Toluene
Poly-glutamic acid
Acetaldehyde
2-Propanone
Hexane
Cyclohexane
253b
295b
0b
352a
0b
1735a
636a
204a
104b
359a
424
29
2
9
20
0b
366b
236
184a
5452a
1784a
277
0b
198
316
17
7
a,b Means with no common superscript differ significantly (P < 0.05), n = 4
Table 2-Production of volatile compounds from aliphatic
group amino acid homopolymers by irradiation
Volatiles
0 kGy
5 kGy
SEM
——— Total ion counts x 103 ———
Poly-alanine
Acetaldehyde
979b
2-Propanone
8502
Acetonitrile
0b
Acetic acid, methyl ester 1106a
Hexane
1600a
Butanal
110a
Methyl cyclopentane
265a
Methyl propionate
0b
Cyclohexane
258a
Poly-glycine
Acetaldehyde
0b
2-Propanone
22555 a
2-Methyl propanal
0b
Hexane
782b
Methyl cyclopentane
174b
Benzene
0b
3-Methyl butanal
0b
2-Methyl butanal
0b
Poly-proline
2-Methyl-1-propene
88a
2-Propanone
4353a
Hexane
1063
Cyclohexane
991a
Hexanal
67a
Toluene
978a
2228a
10974
374a
49b
929b
0b
195b
522a
0b
91
1535
7
109
46
9
17
13
6
419a
3752b
409a
1477a
260a
1008a
91a
78a
115
5402
47
87
21
31
26
18
0b
1227b
944
0b
0b
0b
9
402
100
28
4
60
a,b Means with no common superscript differ significantly (P < 0.05), n = 4
column pressure at 20.5 psi was maintained. The ionization potential of the mass selective detector (Model 5973; Hewlett-Packard Co.) was 70 eV, and the scan range was 18.1 to 350 m/z. Identification of volatiles was achieved by comparing mass spectral
data of samples with those of the Wiley Library (Hewlett-Packard
Co.). Standards, when available, were used to confirm the identification by the mass selective detector. The area of each peak was
integrated using the ChemStation (Hewlett-Packard Co.), and
the total peak area (pA*s x 103) was reported as an indicator of
volatiles generated from the sample.
Odor characteristics
Ten trained sensory panelists characterized overall odor characteristics of the samples. Panelists were selected based on interest, availability, and performance in screening tests conducted
with samples similar to those to be tested. During training, a lexicon of aroma terms to be used on the ballot was developed. Sam2566
Poly-serine
2-Methyl-1-propene
492a
Acetaldehyde
0b
1,1-Oxybis ethane
0b
2-Propanone
4229b
Butanal
193a
Hexane
1315a
Cyclopentanol
122b
Methyl cyclopentane
173
2-Butanone
334a
Cyclohexane
1913a
Toluene
2379a
Poly-threonine
Acetaldehyde
13693 b
1,1-Oxybis ethane
323445b
2-Propanone
0b
Hexane
1391a
Butanal
142b
2-Butanone
276b
Acetic acid ethyl ester
475b
2-Ethoxy butane
0b
Cyclohexane
787a
2,3-Dihydro-1,4-dioxin
0b
1,4-Dioxane
0b
Methyl butyrate
0b
Toluene
61a
2566
SEM
0b
1990a
316a
11555 a
0b
713b
349a
206
118b
0b
0b
20
35
4
864
3
74
5
16
38
20
38
60479 a
117262a
47097 a
788b
228a
845a
4305a
90a
0b
233a
125a
272a
0b
1078
6037
2086
55
11
28
126
3
49
7
3
20
2
a,b Means with no common superscript differ significantly (P < 0.05), n = 4
ples were placed in glass containers, and the sample temperature was brought to 25 °C before samples were tested. All treatments were presented to each panelist, and the order of presentation was randomized.
Statistical analysis
Data were analyzed using the generalized linear model procedure of SAS software (SAS Institute Inc. 1989), and the Student’s
t-test was used to compare differences between irradiated and
nonirradiated means. Mean values and standard error of the
means (SEM) were reported. Significance was defined at P < 0.05.
Results and Discussion
Acidic amino acid group
Methyl cyclopentane and toluene, not found in nonirradiated
polyaspartic acid, were produced by irradiation. The amounts of
2-propanone and hexane in irradiated polyaspartic acid increased, but that of benzene decreased after irradiation. Irradiation of polyglutamic acid newly produced acetaldehyde and increased the amount of 2-propanone ( Table 1). Neither of the
acidic group amino acid homopolymers produced detectable
odor after irradiation, however.
Aliphatic amino acid group
Acetonitrile and methyl propionate were newly produced,
and the amount of acetaldehyde increased from polyalanine after irradiation (Table 2). The amounts of acetic acid methyl ester,
hexane, butanal, and cyclohexane, however, decreased after irradiation. The odor of irradiated polyalanine was characterized as
seaweed odor, but the intensity was very weak.
Irradiation of polyglycine produced 5 new volatiles not found
in the nonirradiated control: acetaldehyde, 2-methyl propanal,
benzene, 2-methyl butanal, and 3-methyl butanal. The amount
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——— Total ion counts x 103 ———
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Volatiles from amino acids by irradiation . . .
Volatiles
0 kGy
5 kGy
SEM
Table 5-Production of volatile compounds from aromatic
group amino acid homopolymers by irradiation
Volatiles
0 kGy
——— Total ion counts x 103 ———
Poly-asparagine
2-Methyl-1-propene
2-Propanone
Hexane
Methyl cyclopentane
Cyclohexane
Benzene
Toluene
Poly-glutamine
2-Methyl-1-propene
Acetaldehyde
1,1-Oxybis ethane
2-Propanone
Butanal
Hexane
Cyclopentanol
Methyl cyclopentane
2-Butanone
Cyclohexane
Toluene
258a
2963a
309b
0b
1923a
0b
0b
0b
729b
813a
419a
0b
1183a
6385a
10
411
42
40
15
11
83
465a
0b
0b
4708b
191a
1336a
121b
170
374a
1926a
2355a
0b
2029a
316a
11102 a
0b
649b
343a
192
114b
0b
0b
21
53
5
648
4
92
5
18
37
24
38
5 kGy
SEM
——— Total ion counts x 103 ———
Poly-tyrosine
Acetaldehyde
Oxirane
2-Methyl butane
Ethanol
2-Propanone
Hexane
Butanal
Methyl cyclopentane
Tetrahydrofuran
2-Methyl-1,3-dioxolane
Benzene
Cyclohexene
2,3-Dihydro-1,4-dioxin
Toluene
2-Ethyl butanal
460b
1258a
349a
2445
1385b
525b
100b
87b
0b
0b
0b
0b
0b
89b
179a
17532 a
0b
0b
741
3774a
751a
230a
243a
1863a
107a
986a
78a
409a
5299a
0b
645
57
78
562
571
34
12
13
128
16
11
2
28
103
31
a,b Means with no common superscript differ significantly (P < 0.05), n = 4
a,b Means with no common superscript differ significantly (P < 0.05), n = 4
Aromatic amino acid group
of 2-propanone decreased, but that of hexane increased after irradiation. Irradiated polyglycine produced a weak seashore odor.
Irradiation did not produce any new volatiles nor increase the
amounts of volatiles in polyproline. Only 2 volatiles, 2-propanone and hexane, remained after irradiation; 2-methyl-1-propene, cyclohexane, hexanal, and toluene detected in nonirradiated polyproline disappeared after irradiation ( Table 2).
Irradiated polyproline produced a solvent-like odor, but its intensity was very weak.
Aliphatic hydroxyl amino acid group
Polyserine produced acetaldehyde and 1,1-oxybis ethane after irradiation (Table 3). The amounts of 2-propanone and cyclopentanol increased, but those of hexane and 2-butanone decreased. Two-Methyl-1-propene, butanal, cyclohexane, and
toluene were detected in nonirradiated control, but were not
found in irradiated polyserine. The odor of irradiated polyserine
was characterized as a cattle barn odor.
New volatiles produced in polythreonine after irradiation included 2-propanone, 2-ethoxybutane, 2,3-dihydro-1,4-dioxin,
and methyl butyrate. The amounts of acetaldehyde, butanal, 2butanone, and acetic acid ethyl ester increased, but those of 1,1oxybis ethane, hexane, cyclohexane, and toluene decreased after
irradiation (Table 3). The odor of irradiated polythreonine was
characterized as a Chinese herbal medicine or as caramel-like.
Amide amino acid group
Irradiation of polyasparagine produced 2 new volatile compounds, methyl cyclopentane and benzene, and increased toluene and hexane. The amount of 2-propanone decreased after irradiation (Table 4). Irradiated polyasparagine, however,
produced no odor.
Acetaldehyde and 1,1-oxybis ethane were newly produced
from polyglutamine after irradiation. The amounts of 2-propanone and hexane increased, but those of 2-methyl-1-propene,
butanal, hexane, 2-butanone, cyclohexane, and toluene decreased after irradiation ( Table 4). The odor of irradiated polyglutamine was hospital odor.
New volatiles such as tetrahydrofuran, 2-methyl-1,3-dioxalone, benzene, cyclohexane, and 2,3-dihydro-1,4-dioxin were
produced after irradiating polytyrosine. The amounts of acetaldehyde, 2-propanone, butanal, methyl cyclopentane, and toluene increased, but those of oxirane, 2-methyl butane, and 2-ethyl butanal decreased (Table 5). The odor of irradiated
polytyrosine was characterized as that of seaweed or seashore.
Basic amino acid group
Irradiation increased the production of acetaldehyde, 2-propanone, hexane, methyl cyclopentane, and toluene, but decreased amounts of 2-methoxy-2-methyl-propane and cyclohexane in polyhistidine (Table 6). Irradiated polyhistidine produced
a sweet odor. Irradiation greatly increased the production of acetaldehyde and also newly produced propanal, butanal, 2-methyl
dioxalone, and benzene from polylysine. The amounts of cyclopentane and toluene increased, but that of cyclohexane decreased (Table 6). The odor of irradiated polylysine was characterized as somewhat sour, coleslaw, or hospital odor.
Sulfur-containing amino acid group
No S-containing amino acid homopolymers are available, so
glutathione (Glu-Cys-Gly) was used to determine the volatiles
produced from cysteine upon irradiation, and Met-Ala for methionine. Irradiation produced 2 sulfur volatiles, carbon disulfide and dimethyl sulfide, in addition to methyl cyclopentane
from glutathione ( Table 7). The odor of irradiated glutathione
was characterized as hard-boiled egg or sulfur, and the odor was
strong.
Irradiation of Met-Ala produced many more new sulfur volatiles than irradiating glutathione. Mercaptomethane, dimethyl
disulfide, methyl thiirane, (methylthio-) ethane, 3-(methylthio)1-propene, 2-methyl-2-(methylthio-) propane, ethanoic acid Smethyl ester, dimethyl disulfide, methyl ethyl disulfide, and 2,4dithiapentane were sulfur volatiles produced from the irradiated
Met-Ala. The amounts of other volatiles were also changed by irradiation, but were minor compared to sulfur volatiles. Irradiation of Met-Gly-Met-Met produced a few more nonsulfur volatiles than Met-Ala and the total amount of sulfur compounds from
the irradiated Met-Gly-Met-Met was greater than that from the
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Food Chemistry and Toxicology
Table 4-Production of volatile compounds from amide
group amino acid homopolymers by irradiation
Volatiles from amino acids by irradiation . . .
Table 6-Production of volatile compounds from basic
group amino acid homopolymers by irradiation
Volatiles
0 kGy
5 kGy
SEM
ion counts x
103 ———
385a
969b
992b
98866 a
501b
106
65b
2277a
0b
0b
1382a
2137a
19884 b
1037a
136
350a
0b
119a
18
82
196
2181
36
13
7
28
15
116a
0b
0b
24797
634b
0b
242b
90006
2161a
0b
0b
122b
0b
11729 a
460a
35641
1484a
252a
775a
69419
0b
284a
310a
469a
8
421
16
5693
35
8
33
8755
20
40
2
13
——— Total
Food Chemistry and Toxicology
Poly-histidine
4,4-Dimethyl-2-oxetanone
Acetaldehyde
2-Propanone
2-Methoxy-2-methyl-propane
Hexane
Butanal
Methyl cyclopentane
Cyclohexane
Toluene
Poly-lysine
2-Methyl-1-propene
Acetaldehyde
Propanal
2-Propanone
Hexane
Butanal
Methyl cyclopentane
Tetrahydrofuran
Cyclohexane
2-Methyl dioxolane
Benzene
Toluene
a,b Means with no common superscript differ significantly (P < 0.05), n = 4
irradiated Met-Ala because of higher methionine content per
unit weight (Table 7). The odor of irradiated Met-Ala was much
stronger than that of glutathione and was characterized as boiled
cabbage, sulfury, or rotten vegetable-like. The odor characteristics of Met-Gly-Met-Met were also similar to those of Met-Ala. Table 7 also suggests that methionine produced far greater
amounts of sulfur compounds than cysteine, and is the most important amino acid in the production of irradiation off-odor.
Results and Discussion
M
ANY NEW VOLATILES WERE GENERATED AND THE
amounts of volatiles produced from amino acid homopolymers were changed after irradiation. The production of many
new volatiles from amino acids upon irradiation indicated that
more than 1 site in amino acid side chains was susceptible to free
radical attack and many volatiles were apparently produced by
the secondary chemical reactions after the primary radiolytic
degradation of side chains. Only sulfur-containing volatiles,
however, produced strong odor that was similar to or close to irradiation odor. The perception of odor from samples containing
sulfur volatiles changed greatly depending upon their composition and amounts present in the sample. Although volatiles generally found in all amino acid homopolymers could provide dilution effect or produce interactions with sulfur compounds, their
roles in the odor characteristics of irradiated sulfur-containing
amino acids are expected to be minor. The result of this study
also indicated that sulfur compounds produced by S-containing
amino acids such as cysteine and methionine played the major
role in irradiation odor.
The volatile profiles and sensory characteristics of amino acids (Tables 1 to 8) clearly explained why irradiation odor was different from lipid oxidation odor, and why lipid oxidation was responsible for only a small part of the off-odor in irradiated meat
(Ahn and others 1997, 1998b, 1999). Patterson and Stevenson
(1995) identified dimethyl trisulfide and bis(methylthio-)methane as the most potent off-odor sulfur compounds in irradiated
2568
Table 7-Production of volatile compounds from sulfurcontaining amino acid dimer or oligomers by irradiation
Volatiles
0 kGy
Glutathione (g-Glu-Cys-Gly)
Carbon disulfide
0b
Hexane
316b
Methyl cyclopentane
0b
Cyclohexane
119a
Dimethyl disulfide
0b
Met-Ala
2-Methyl-1-propene
614a
Acetaldehyde
0b
Mercaptomethane
0b
2-Propanone
1244a
Dimethyl sulfide
0b
2-Methyl propanol
0b
Hexane
281b
Methyl thiirane
0b
(Methylthio) ethane
1376a
2-Ethoyxy2-methyl propane1
299a
Acetic acid ethyl ester
3290
Cyclohexane
1565a
3-(Methylthio)-1-propene 0b
Ethanoic acid,
S-methyl ester
0b
2-Methyl-2-(methylthio)
propane
86a
Dimethyl disulfide
5043b
Methyl benzene
591a
Methyl ethyl disulfide
0b
2,4-Dithiapentane
0b
Met-Gly-Met-Met
2-Methyl-1-propene
270a
Acetaldehyde
2264a
Mercaptomethane
0b
Pentanal
0b
Dimethyl sulfide
0b
2-Propanone
4010a
Acetonitrile
3485a
Hexane
285b
2,2-Oxybis propane
17951 a
(Methylthio) ethane
0b
2-Butanone
206a
Acetic acid ethyl ester 116873a
Cyclohexane
988a
Benzene
0b
1-Heptanethiol
0b
3-(Methylthio)-1-propene 0b
Ethanethioic acid,
S-methyl ester
0b
2-Butanamine
0b
2-Methyl-2-(methylthio)
propane
92b
Dimethyl disulfide
1430b
Methyl ethyl disulfide
0b
Ethyl benzene
0b
1,3-Dimethyl benzene
0b
1,4-Dimethyl benzene
0b
Isopropyl benzene
0b
2568
SEM
589a
496a
82a
0b
214a
24
39
5
2
47
0b
2910a
11842 a
0b
166244a
114a
1146a
4177a
0b
11
230
709
456
6183
3
47
174
47
344b
4467
0b
186a
114
415
13
11
106a
7
0b
346229a
0b
2221a
825a
1
9385
23
80
25
0b
0b
17325 a
341a
201541a
0b
356b
780a
3843b
2053a
0b
77893 b
0b
210a
94a
122a
8
224
866
18
939
289
414
26
183
15
35
4084
21
1
1
1
170a
156a
8
6
149a
351320a
1935a
38116 a
60346 a
11550 a
725a
2
1247
15
322
823
164
20
a,b Means with no common superscript differ significantly (P < 0.05), n = 4
chicken meat, but our data indicated that many other sulfur
compounds could be produced from methionine and cysteine
(Table 7). The volatility of aroma compounds depends on the vapor-liquid partitioning of volatile compounds, which determines
the affinity of volatile molecules for each phase (Buttery and others 1973), and the interactions among food components such as
carbohydrates and proteins affect the release of volatile compounds in foods (Godshall 1997). Physicochemical conditions of
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——— Total ion counts x 103 ———
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Volatiles from amino acids by irradiation . . .
Table 8-Major volatile compounds from irradiated amino acid homopolymers and oligomers and their odor characteristics
Amino acid polymer
Major volatiles
Odor characteristics
Poly-aspartic acid
Poly-glutamic acid
Poly-alanine
Poly-glycine
Poly-proline
Poly-serine
Poly-threonine
2-propanone, methyl cyclopentane, toluenea
acetaldehyde, 2-propanone
acetonitrile, methyl propionate, acetaldehyde, 2-propanone
acetaldehyde, 2-methyl propanal, 2-methyl butanal, 3-methyl propanal
2-propanone, hexane
acetaldehyde, 1,1-oxybis ethane, 2-propanone
acetaldehyde, 2-propanone, acetic acid ethyl ester, 2-ethoxy butane,
2,3-dihydro-1,4-dioxin, 1,4-dioxin, methyl butyrate, 1,1-oxybis ethane
methyl cyclopentane, benzene, toluene
acetaldehyde, 1,1-oxybis ethane, 2-propanone
acetaldehyde, tetrahydrofuran, 2-methyl-1,3-dioxalane, benzene, toluene,
cyclohexane, 2,3-dihydro-1,4-dioxin
2-methoxy-2-methyl propane, toluene
acetaldehyde, propanal, butanal, 2-methyl dioxalone, benzene, 2-propanone
carbon disulfide, dimethyl disulfide, methyl cyclopentane
acetaldehyde, mercaptomethane, dimethyl sulfide, methyl thiirane,
3-(methylthio)-1-propene, ethanoic acid- S-methyl ester, dimethyl disulfide,
methyl ethyl disulfide, 2,4-dithiapentane, 2-methyl propanal
mercaptomethane, pentanal, dimethyl sulfide, (methylthio)-ethane, benzene,
1-heptanethiol, 3-(methylthio)-1-propene, ethanoic acid-S-methyl ester,
dimethyl disulfide, methyl ethyl disulfide, 2-butanamine, 1,3-dimethyl benzene,
1,4-dimethyl benzene, isopropyl benzene, ethyl benzene
no odor
sweet, honey
seaweed
seashore odor
organic solvent
cattle barn odor
Chinese herbal medicine
Poly-histidine
Poly-lysine
Glutathione
Met-Ala
Met-Gly-Met-Met
no odor
hospital odor
seaweed or seashore
sweet
coleslaw, sour
hard-boiled eggs, sulfury
boiled eggs, sulfury
rotten vegetable
boiled cabbage, sulfury,
rotten vegetable
a Volatiles written in italic did not produce detectable odor at the levels found in the samples
foods, which influence conformation of proteins, also are closely
related to flavor release (Lubbers and others 1998). Jo and Ahn
(1999) reported that the amount of volatiles released from oil
emulsion correlated negatively with fat content. The release of
nonpolar hydrocarbons was not influenced, but polar compounds, such as aldehydes, ketones, and alcohols, were greatly
influenced by water. This indicated that the relative amounts of
volatile compounds released from meat systems could be significantly different from those in the aqueous system tested here.
However, the results from this study confirmed the sources of
volatile compounds critical to irradiation odor reported earlier by
Jo and Ahn (2000).
Hashim and others (1995) described the characteristic of irradiation odor as a “bloody and sweet” aroma, and Ahn and others
(2000) described it as a “barbecued corn-like” odor. No single
group amino acid homopolymers provided such odor characteristics. The odor intensity of sulfur-containing amino acids was
much stronger and stringent than that of other amino acid
groups. This indicated that sulfur compounds were the most influential in irradiation off-odor, but volatiles from other amino
acid groups also played an important role in overall odor perception. Sulfur compounds have very low odor thresholds and most
of them are considered to be the major cause of off-odor. However, some sulfur compounds such as 2-pentylthiophene are important for freshly cooked meat flavor (Tang and others 1983).
The amounts of aldehydes, especially those of hexanal and
pentanal, are highly correlated with oxidation of lipids (Ahn and
others 1998a). This study indicated that some aldehydes such as
acetaldehyde, 2-methyl propanal, 3-methyl butanal, and 2-methyl butanal could be newly generated from amino acid side
chains after irradiation. Benzene and toluene (methyl benzene)
are considered as carcinogens. Irradiation of glycine, asparagine,
lysine, and tyrosine newly produced benzene, but irradiation reduced or destroyed toluene in nonirradiated proline, asparagine,
glutamine, lysine, serine, and tyrosine. Although toluene was detected in both irradiated and nonirradiated broiler meats (Du
and others 2001a, 2001b), it is difficult to assess its human
health implications. It is apparent, however, that toluene could
be produced from the components naturally present in meat
even without irradiation.
Conclusion
S
ULFUR COMPOUNDS PRODUCED FROM THE SIDE CHAINS OF ME-
thionine and cysteine were the most important volatiles for
off-odor production in irradiated meat. Sulfur compounds were
not only produced by the radiolytic cleavage of side chains (primary reaction), but also by the secondary reactions of primary
sulfur compounds with other compounds around them. The
amounts and kinds of sulfur compounds produced from irradiated methionine and cysteine indicated that methionine is the
major amino acid responsible for irradiation off-odor. The total
amount of sulfur compounds produced from cysteine is only
about 0.25 to 0.35% of methionine even after the proportion of
cysteine or methionine in each of the dimmer, trimer, or tetramer
was considered. Therefore, the contribution of methionine to the
irradiation odor is far greater than that of cysteine.
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MS 20010619 Submitted 11/10/01, Accepted 1/10/02, Received 3/25/02
Journal paper number J-19616 of the Iowa Agriculture and Home Economics Experiment
Station, Ames, IA 50011-3150. Project No. 6523, supported by the National Research Initiative Competitive Grant.
Author is with the Dept. of Animal Science, Iowa State Univ., Ames, IA 500113150. Direct queries to author (E-mail: duahn@iastate.edu).
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