JFS: Food Chemistry and Toxicology
Sources and Mechanisms of Carbon
Monoxide Production by Irradiation
ABSTRACT: The sources and mechanisms of gas production by irradiation were determined using model systems
prepared with fatty acid, phospholipids, oil, sugars, glycolysis and TCA cycle intermediates, nucleic acids, amino
acid monomers and homopolymers, and proteins. The model systems were irradiated at 0, 2.5, 5, or 10 kGy using a
linear accelerator and the amounts of CO, CO2, and CH4 produced were determined using gas chromatography. The
productions of CO, CO2, and CH4 in all samples were irradiation-dose dependent. Glycine, asparagine, and glutamine
were the major sources of CO production among amino acids, and glyceraldehydes, pyruvate, and ␣-ketoglutarate
were the major sources of CO among glycolysis intermediates. Phosphatidyl choline, phosphatidyl ethanolamine,
and lysophosphatidyl choline produced the greatest amounts of CO among the phospholipids. The major sources of
CO2 production were pyruvate, threoine, and methionine, and those of CH4 were methionine and acetone. The amounts
of CO produced from these sources were significant, and the production of gas compounds via radiolytic degradation appears to be closely related to the structure of molecules.
Keywords: carbon monoxide, carbon dioxide, methane, model system, irradiation
Introduction
C
olor is a major sensory attribute determining consumer accep
tance of meat. Consumers expect the color of uncured cooked
light meats such as oven-roasted poultry breast meat or poultry
breast rolls to be white. Therefore, if those meats show pink or red
color, consumers suspect that they are contaminated or undercooked. Millar and others (1995) found that irradiated chicken
breasts had a definite color change from the usual brown/purple to
a pink/red, but the degree of redness varied depending upon various factors such as irradiation dose, animal species, muscle type,
and packaging type (Satterlee and others 1971; Shahidi and others
1991; Millar and others 1995; Luchsinger and others 1996; Nanke
and others 1998).
Nanke and others (1998) proposed the color compound in irradiated meat to be an oxymyoglobin-like pigment. However, the red
pigment cannot be oxymyoglobin because the red color formed by
irradiation is mainly produced under anoxic conditions, and the
increase of redness was dose-dependent (Jo and others 2000). Nam
and Ahn (2002a, 2002b) characterized the pink pigment formed in
irradiated raw and cooked turkeys as CO-myoglobin (CO-Mb).
Furuta and others (1992) and Woods and Pikaev (1994) reported that
a considerable amount of CO was produced by radiolysis of organic
components in irradiated frozen meat and poultry. CO has a very
strong affinity to heme pigments and thus easily forms CO-Mb
complex, which increases the intensity of red meat and blood color
significantly. The affinity of CO to heme pigments is significantly
influenced by the valence of heme iron and the oxidation-reduction
potential (ORP) of meat determines the status of iron in heme pigments. Our preliminary study indicated that irradiation of meat
decreased ORP (increased reducing potential) and produced gaseous compounds that can act as a 6th ligand of myoglobin (Hannah
MS 20040079 Submitted 2/10/04, Revised 3/15/04, Accepted 4/5/04. Authors
Lee and Ahn are with the Dept. of Animal Science, Iowa State Univ., Ames, IA
50011-3150. Direct inquiries to author Ahn (E-mail: duahn@iastate.edu).
© 2004 Institute of Food Technologists
Further reproduction without permission is prohibited
and Simic 1985; Nam and Ahn 2002a,b). Therefore, irradiation generates favorable conditions for CO-Mb complex formation, which
intensifies the redness of heme pigments.
Although CO-Mb was considered as the major pigment responsible for pinking in irradiated meat, no attempt has been made to elucidate the sources and mechanisms of CO production by irradiation.
In addition to CO, this study also was expected to show the production
of other gas compounds such as CO2 and CH4 because the production of these compounds may provide significant clues in understanding the mechanism of CO production in irradiated meat.
The objectives of this study were to determine the sources of CO
production and to elucidate the mechanisms of CO production in
meat by irradiation.
Materials and Methods
Sample preparation
Model systems were prepared with various components generally
found in meat: fatty acids and oils (oleic acid, linoleic acid, linolenic
acid, phosphatidyl choline, corn oil, and fish oil), carbohydrates
(glucose, fructose, starch, and glycogen), glycolysis and citric acid
(TCA) cycle intermediates (glucose-6-phosphate, acetone, pyruvate,
lactate, ␣-ketoglutarate, citrate, oxaloacetate, glyceraldehydes, adenosine-5′ triphosphate, and 3-phosphate glycerol), nucleic acids
(adenine, guanine, cytosine, uracil, and thymine), amino acid
monomers (glycine, leucine, threoine, lysine, histidine, tyrosine,
tryptophan, glutamate, aspartate, asparagine, cysteine, and methionine), amino acid homopolymers (glycine, leucine, threonine,
lysine, tyrosine, glutamate, aspartate, asparagine, glutathione, and
met-gly-met-met), and proteins (albumin and hemoglobin).
All chemicals were purchased from Sigma Chemical Co. (St. Louis, Mo., U.S.A.). For fatty acids and oils, oil-in-water emulsion systems were prepared by blending (Waring blender; 22000 rpm for 2
min; Dynamics Corp. America Co., New Hartford, Conn., U.S.A.)
0.8 g of fatty acid or oil with 80 mL deionized distilled water. PhosVol. 69, Nr. 6, 2004—JOURNAL OF FOOD SCIENCE C485
Published on Web 7/27/2004
Food Chemistry and Toxicology
E.J. LEE AND D.U. AHN
Mechanism of carbon monoxide production . . .
Table 1—The production of CO, CO2, and CH 4 from fatty acids and oils by irradiationa,b
Unit (ppmd)
0 kGy
2.5 kGy
5 kGy
Food Chemistry and Toxicology
CO
Oleic acid
Linoleic acid
Linolenic acid
Phosphatidyl choline
Phosphatidyl ethanolamine
Lysophosphatidyl choline
Corn oil
Fish oil
S.E.M.
18.98 y
12.58 by
26.57 y
28.20 dy
24.73 cy
63.45 cx
34.38 y
22.56 by
5.75
20.93 z
26.57 az
22.78 z
276.90 cw
130.69 by
195.77 bx
47.72 z
37.42 az
9.11
20.39 z
25.81 ayz
30.37 yz
399.68 bv
177.87 ax
207.48 bw
31.78 yz
48.05 ay
5.75
CO2
Oleic acid
Linoleic acid
Linolenic acid
Phosphatidyl choline
Phosphatidyl ethanolamine
Lysophosphatidyl choline
Corn oil
Fish oil
S.E.M.
179.28 bx
180.37 cx
155.42 bxy
133.41 dxy
97.61 dy
128.53 bxy
108.46 dxy
116.38 cxy
17.03
466.70 awx
316.70 by
199.35 bz
509.22 cw
611.17 cv
174.95 bz
440.35 cwx
368.76 bwy
30.37
493.28 axy
413.23 abxy
385.58 ay
703.36 bvw
785.25 bv
163.23 bz
604.12 bwx
485.14 bxy
51.08
CH4
Oleic acid
Linoleic acid
Linolenic acid
Phosphatidyl choline
Phosphatidyl ethanolamine
Lysophosphatidyl choline
Corn oil
Fish oil
S.E.M.
0.00
0.00
0.00 b
0.00 c
0.00b
0.00
0.00
0.00
0.00
0.00
0.00
0.00 b
0.00 c
0.00b
0.00
0.00
0.00
0.00
0.00 y
0.00 y
0.00 by
11.06 bx
0.00 by
0.00 y
0.00 y
0.00 y
1.74
10 kGy
S.E.M.c
16.81 z
20.07 abz
37.96 z
472.13 ax
185.79 ay
608.46 aw
37.20 z
36.66 az
9.54
3.04
2.49
6.07
10.30
7.16
14.32
4.01
3.58
548.81 az
468.00 az
423.86 az
1253.80 ay
1624.51 ax
3219.64 aw
731.02 az
778.74 az
122.67
42.08
36.66
40.46
55.75
42.84
150.76
35.14
39.05
0.00 z
0.00 z
4.34 ay
23.64 aw
13.34 ax
0.00 z
0.00 z
0.00 z
0.54
0.00
0.00
0.33
2.93
0.22
0.00
0.00
0.00
aDifferent letters (a-d) within a row with the same sample indicate statistically significant difference (P < 0.05).
phatidylcholine, phosphatidylethanolamine, or lysophosphatidyl
choline (100 mg dissolved in chloroform) was evaporated from chloroform to a thin film on the wall of a 40-mL sample vial. The vial was
placed under a stream of nitrogen to remove any chloroform. Phospholipid liposome systems were prepared by hydrating each phospholipid with 10 mL water by gently shaking for 15 min. The milky
suspension was then vortex-mixed to disperse the phospholipid
before use. For water-soluble compounds, an aqueous solution of
each component (10 mg/mL) was prepared.
Four 5-mL portions of 1% solutions (1 from each replication)
were transferred to scintillation vials and irradiated at 0, 2.5, 5, or
10 kGy using a linear accelerator (Model 10/15; San Diego, Calif.,
U.S.A.). The energy and power level used were 10 MeV and 30 kW
(2 parallel 15 kW e-beams), respectively, and the average dose rate
was 99.3 kGy/min. The max/min ratios were 1.05, 1.02, and 1.04
(average) for 2.5, 5, and 10 kGy, respectively. To confirm the target
dose, 4 alanine dosimeters per cart were attached to the top and
bottom surfaces of a sample vial. The alanine dosimeter was read
using a Bruker e-Scan II (Bruker Instruments Inc., Billerica, Mass.,
U.S.A.). Four replications were prepared for all model systems.
Table 2—The production of CO, CO2, and CH4 from carbohydrates by irradiationa,b
Unit (ppmd)
0 kGy
2.5 kGy
CO
Glucose
Fructose
Starch
Glycogen
S.E.M.
15.08 c
13.67 d
13.77 c
13.88 d
2.39
37.42 bcy
53.47 cx
40.46 ay
54.23 cx
3.47
CO2
Glucose
Fructose
Starch
Glycogen
S.E.M.
75.92 by
136.44 bx
146.42 dx
128.85 cx
11.17
259.00 ay
294.47 axy
340.35 cx
295.88 bxy
16.59
CH4
Glucose
Fructose
Starch
Glycogen
S.E.M.
0.00
0.00
0.00
0.00 b
0.00
0.00
0.00
0.00
0.00 b
0.00
5 kGy
10 kGy
SEMc
57.48 aby 85.47 ax
86.55 bx 116.92 ax
33.95 bz
32.00 by
74.62 bx 115.51 ax
4.45
14.32
9.76
8.79
1.95
4.77
292.30 a
359.33 a
474.51 b
317.57 b
55.64
373.97 ay
394.25 ay
960.20 ax
434.71 ay
61.39
54.12
35.68
32.00
24.84
0.00
0.00
0.00
0.00 b
0.00
0.00 y
0.00 y
0.00 y
3.69 ax
0.22
0.00
0.00
0.00
0.11
aDifferent letters (a-d) within a row with the same sample indicate statistically
Determination of gas compounds
To identify the gaseous compounds produced by irradiation, CO
gas was purchased from Aldrich (Milwaukee, Wis., U.S.A.), and CH4,
and CO2 were purchased from Praxair (Danbury, Conn., U.S.A.). The
standard gases were analyzed using a gas chromatograph (GC,
Model 6890; Hewlett Packard Co., Wilmington, Del., U.S.A.) with a
flame ionization detector (FID). The method of Furuta and others
(1992) was modified for the detection of carbon-related gases. The
control or irradiated sample (5 mL) was placed in a 20-mL glass vial.
C486
JOURNAL OF FOOD SCIENCE—Vol. 69, Nr. 6, 2004
significant difference (P < 0.05).
bDifferent letters (x-z) within a column with the same irradiation dose indicate
statistically significant difference (P < 0.05).
cS.E.M., standard error of the means (n = 4).
dGas concentration in headspace (19 mL) from 5 mL 1% sample solution.
To minimize experimental errors due to air incorporation, the vials
were flushed with helium gas (40 psi) for 5 s before irradiation. In
our previous study, irradiated turkey meat samples were microwaved to release gas compounds from meat (Nam and Ahn 2002a,
URLs and E-mail addresses are active links at www.ift.org
Mechanism of carbon monoxide production . . .
Table 3—The production of CO, CO2, and CH4 from glycolysis intermediates by irradiationa,b
Unit (ppmd)
2.5 kGy
46.64 cx
17.79 by
14.43 by
27.44 bxy
45.88 bx
22.02 xy
14.53 dy
36.12 dxy
22.56 abxy
14.43 dy
6.18
87.64 by
27.98 bz
37.96 bz
25.27 bz
73.75 aby
24.40 z
29.83 cz
168.44 cx
24.73 az
28.74 cz
9.87
58.57 dy
114.75 bcy
408.68 by
186.33 by
368.00 dy
176.25 cy
309921.00 ax
194.47 dy
41.00 ay
169.74 dy
8283.42
0.00
0.00 d
0.00 b
0.00 b
0.00
0.00
0.00 b
0.00 b
0.00 b
0.00
0.00
5 kGy
114.43 by
26.36 by
46.64 by
22.02 by
68.87 aby
20.39 y
41.21 by
345.23 bx
15.51 by
42.62 by
23.21
10 kGy
171.15 aw
45.88 axyz
91.97 axy
38.50 ayz
99.57 ax
22.02 yz
63.77 axyz
740.24 av
21.15 abyz
91.43 axy
13.12
SEMc
9.54
3.36
10.30
3.15
9.22
3.47
3.47
36.98
2.06
3.80
154.01 cy
216.92 by
2508.46 by
198.81 by
2280.70 cy
1964.21 by
277911.00 ax
495.45 cy
35.25 ay
301.30 cy
1484.93
295.34 by
210.74 by
5106.62 by
184.17 by
4089.81 by
2802.39 by
290112.00 ax
648.92 by
43.17 ay
524.95 by
1671.69
413.56 az
350.11 az
15352.50 ay
252.71 az
6870.72 az
4328.10 az
316309.00 ax
992.95 az
48.05 az
932.54 az
2931.35
20.72
30.26
2040.46
11.93
193.28
334.06
12653.70
29.07
3.80
41.00
0.00 z
30.37 cx
0.00 bz
3.04 by
0.00 z
0.00 z
0.00 bz
0.00 bz
0.00 bz
0.00 z
0.43
0.00 z
58.57 bw
5.86 bx
3.36 by
0.00 z
0.00 z
0.00 bz
0.00 bz
0.00 bz
0.00 z
0.65
0.00 z
130.69 ax
27.98 ay
10.74 az
0.00 z
0.00 z
4.23 az
27.12 ay
4.12 az
0.00 z
4.88
0.00
2.71
6.18
1.19
0.00
0.00
0.11
1.08
0.22
0.00
a Different letters (a-d) within a row with the same sample indicate statistically significant difference (P < 0.05).
b Different letters (v-z) within a column with the same irradiation dose indicate statistically significant difference (P < 0.05).
cS.E.M., standard error of the means (n = 4).
d Gas concentration in headspace (19 mL) from 5 mL 1% sample solution.
2002b). To provide the same conditions as in irradiated meat, all
samples were microwaved for 10 s at full power. Ten minutes after
microwave heating, the headspace gas of each sample (200 ␮L) was
withdrawn using an airtight syringe and injected into a splitless inlet
of a GC (Model 6890; Hewlett Packard Co.). A Carboxen-1006 Plot
column (30 m ⫻ 0.32-mm inner dia; Supelco, Bellefonte, Pa., U.S.A.)
was used. Helium was used as a carrier gas at a constant flow of 1.8
mL/min and oven conditions were set at 120 ⬚C. A FID equipped
with a nickel catalyst (Hewlett Packard Co.) was used for the methanization of CO and CO2, and the temperatures of inlet, detector,
and nickel catalyst were 250 ⬚C, 280 ⬚C, and 375 ⬚C, respectively.
Detector (FID) air, H2, and make-up gas (He) flows were 350, 35, and
40 mL/min, respectively. The identification of gaseous compounds
was achieved using standard gases and a gas chromatography/mass
spectrometer (GC/MS), and the area of each peak was integrated by
using Chemstation software (Hewlett Packard Co.). To quantify the
amount of gases released, peak areas (pA * second) were converted to the concentration (ppm) of gas in the sample headspace (15
mL) using CO2 concentration (330 ppm) in air.
Statistical analysis
Data were analyzed using the generalized linear model procedure of SAS software (SAS Inst. 1995); Student-Newman-Keul’s
URLs and E-mail addresses are active links at www.ift.org
multiple range test was used to determine significant differences
between the mean values of treatments. Mean values and standard
error of the means (SEM) were reported. Significance was defined
at P < 0.05.
Results and Discussion
Sources of gas production by irradiation
The production of CO, CO2, and CH4 in samples prepared with
fatty acid, phospholipid, or plant oil were irradiation-dose dependent (Table 1). Ionizing radiation is known to generate hydroxyl
radicals in aqueous (Thakur and Singh 1994) or oil emulsion systems (O’Connell and Garner 1983). The hydroxyl radical is the most
reactive oxygen species. It can initiate lipid oxidation by abstracting a hydrogen atom from a fatty acyl chain of a polyunsaturated
fatty acid and form a lipid radical. After the initial cleavage at the
weakest bond of fatty acids or their esters by irradiation, a variety
of compounds are formed by the subsequent chemical reactions.
The scission at the acyl-oxygen bond among typical compounds
accounts for the formation of a major aldehyde, CO, and alcohol or
water and that of the alkyl-oxygen bond generated free fatty acid,
CO2, the Cn–1 alkane, and, possibly, some short-chain hydrocarbons
(Josephson and Peterson 2000).
Vol. 69, Nr. 6, 2004 —JOURNAL OF FOOD SCIENCE
C487
Food Chemistry and Toxicology
CO
Glucose-6-phosphate
Acetone
Pyruvate
Lactate
/alpha/-Ketoglutarate
Citrate
Oxaloacetate
Glyceraldehyde
Glyceraldehyde-3-phosphate
Adenosine-5-triphosphate
S.E.M.
CO2
Glucose-6-phosphate
Acetone
Pyruvate
Lactate
/alpha/-Ketoglutarate
Citrate
Oxaloacetate
Glyceraldehyde
Glyceraldehyde-3-phosphate
Adenosine-5-triphosphate
S.E.M.
CH4
Glucose-6-phosphate
Acetone
Pyruvate
Lactate
/alpha/-Ketoglutarate
Citrate
Oxaloacetate
Glyceraldehyde
Glyceraldehyde-3-phosphate
Adenosine-5-triphosphate
S.E.M.
0 kGy
Mechanism of carbon monoxide production . . .
Table 4—The production of CO, CO2, and CH4 from nucleic
acids by irradiationa,b
0 kGy
Unit (ppmd)
2.5 kGy
5 kGy
Food Chemistry and Toxicology
CO
Adenine
Guanine
Cytosine
Uracil
Thymine
S.E.M.
11.28
13.12 b
14.64 c
15.73 b
12.58 b
2.06
16.05
19.31 ab
24.19 b
27.98 b
22.23 ab
3.04
CO2
Adenine
Guanine
Cytosine
Uracil
Thymine
S.E.M.
147.29 w
49.13 cy
28.20 bz
131.24 cw
110.95 cdx
6.40
179.83 y
48.59 cz
30.37 bz
271.69 bx
180.37 bcy
12.15
170.61 y
73.21 bz
37.42 az
305.64 bw
221.26 bx
14.86
CH4
Adenine
Guanine
Cytosine
Uracil
Thymine
S.E.M.
0.00
0.00 b
0.00 b
0.00 b
0.00 b
0.00
0.00
0.00 b
0.00 b
0.00 b
0.00 b
0.00
0.00
0.00 b
0.00 b
0.00 b
0.00 b
0.00
16.16 y
22.34 abxy
30.91 bx
29.61 bxy
16.81 aby
3.25
Table 5—The production of CO, CO2, and CH4 from amino acid
monomers by irradiationa,b
Unit (ppmd)
10 kGy
SEMc
15.73 z
28.20 ayz
44.25 ay
69.74 ax
29.83 ayz
4.77
2.39
3.04
2.71
3.80
3.80
0 kGy
CO
Glycine
Leucine
Threonine
Lysine
Histidine
Tyrosine
Tryptophan
Glutamate
Aspartate
Asparagine
Glutamine
Cysteine
Methionine
S.E.M.
2.5 kGy
15.51 cyz
15.73 byz
20.07 bcxyz
19.52 xyz
31.45 w
23.10 bwxyz
25.27 wxyz
23.64 wxyz
27.66 wx
26.03 dwxyz
15.18 cy
26.90 awxy
21.48 bwxyz
2.39
49.53 by
17.68 bz
31.45 abyz
28.20 yz
22.02 z
47.72 ay
22.56 z
31.45 yz
20.39 z
80.26 cx
48.27 by
18.44 bz
21.15 bz
5.31
195.23 yz
136.44 az
41.76 az
677.66 ax
324.30 ay
49.67
19.85
4.66
1.95
40.89
28.20
0.00 y
4.66 ax
3.80 ax
3.80 ax
3.80 ax
0.33
0.00
0.22
0.11
0.00
0.00
CO2
Glycine
Leucine
Threonine
Lysine
Histidine
Tyrosine
Tryptophan
Glutamate
Aspartate
Asparagine
Glutamine
Cysteine
Methionine
S.E.M.
154.56 dxy
126.14 cy
132.65 cxy
134.82 cxy
141.54 cxy
143.49 bxy
172.99 bx
154.88 dxy
164.32 dxy
165.40 dxy
84.60 dz
169.52 dx
146.42 dxy
8.68
1191.22 cw
177.12 bz
3261.72 bu
317.25 byz
223.43 cz
375.05 ayz
264.97 bz
875.27 cx
1301.30 cw
972.89 cx
538.83 cy
393.71 cyz
2633.73 cv
66.49
Phosphatidyl choline (PC), phosphatidyl ethanolamine (PE), and
lysophosphatidyl choline (LPC) produced the highest amounts of
CO among fatty acids and oils. The greatest amounts of CO2 were
generated from PC, PE, and LPC, and the production of CO2 from
LPC at 10 kGy were about 10 times greater than that at 0 kGy. Small
amounts of CH4 were detected only in linoleic acid, PC, and PE
after >5 kGy irradiation.
The proposed mechanism of CO and CO2 production from phosphoglycerides is that hydroxyl radical generated by high-energy
radiation breaks the ester bonds between fatty acids and glycerol
1st, and then the -CO- group or carboxylic group of fatty acids is
further degraded to produce either CO or CO2 gas. A glycerol backbone, 2 fatty acid chains, or a fatty acid and a hydrogen, a phosphate, and a choline or ethanolamine, are the common denominators of PC, PE, and LPC. The susceptibility of certain bonds to
radiolytic degradation or any other chemical reactions is decided
by bond strength or bond dissociation enthalpy. The bond strength
in a molecule is dependent on its component atoms, and the bond
strength of a double bond is greater than that of a single bond because of electron localization (Atkins 1979; Halliwell and Gutteridge
1989a). This, in turn, weakens the bond strength of adjacent atoms
with single bonds. Because of the C = O double-bond in the carboxyl
end of a fatty acid, the ester bonds of fatty acids to the glycerol
backbone are the most susceptible to radiolytic degradation in
phosphoglycerides.
Triglycerides such as corn oil and fish oil produced much lower
amounts of CO and CO2 than phosphoglycerides because triglycerides are nonpolar and have no direct contact with water molecules, whereas phosphoglycerides have a polar end, which allows
direct contact with water. This provides an important explanation
of why phosphoglycerides in a liposome system (aqueous) produce
greater amounts of CO and CO2 than triglycerides and free fatty
acids in an oil-in-water emulsion system. Irradiation produces
CH4
Glycine
Leucine
Threonine
Lysine
Histidine
Tyrosine
Tryptophan
Glutamate
Aspartate
Asparagine
Glutamine
Cysteine
Methionine
S.E.M.
0.00 b
0.00 d
0.00 c
0.00 d
0.00 b
0.00 b
0.00
0.00 b
0.00 b
0.00
0.00
0.00 b
0.00 d
0.00
0.00 bz
6.62 cz
0.00 cz
23.32 cy
0.00 bz
0.00 bz
0.00z
0.00 bz
0.00 bz
0.00 z
0.00 z
0.00 bz
245.66 cx
2.82
aDifferent letters (a-d) within a row with the same sample indicate statistically
significant difference (P < 0.05).
bDifferent letters (x-z) within a column with the same irradiation dose indicate
statistically significant difference (P < 0.05).
cS.E.M., standard error of the means (n = 4).
dGas concentration in headspace (19 mL) from 5 mL 1% sample solution.
C488
JOURNAL OF FOOD SCIENCE—Vol. 69, Nr. 6, 2004
5 kGy
10 kGy
SEMc
52.39 bx
72.13 ax
15.51 bz
26.36 ayz
30.37 abyz 41.21 ay
21.69 yz
26.36 yz
22.23 yz
31.78 yz
38.29 axyz 38.83 ayz
20.93 yz
23.32 z
20.39 yz
32.00 yz
40.13 xy
38.29 yz
97.61 bw
123.10 aw
55.31 bx
72.99 ax
19.85 byz
32.32 ayz
20.61 byz
37.42 ayz
5.10
3.58
3.90
1.95
3.90
3.25
2.93
4.23
2.82
4.99
7.27
3.47
5.31
1.95
2.39
1715.84 bv 2531.78 awx 62.91
172.45 bz
248.92 az
11.82
3954.99 bt 10868.60 au 459.44
299.13 bz
472.13 az
12.26
740.78 bxy 2416.49 awx 118.76
375.05 az
435.47 az
24.73
218.87 bz
379.61 az
25.38
1080.26 bw 2011.39 axy 57.48
1681.13 bv 3426.03 aw 113.23
1805.86 bv 3391.33 aw 125.49
863.34 bwx 1511.17 axyz 47.29
518.76 byz 1112.58 ayz 34.16
3260.09 bu 7074.68 av 110.63
97.94
304.45
0.00 bz
11.39 bx
3.04 by
43.38 bw
0.00 bz
0.00 bz
0.00 z
0.00 bz
0.00 bz
0.00 z
0.00 z
0.00 bz
461.50 bv
0.54
4.12 az
22.56 ay
8.13 az
96.53 ax
4.77 az
4.45 az
0.00 z
5.53 az
4.12 az
0.00 z
0.00 z
4.66 az
888.61 aw
2.49
0.22
0.43
0.33
0.76
0.11
0.33
0.00
0.54
0.11
0.00
0.00
0.22
5.97
aDifferent letters (a-d) within a row with the same sample indicate statistically
significant difference (P < 0.05).
bDifferent letters (t-z) within a column with the same irradiation dose indicate
statistically significant difference (P < 0.05).
cS.E.M. = standard error of the means (n = 4).
dGas concentration in headspace (19 mL) from 5 mL 1% sample solution.
hydroxyl radicals by splitting water molecules and the half-life of the
free radicals are very short (10–6 s) (Halliwell and Gutteridge 1989b).
Therefore, the hydroxyl radicals produced by irradiation cannot
travel far, and the chemical reaction should be instantaneous and
site-specific.
The productions of CO in glucose, fructose, and glycogen were
irradiation dose–dependent, and the major sources of CO in carbohytrates were fructose and glycogen (Table 2). Starch was the major
source of CO2 production, and glycogen was the only carbohytrate
that generated CH4 gas at 10 kGy. Among glycolysis intermediates,
glyceraldehyde was the major source of CO production, and much
CO gas was also detected in glucose-6-phosphate, pyruvate, and
adenosine-5-triphosphate by irradiation. Pyruvate, ␣-ketoglutarate,
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Mechanism of carbon monoxide production . . .
Table 6—The production of CO, CO2, and CH4 from protein and amino acid homopolymers by irradiationa,b
Unit (ppmd)
CO
Albumin
Hemoglobin
Glycine homopolymer
Threonine homopolymer
Lysine homopolymer
Tyrosine homopolymer
Glutamate homopolymer
Aspartate homopolymer
Asparagine homopolymer
Glutathione
Met-Gly-Met-Met
S.E.M.
22.02 dxy
39.91 dxy
15.18 dy
46.42 dx
26.36 cxy
21.48 bxy
32.86 bxy
24.19 cxy
31.24 dxy
47.18 abx
13.56 dy
6.51
CO2
Albumin
Hemoglobin
Glycine homopolymer
Threonine homopolymer
Lysine homopolymer
Tyrosine homopolymer
Glutamate homopolymer
Aspartate homopolymer
Asparagine homopolymer
Glutathione
Met-Gly-Met-Met
S.E.M.
144.79 dz
252.71 dxyz
311.61 dwxy
429.50 dw
340.56 awxy
141.87 cz
170.28 bz
131.78 dz
345.99 cwxy
383.73 cwx
222.89 cyz
36.44
CH4
Albumin
Hemoglobin
Glycine
Threonine
Lysine
Tyrosine
Glutamate
Aspartate
Asparagine
Glutathione
Met-Gly-Met-Met
S.E.M.
0.00 d
0.00 d
0.00 c
0.00 b
0.00 c
0.00 b
0.00 c
0.00 b
0.00
0.00 b
0.00 d
0.00
2.5 kGy
5 kGy
10 kGy
SEMc
95.77 cyz
101.95 cy
46.64 cz
76.25 cyz
52.93 byz
50.43 ayz
70.28 ayz
78.63 byz
473.97 cx
43.17 bz
77.55 cyz
11.61
164.86 by
188.50 by
60.20 bz
91.43 bz
68.33 bz
47.72 z
67.03 az
77.33 bz
823.21 bx
48.81 abz
101.74 bz
13.45
227.22aw
259.00 av
77.87 az
146.42 ax
112.26 ay
65.94 z
77.01 az
122.34 axy
962.04 au
56.18 az
144.25 ax
8.89
9.54
11.71
3.47
2.60
6.62
6.94
3.15
4.77
24.08
2.60
4.34
279.07 cyz
492.19 cwx
531.45 cvwx
605.75 cvw
326.79 ay
459.33 bx
207.16 az
460.41 cx
628.31 bv
521.15 bvwx
428.20 bx
31.56
427.12 by
705.32 bvw
638.07 bwx
797.51 bvw
353.90 ay
498.37 bxy
228.85 az
832.43 bv
638.61 bwx
469.63 bcy
454.99 by
42.73
517.35 ay
908.89 auv
776.57 avwx
915.73 auv
472.67 ay
822.99 avw
244.36 az
1048.81 atu
1171.15 at
657.05 awxy
589.48 axy
54.01
28.74
39.70
20.17
29.07
50.00
47.61
11.93
45.77
52.82
36.33
31.67
35.03 bx
20.93 cxy
0.00 cy
0.00 by
0.00 cy
0.00 by
0.00 cy
0.00 by
0.00 y
0.00 by
369.04 cy
5.42
37.74 byz
45.01 by
2.28 bz
2.82 bz
6.18 bz
0.00 bz
9.87 bz
0.00 bz
0.00 z
0.00 bz
728.09 bx
8.79
68.87 ay
91.65 ax
5.31 az
10.74 az
9.44 az
6.18 az
14.64 az
5.75 az
0.00 z
4.66 az
1336.23 aw
6.18
8.46
3.58
0.11
0.87
0.65
0.33
0.43
0.33
0.00
0.11
15.18
Food Chemistry and Toxicology
0 kGy
a Different letters (a-d) within a row with the same sample indicate statistically significant difference (P < 0.05).
b Different letters (t-z) within a column with the same irradiation dose indicate statistically significant difference (P < 0.05).
cS.E.M. = standard error of the means (n = 4).
d Gas concentration in headspace (19 mL) from 5 mL 1% sample solution.
citrate, and adenosine-5-phosphate were the major sources of CO2
production among glycolysis intermediates by irradiation and were
closely related to their molecular structures (Table 3). Oxaloacetate,
␣-ketoglutarate, and citrate have 2 or 3 carboxyl groups in their
molecular structures. It was assumed that CO2 gas was originated
from the carboxyl groups during irradiation. Oxaloacetate produced
very high amounts of CO 2 in all irradiation treatments, but the
amounts among irradiation treatments were not significant. This
indicates that bond strength of carboxylic groups in oxaloacetate is
weaker than others and can be broken even by relatively low energy
radiation such as microwave heating. Acetone was the major source
of CH4 gas, and small amounts of CH4 gas were also detected in
pyruvate and lactate when irradiated at 10 kGy. Acetone, pyruvate,
and lactate have 1 or 2 methyl groups in their molecular structures,
and it was presumed that CH4 gas was produced from the methyl
groups of these compounds via radiolytic processes.
The nucleic acids adenine, guanine, cytosine, uracil, and thymine
were more stable than other compounds and produced only limited
amounts of CO, CO2, and CH4 gases (Table 4). Only very small
amounts of CO were generated from nucleic acids and the amounts
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of CO2 produced from uracil and thymine were irradiation dose–
dependent. Small amounts of CH4 were detected only when nucleic
acids were irradiated at 10 kGy.
Table 5 shows the production of CO, CO2, and CH4 from amino
acid monomers by irradiation. Large amounts of CO gas were detected in asparagine, glutamine, and glycine. This indicated that
amino acids that contained amide as a side chain produced a large
amount of CO gas after irradiation. Most amino acid monomers
produced great amounts of CO2, but threoine and methionine produced the highest amount of CO2. Methionine, lysine, and leucine
were the major sources of CH 4 production among amino acid
monomers by irradiation. The amounts of CH4 produced from
methionine were 10 times greater than those of lysine. Methionine
and leucine have also 1 or 2 methyl groups in their molecular structures. The proposed mechanism of CO, CO2, and CH4 production
by irradiation is that the methyl group in methionine can be broken down easily by irradiation because the bond energy of the
methyl group bound to sulfur atom is weaker than that of others
(Simic and others 1992). Therefore, methionine generated the largest
amount of CH4 gas by irradiation, even though methionine has only
Vol. 69, Nr. 6, 2004 —JOURNAL OF FOOD SCIENCE
C489
Mechanism of carbon monoxide production . . .
Food Chemistry and Toxicology
1 methyl group in its structure. This result agrees with the data from
acetone, pyruvate, and lactate, which also have methyl groups.
Amino acid homopolymers generated higher amounts of gases
than others (Table 6). Asparagine homopolymer was the major source
of CO among amino acid homopolymers, and the amount of CO in
asparagine homopolymer was 10 times higher than those of asparagine monomer. Large amounts of CO were also produced from albumin and hemoglobin upon irradiation. The major sources of CO2
in amino acid homopolymers were aspartate homopolymer and
asparagine homopolymer. Hemoglobin, albumin, threonine homopolymer, and glycine homopolymer also produced much CO2 gas.
Met-gly-met-met was the major source of CH4 and albumin, and
hemoglobin also generated much CH4. The results with amino acid
homopolymers generally agreed with those of amino acid monomer.
Conclusions
A
sparagine homopolymer, glyceraldehydes, and phospholipids
produced the greatest amounts of CO gas by irradiation. The
amounts of CO produced from these sources were large enough to
react with most of the heme pigments present in light meats such
as poultry breast and pork loin, and the production of gas compounds (CO, CO2, and CH4) via the radiolytic degradation was closely related to the chemical structure of molecules.
References
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