JFS:
Food Chemistry and Toxicology
Production of Volatiles from
Fatty Acids and Oils by Irradiation
E.J. LEE AND D.U. AHN
Food Chemistry and Toxicology
ABSTRACT: To understand the mechanisms of off-odor production in irradiated meat, the volatile compounds
produced from individual fatty acids by irradiation were identified. Nonirradiated oil emulsions prepared with
polyunsaturated fatty acids (PUFAs) produced many volatile compounds, but the amounts of volatiles generally
decreased after irradiation. Although volatile profiles of fatty acid emulsions were changed by irradiation, the odor
characteristics and intensity between irradiated and nonirradiated fatty acid emulsions were not different.
Thiobarbituric acid-reactive substances (TBARS) values indicated that irradiation accelerated lipid oxidation during subsequent storage, but the volatiles produced by lipid oxidation were not the major contributors of off odor in
irradiated samples.
Keywords: volatiles, fatty acid, irradiation, TBARS, odor characteristics
Introduction
I
RRADIATION IS AN EFFECTIVE METHOD FOR PATHOGEN CONTROL IN RAW
meat. It is permitted for use in both poultry and red meats. A major concern, however, is its effect on meat quality. When molecules
absorb ionizing energy, they become reactive and form ions or free
radicals that react to form stable radiolytic products (Woods and
Pikaev 1994). These reactive substances oxidize myoglobin and fat
and thus cause discoloration, rancidity, and off odor in meat (Murano 1995).
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 (PUFA) and form a lipid radical. In the presence of oxygen, the
lipid radical rapidly reacts with oxygen to form a peroxyl radical
which, in turn, can extract a hydrogen atom from another fatty acyl
chain, yielding a new free radical that can perpetuate the chain
reaction and a lipid hydroperoxide that can be degraded into various volatile compounds after a series of secondary reactions (Gray
1978; Enser 1987).
Kanatt and others (1998) reported that irradiation increased 2thiobarbituric acid-reactive substances (TBARS) values and carbonyl content in ground chicken meat. Ahn and others (1997) reported that irradiation increased lipid oxidation in raw turkey breast
and thigh meats that were aerobically packaged, but had limited
effects on the formation of total volatiles during storage at 4 °C for
7 d or longer.
Lipid oxidation is known to generate aldehydes, ketones, hydrocarbons, esters, furans, and lactones, which can be responsible for
rancid flavors and sensory defects in meat products (Ladikos and
Lougovois 1990). Aldehydes contributed the most to oxidation flavor and rancidity in cooked meat (Shahidi and Pegg 1994). Hexanal was the predominant aldehyde produced by lipid oxidation,
and hexanal content correlated the best with TBARS of meat (Ang
and Lyon 1990; Spanier and others 1992; Ahn and others 1998a,
1998b, 1999b).
The objective of this study is to determine the volatile com-
70
JOURNAL OF FOOD SCIENCE—Vol. 68, Nr. 1, 2003
pounds produced from individual fatty acids by irradiation as a step
toward understanding the mechanisms of off-odor production in
irradiated meat.
Materials and Methods
Sample preparation
Selected fatty acids generally found in meat (palmitoleic, oleic,
linoleic, linolenic, and arachidonic), corn oil (refined), and fish oil
(refined) were purchased from Sigma Chemical Co. (St. Louis, Mo.,
U.S.A.) and used to determine their contribution to lipid oxidationdependent production of volatiles by irradiation. An oil-in-water
emulsion system was used in this study because it can increase the
surface area of fatty acid or oil. Oil emulsion was prepared by blending 0.5 g of fatty acid or oil in 50 mL deionized distilled water (Waring blender; 22,000 rpm for 2 min; Dynamics Corp. America Co.,
Conn., U.S.A.). An aliquot of oil emulsion sample (15 mL) was transferred to a scintillation vial and irradiated at 0 or 5.0 kGy using a
Linear Accelerator (Circe IIIR; Thomson CSF Linac, Saint-Aubin,
France). The energy and power level used were 10 MeV and 10 kW,
respectively, and the average dose rate was 99.3 kGy/min. The max/
min ratio was approximately 1.39 (avg.). To confirm the target dose,
2 alanine dosimeters per cart were attached to the top and bottom
surface of a sample vial. The alanine dosimeter was read using a
104 Electron Paramagnetic Resonance Instrument (Bruker Instruments Inc., Billerica, Mass., U.S.A.). Immediately after irradiation,
2-mL portions of the oil emulsion were transferred to sample vials,
flushed with helium gas (99.999%) for 5 s at 40 psi, and capped.
Two of them were used to obtain volatile profiles, two for TBARS,
and the rest were used to determine odor characteristics. Volatile
profiles, TBARS, and odor characteristics of irradiated and nonirradiated oil emulsions were compared. Fatty acid compositions of
corn oil and fish oil were analyzed using the method described by
Nam and others (2001).
TBARS analysis
Oil emulsion (1 mL) was transferred to a 13 ⫻100 mm disposable
glass tube and butylated hydroxyanisole (50 ␮L, 7.2% in ethanol)
© 2003 Institute of Food Technologists
Volatiles from fatty acid by irradiation. . .
0 kGy
Volatiles
1-Pentene
Pentane
1-Methoxy-2-methyl-1-propene
2-Methyl pentane
3-Methyl pentane
2,2-Dimethyl pentane
2,3-Dimethyl pentane
3,3-Dimethyl pentane
1-Hexene
Hexane
2-Methyl hexane
3-Methyl hexane
3-Ethyl hexane
2,4-Dimethyl hexane
1-Octene
Octane
2-Octene
3-Octene
3-Methyl octane
2,6-Dimethyl octane
1-Heptene
Heptane
2,6-Dimethyl heptane
1,2,4-Trimethyl heptane
Ethyl benzene
1,3-Dimethyl benzene
2,2,3-Trimethyl butane
3-Nonen-1-ol
Undecanenitrile
Octahydro-1H-indene
1,3-Cyclopentadiene
4-Methyl cyclopentene
3-Methyl cyclopentane
Methyl cyclopentane
1,1,3-Trimethyl cyclopentane
Cyclohexane
Cyclohexene
Methyl cyclohexane
1,3-Dimethyl cyclohexane
Ethyl cyclohexane
1,1,3-Trimethyl cyclohesane
1,2,4-Trimethyl cyclohexane
1,2,3,5-Tetramethyl cyclohexane
1-Ethyl-3-methyl cyclohexane
Propyl cyclohexane
1-Ethyl-2,3-dimethyl cyclohexane
Butyl cyclohexane
1,1,2,3-Tetramethyl cyclohexane
1-Methyl-4-(1-methylethyl)
cyclohexane
1,1,4-Trimethyl cyclohexane
1,2-Dimethyl cyclooctane
Total volatiles
Odor characteristics
5 kGy
SEM
total ion counts ⫻ 104
0b
0b
81a
870a
5163a
3850a
256a
579
0b
10691 a
288a
986a
211
50b
0b
56b
305
280b
518
1467
0b
127
120b
445
767a
4246a
632a
1010
2476
1231
114a
0b
0b
32889 a
813
9492a
0b
526a
774a
490a
910
1238
577
5092a
5539
707
975
3447
952
809
2116
104749
chickeny,
fishy,
wet dog
66a
571a
0b
514b
2875b
2808b
165b
595
88a
6466b
178b
272b
176
74a
162a
80a
296
413a
456
1464
300a
133
218a
362
188b
903b
436b
952
2254
1104
0b
214a
117a
11605 b
799
2922b
91a
348b
233b
385b
768
1074
474
3443b
4691
635
947
2123
829
1
18
2
37
234
176
11
25
2
479
12
22
12
3
6
5
17
25
44
277
10
6
12
26
16
119
26
166
457
221
2
6
3
1235
153
269
2
17
18
21
54
99
38
401
599
110
214
593
116
725
129
2064
457
59615
chickeny,
fishy,
metallic,
rancid
a,b Means with no common superscript differ significantly (P ⬍ 0.05), n ⫽ 4
and thiobarbituric acid (TBA, 20 mM)/trichloroacetic acid (TCA,
15% wt/vol) solution (2 mL) were added. The mixture was vortexed
and then incubated in a boiling water bath for 15 min to develop color. The sample was then cooled in cold water for 10 min, mixed, and
centrifuged for 30 min at 3000 ⫻ g. The absorbance of the resulting
supernatant solution was determined at 531 nm against a blank
containing 1 mL of deionized distilled water and 2 mL of TBA/TCA
solution. If the absorbance of supernatant solution after color development was above 1.0, the solution was diluted properly with
water and TBA/TCA mixture (1:2) until the absorbance became ⬍ 1.0. The amounts of TBARS were expressed as milligrams of
malondialdehyde per L of oil emulsion.
Determination of volatile compounds
A purge-and-trap apparatus (Precept II and Purge & Trap Concentrator 3000; Tekmar-Dohrmann, Cincinnati, Ohio, U.S.A.) connected to a gas chromatograph/mass spectrometer (GC/MS;
Hewlett-Packard Co., Wilmington, Del., U.S.A.) was used to analyze volatiles produced (Ahn and others 2001). Oil emulsion samples (2 mL) were placed in 40-mL sample vials, and the vials were
flushed with helium gas (40 psi) for 5 s. The maximum waiting
time of a sample in a refrigerated (4 °C) holding tray was less than
6 h to minimize oxidative changes before analysis (Ahn and others
1999a). The sample was purged with helium gas (40 mL/min) for
12 min at 40 °C. Volatiles were trapped using a Tenax-charcoalsilica column ( Tekmar-Dohrmann) 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 ⫻ 0.25 mm i.d., 1.4 ␮m nominal), an
HP-1 column (52.5 m ⫻ 0.25 mm i.d., 0.25 ␮m nominal; HewlettPackard), and an HP-Wax column (7.5 m ⫻ 0.25 mm i.d., 0.25 ␮m
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 2.5 min. After that, the oven
temperature was increased to 15 °C at 2.5 °C/min, increased to
45 °C at 5 °C/min, increased to 110 °C at 20 °C/min, increased to
210 °C at 10 °C/min, and then was held for 2.5 min at the temperature. Constant column pressure at 20.5 psi was maintained. The
ionization potential of mass selective detector (Model 5973;
Hewlett-Packard) 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). 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), and the total
peak area (pA*s ⫻ 104) 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. Samples were
placed in glass scintillation vials, 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. Sensory panelists were asked to write all odor characteristics that they could recognize and the odor characteristics that all
panelists agreed on were used to determine the odor intensity of
emulsions.
Statistical analysis
Data were analyzed using the generalized linear model procedure of SAS software (SAS Institute Inc. 1989); 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.
Vol. 68, Nr. 1, 2003—JOURNAL OF FOOD SCIENCE
71
Food Chemistry and Toxicology
Table 1—Volatile compounds in irradiated oil emulsion prepared with arachidonic acid
Volatiles from fatty acid by irradiation. . .
Table 2—Volatile compounds in irradiated oil emulsion prepared with linolenic acid
0 kGy
Volatiles
5 kGy
total ion counts ⫻
0b
Food Chemistry and Toxicology
Propanal
2,3-Dimethyl butane
531
2,2,3-Trimethyl butane
17893
1-Pentene
82b
2-Pentene
3517a
2-Methyl-1-pentene
0b
3-Methyl-2-pentene
0b
Pentane
611a
2-Methyl pentane
8092
3-Methyl pentane
44759
2,3-Dimethyl pentane
1224
3,3-Dimethyl pentane
2410b
2,2,4,4-Tetramethyl pentane
354
1-Hexene
0b
2-Hexene
0b
3,3-Dimethyl-1-hexene
95b
Hexane
79779
2-Methyl hexane
1578
2,4-Dimethyl hexane
232
2,5-Dimethyl hexane
138a
3-Methyl hexane
2385a
4-Propyl-3-heptene
1258a
Heptane
968
2-Methyl heptane
136
2,6-Dimethyl heptane
209
3-Methyl heptane
150
3-Methyl heptane
2996
Octane
311a
3-Methyl octane
1119
3-Ethyl-1-octene
350a
2,5-Octadiene
100b
Piperidine
511
Ethyl benzene
6791a
1,4-Dimethyl benzene
34507 a
p-Xylene
15141 a
3-Nonen-1-ol
1512a
Undecanenitrile
4169a
cis-Octahydro-1H-indene
2055a
o-Menth-8-ene
892a
1-Ethyl-1-methyl cyclopropane
0b
1,2-Dimethyl cyclopropane
561a
1,1,2-Trimethyl-2-cyclopropane
1254a
Methyl cyclopentane
168782a
Ethyl cyclopentane
205a
3-Methyl cyclopentene
0b
3-Methyl-1-cyclopentene
0b
1,3-Dimethyl cyclopentane
202b
1,2-Dimethyl cyclopentane
247a
1,2,3-Trimethyl cyclopentane
100a
Cyclohexene
78560 a
Methyl cyclohexane
3377a
Ethyl cyclohexane
2156a
Propyl cyclohexane
11800 a
Butyl cyclohexane
1539a
1,2-Dimethyl cyclohexane
624a
1,3-Dimethyl cyclohexane
3025a
1,4-Dimethyl cyclohexane
873a
1,1,3-Trimethyl cyclohexane
2098a
1,2,3-Trimethyl cyclohexane
1080
1,2,4-Trimethyl cyclohexane
2863
1,1,2,3-Tetramethyl cyclohexane
3688a
1,1,4,4-Tetramethyl cyclohexane
3141a
1-Ethyl-2,3-dimethyl cyclohexane
966a
1-Methyl-4-(1-methyle)-cyclohexane 2068a
4-Methyl cyclohexene
812a
Total volatiles
526876
Odor characteristics
fishy,
glue-like,
fruity
SEM
104
1519a
32
321
65
12065
2336
136a
13
196b
215
a
473
56
467a
58
185b
44
5285
975
31244
4795
756
141
4908a
552
60
189
a
646
84
811a
94
212a
20
65482
6067
958
187
175
30
0b
6
1376b
258
b
826
105
955
138
109
11
170
25
140
16
1869
352
142b
36
771
126
b
200
38
172a
16
354
57
1276b
156
b
6814
693
5503b
477
991b
111
2637b
400
b
1245
156
598b
65
461a
47
319b
39
b
768
92
123599b 11090
101b
18
1538a
158
a
2829
294
905a
90
138b
27
0b
7
b
33688
7718
1832b
328
1273b
168
6856b
797
b
963
142
381b
55
746b
142
509b
63
1344b
201
756
100
1889
294
2301b
275
b
1814
269
623b
81
1031b
116
522b
66
339233
beany, fishy,
painty, fruity,
metallic
a,b Means with no common superscript differ significantly (P ⬍ 0.05), n ⫽ 4
72
JOURNAL OF FOOD SCIENCE—Vol. 68, Nr. 1, 2003
Table 3—Volatile compounds in irradiated oil emulsion prepared with linoleic acid
0 kGy
5 kGy
SEM
total ion counts ⫻ 104
Volatiles
1,1-Oxybis ethane
Butane
2,3-Dimethyl butane
Butanal
1-Pentene
2-Pentene
Pentane
3-Methylene pentane
3-Methyl pentane
Pentanal
1-Hexene
2-Hexene
3-Hexene
Hexane
Hexanal
1-Heptene
Heptane
2-Heptenal
1-Heptyne
1-Octene
2-Octene
Octane
2-Propyl furan
2-Buthyl furan
Benzene
Ethyl benzene
1,3-Dimethyl benzene
1,4-Dimethyl benzene
1,2-Dimethyl cyclopropane
Methyl cyclopentane
1-Methyl cyclopentene
Cyclohexane
Total volatiles
Odor characteristics
1600a
352b
150a
1204a
79b
0b
4994b
0b
4339a
29390 a
27772 a
0b
62678 a
131b
47947 a
0b
141b
92a
0b
0b
103a
0b
65a
560a
302a
1318a
4717a
1184a
0b
9111a
0b
182a
198411
glue-like,
metallic,
cardboard
96b
49
1336a
34
112b
5
205b
50
958a
9
232a
2
10743 a
174
70a
5
1328b
113
b
3833
1208
9213b
664
132a
4
27231 b 1047
a
494
10
6399b 4980
644a
22
203a
9
b
0
12
101a
2
108a
8
0b
6
a
70
8
0b
3
150b
39
0b
13
266b
83
1106b
763
229b
92
525a
3
b
2599
332
110a
3
119b
9
68612
glue-like,
metallic,
cardboard,
rancid
a,b Means with no common superscript differ significantly (P ⬍ 0.05), n ⫽ 4
Results and Discussion
T
ABLES 1 TO 3 SHOW THE VOLATILES PRODUCED FROM OIL EMULSIONS
of PUFAs (arachidonic, linolenic, and linoleic acid) before and
after irradiation. Many new volatiles were generated from oil emulsions of PUFAs by irradiation: pentane, 1-heptene, 4-methyl cyclopentene, and 1-octene were newly generated from arachidonic
acid; 3-methyl-1-cyclopentene, propanal, and 3-methyl cyclopentene from linolenic acid; 1-heptene, 1,2-dimethyl cyclopropane,
and 2-pentene from linoleic acid. The amounts of most of the volatiles in the oil emulsions of PUFAs, however, decreased after irradiation: methyl cyclopentane, cyclohexane, hexane, and 1,3-dimethyl benzene decreased the most in arachidonic acid emulsion;
methyl cyclopentane, cyclohexene, 1,4-dimethyl benzene, hexane,
and 3-methyl pentane in linolenic acid emulsion; and hexanal, 3hexene, pentanal, and 1-hexene in linoleic acid emulsion. Nawar
(1986) reported that a series of dienes, trienes, and tetraenes were
formed from unsaturated triacylglycerols by irradiation at 60 kGy
under vacuum conditions. Although the irradiation dose used in
this study is much lower, the number of volatiles produced from
fatty acid emulsions was greater than that of Nawar (1986). Instead of irradiating oil or fatty acids directly, this study used an oilin-water emulsion system because the emulsion system would be
Volatiles from fatty acid by irradiation. . .
0 kGy
Volatiles
Propanal
1-Pentene
Pentane
Pentanal
1-Hexene
Hexane
Hexanal
1-Heptene
Heptane
1-Octene
Octane
Toluene
Methyl cyclopentane
Total volatiles
Odor characteristics
5 kGy
total ion counts ⫻
0b
0b
0b
141a
0b
842b
65a
0b
370b
0b
335b
65a
374a
2192
bloody,
fishy,
antiseptic
SEM
104
709a
8
127a
1
974a
19
0b
8
120a
2
a
2095
212
0b
4
403a
8
5832a
125
a
308
10
3649a
145
0b
1
94b
11
14311
bloody,
fishy,
rancid,
boiled beef,
buttery
a,b Means with no common superscript differ significantly (P <0.05), n = 4
Table 5—Volatile compounds in irradiated oil emulsion prepared with palmitoleic acid
0 kGy
Volatiles
1-Pentene
Pentane
3-Methyl pentane
1-Hexene
2-Hexene
Hexane
1-Heptene
Heptane
1-Octene
Methyl bezene
Methyl cyclopentane
Methyl cyclohexane
Ethyl cyclopentane
Total volatiles
Odor characteristics
5 kGy
Table 6—Volatile compounds in irradiated oil emulsion prepared with corn oil
SEM
total ion counts ⫻ 104
0b
148a
4
0b
890a
7
a
b
94
0
3
0b
392a
4
0b
73a
0
955b
4384a
41
b
a
0
77
1
0b
83a
1
0b
198a
5
121a
0b
2
a
b
476
245
12
0b
204a
3
0b
131a
2
1646
6825
metallic,
metallic,
cardboard fishy,
rancid
a,b Means with no common superscript differ significantly (P ⬍ 0.05), n ⫽ 4
more appropriate to study the mechanisms of volatile production
in meat and meat products. Benzene and toluene (methyl benzene) were detected in irradiated fatty acids or oil emulsions. Du
and others (2001a, 2001b) detected benzene and toluene in both
irradiated and nonirradiated broiler meats. Irradiation of amino acid
homerpolymer (Ahn 2002), and liposome containing amino acid
homopolymers (Ahn and Lee 2002) also produced benzene and
toluene. This indicated that benzene and toluene were produced
from the components naturally present in meat even without irradiation. Sensory panelists described the odor characteristics of fatty
acid emulsions as “fishy” and “metallic,” and the intensity and characteristics of odor from irradiated fatty acid emulsions were not
different from those of nonirradiated emulsions.
The volatile profiles produced from oil emulsion prepared with
oleic and palmitoleic acids were similar. Volatiles such as pentane,
propanal, 1-heptene, 1-octene, and 1-pentene, were newly generated from oil emulsion prepared with oleic or palmitoleic acid by
0 kGy
Volatiles
Pentane
Hexane
1-Heptene
Heptane
1-Octene
2-Octene
Octane
Methyl cyclopentane
Total volatiles
Other characteristics
5 kGy
SEM
total ion counts ⫻ 104
0b
247a
3
237b
290a
3
0b
506a
4
b
a
0
163
1
0b
111a
2
0b
101a
2
0b
234a
6
121
120
2
358
1772
fishy,
fishy,
alcohol, alcohol,
metallic
metallic
a,b Means with no common superscript differ significantly (P ⬍ 0.05), n ⫽ 4
Table 7—Volatile compounds in irradiated oil emulsion prepared with fish oil
0 kGy
5 kGy
SEM
total ion counts ⫻ 104
Volatiles
Propanal
2-Propenal
Butanal
1-Pentene
1-Penten-3-one
2-Pentene
Pentane
Pentanal
Hexane
2-Hexene
Heptane
Octane
2,5-Octadiene
2,4-Octadiene
1,3,6-Octatriene
2-Methyl furan
2-Ethyl furan
2-Propyl furan
Alpha-pinene
Benzene
1,2-Dimethyl cyclopropane
Methyl cyclopentane
2-Propenyl cyclopentane
1,3-Cyclopentadiene
3-Ethylidene cyclohexene
Total volatiles
Odor characteristics
3812a
179
465a
103a
87a
1145a
154b
323a
757a
121a
458b
472b
2173a
1036a
128a
85a
4649a
170a
128a
121a
596a
162a
435a
97a
109a
17965
green grass,
fishy,
leather-like
2408b
134
287
67
175b
12
0b
4
0b
10
b
0
19
307a
6
81b
11
645b
24
0b
2
578a
22
728a
71
886b
112
b
464
52
0b
3
0b
1
0b
94
b
65
5
65b
11
0b
3
0b
13
b
78
3
0b
16
0b
5
0b
7
6767
green grass,
fishy,
leather-like,
rancid,
metallic,
painty
a,b Means with no common superscript differ significantly (P ⬍ 0.05), n ⫽ 4
irradiation (Tables 4 and 5). The amount of hexane increased and
that of methyl cyclopentane decreased in both oleic and palmitoleic acid emulsions by irradiation. The odor characteristics of oil
emulsion prepared with monounsaturated fatty acids (MUFAs)
were similar to those of PUFAs, but the odor intensity of MUFA
emulsions was weaker than that of PUFA emulsions.
The volatile results of MUFA and PUFA emulsions indicated that
irradiation newly produced or increased the amounts of 1-hexene,
1-heptene, 1-octene, and 1-pentene, which were known as the irradiation-dependent volatiles. Buttery and others (1973) reported
that, as the carbon number (molecular weight) of volatile comVol. 68, Nr. 1, 2003—JOURNAL OF FOOD SCIENCE
73
Food Chemistry and Toxicology
Table 4—Volatile compounds in irradiated oil emulsion prepared with oleic acid
Volatiles from fatty acid by irradiation. . .
Food Chemistry and Toxicology
pounds increased, so did the reduction rates of volatile compounds
because the increase of the carbon number sharply decreased the
air-to-solution partition coefficient. The presence of large quantities of hydrocarbons, aldehydes and ketones in nonirradiated oil
emulsions from PUFAs indicated that a significant degree of oxidative process had progressed in the oil emulsions before irradiation.
The decrease of volatiles from the PUFA emulsions by irradiation
suggested that the secondary products of lipid oxidation in the fatty
acid emulsions could have reacted with the radiolytic products in
the fatty acids to produce different volatiles or nonvolatile molecules. Consequently, the amounts of many volatiles were decreased by irradiation due to the secondary chemical reactions. In
the MUFA emulsion, however, direct impact of electron energy
should have broken acyl bonds, and produced short-chain hydrocarbons, and generated new volatiles.
A few volatiles were detected in nonirradiated oil emulsion prepared with corn oil, and 1-heptene, octane, pentane, and heptane
were newly generated after irradiation (Table 6). Nonirradiated
emulsion prepared with fish oil produced many volatiles, but the
amounts changed after irradiation (Table 7). The amounts of octane, pentane, heptane, and 2-propenal in emulsion prepared with
fish oil increased significantly after irradiation, but the increased
amounts were relatively small. The production of 2,5-octadiene
and propanal in emulsion prepared with fish oil decreased, and 2ethylfuran, 2-pentene, 1,2-dimethyl cyclopropane, and 2,4-octadiene disappeared after irradiation. Corn oil emulsion generated
many new volatiles, while fish oil emulsion decreased the number
and amount of volatiles by irradiation as in MUFA and PUFA emulsions (Tables 1 to 5). The fatty acid compositions showed that corn
oil had a much higher portion of MUFA than fish oil (Table 8) and
corn oil had almost no lipid oxidation products before irradiation.
The result of volatile analysis in corn and fish oils indicated that the
volatile profiles of irradiated oils were influenced by the composition, the length of carbon chain, and the number of double bonds
of fatty acids in oils. Gunstone (1991) reported that when the relative autooxidation rate of fatty acid with one double bond was 1, the
rates of fatty acid oxidation with 2 and 3 double bonds were 27 and
77, respectively. Therefore, the replacement of saturated fatty acid
(SAFA) and MUFA by diunsaturated fatty acid (DUFA) and PUFA
should increase the susceptibility of fatty acids to lipid oxidation
significantly.
Irradiation influenced the TBARS values of fatty acids emulsions
(Figure 1). Diehl (1995) indicated that irradiation of aqueous systems produced hydrogen peroxide, particularly in the presence of
oxygen. During post-irradiation storage, hydrogen peroxide gradually disappears while other constituents of the system are oxidized. Obviously, some oxidized compounds not present, or
present at lower concentrations immediately after irradiation, can
increase after hours or days after irradiation. In this study, TBARS
values of irradiated emulsion samples immediately after irradiation were lower than those of nonirradiated samples. After 5 d of
storage at 4 °C, however, irradiated samples developed higher
TBARS values than nonirradiated emulsions. Arachidonic acid, linolenic acid, and fish oil, which had a high proportion of multidouble-bonded fatty acids, had accelerated lipid oxidation after irradiation. Shahidi and Pegg (1994) reported that aldehydes
contributed the most to oxidation flavor and rancidity in cooked
meat, and hexanal was the predominant volatile aldehyde. Among
the volatiles of emulsion prepared with arachidonic acid, linolenic acid, or fish oil, aldehydes including 2-propenal, propanal, butanal, pentanal, and hexanal increased the most during the storage
(Table 9). Hexanal was produced only in emulsion prepared from
arachidonic acid and propanal only from linolenic acid, indicating
74
JOURNAL OF FOOD SCIENCE—Vol. 68, Nr. 1, 2003
Table 8—Fatty acid composition of corn oil and fish oil1
Fatty acid
C14:0
C15:0
C16:0
C16:1
C17:0
C18:0
C18:1
C18:2
C18:3
C20:0
C20:2
C20:3
C20:4
C20:5
C22.5
C22:6
Unidentified
Corn oil (%)
trace
trace
11.41
trace
trace
1.87
30.83
55.89
trace
trace
trace
trace
trace
trace
trace
trace
trace
Fish oil (%)
7.25
0.56
20.38
10.07
0.40
3.50
10.88
1.75
3.29 1
0.53
1.56
2.04
0.57
11.62
2.20
11.47
11.93
1 Values are average of duplicated analysis
Figure 1—TBARS of nonirradiated and irradiated oil emulsions prepared with fatty acids and oils during storage
(different letters within a same storage time differ significantly).
Volatiles from fatty acid by irradiation
Table 9—Production of aldehydes and TBARS values in irradiated and nonirradiated emulsions prepared with arachidonic acid and linolenic acid, and fish oil during storage
0kGy
5kGy
Linolenic acid
SEM
0kGy
total ion counts ⫻
Day 0
2-Propenal
Propanal
Butanal
Pentanal
Hexanal
Total aldehydes (%)
TBARS (mg/kg)
0
0
0
0
0
0
2.58a
0
0
0
0
0
0
1.41b
–
–
–
–
–
0.17
Day 10
2-Propenal
Propanal
Butanal
Pentanal
Hexanal
Total aldehydes (%)
TBARS(mg/kg)
11435 b
794a
0b
1180b
28864 b
33.2
143.43
27531 a
0b
223a
2494a
58702 a
47.9
140.10
958
13
5
94
2302
–
1.44
Fish oil
5kGy
0
0b
0
0
0
0
4.51a
SEM
0kGy
5kGy
SEM
104
0
1519a
0
0
0
0.5
1.27b
–
32
–
–
–
–
0.03
4426a
3393b
32297
30809
117a
0b
0
0
0
0
9.5
6.8
a
103.68
76.37b
280
1083
10
–
–
–
1.40
179
0
465a
323a
0
5.3
2.27
287
0
175b
81b
0
7.8
2.38
7070a
2775b
24403 a
10899 b
1314a
455b
580a
248b
0
0
79.4
87.1
a
54.26
26.86b
67
–
12
11
–
–
0.13
Food Chemistry and Toxicology
Arachidonic acid
Volatiles
386
744
21
31
–
–
0.28
n⫽4
that n-3 PUFAs are the source of propanal and n-6 PUFAs of hexanal. Fish oil that contains both n-3 and n-6 PUFAs produced both
propanal and hexanal. Longer storage time increased the amount
of aldehydes and TBARS values in these oil emulsions, but irradiation had minimal effect on the increase of aldehydes and TBARS.
Conclusion
I
RRADIATION PRODUCED A FEW NEW VOLATILES AND INCREASED THE
amount of 1-hexene, 1-heptene, 1-octene, and 1-pentene, which
were known as the irradiation-dependent volatiles, in oil emulsion
of MUFAs and PUFAs. These volatiles, however, had little effect on
the sensory characteristics of oil emulsion. The amounts of aldehydes, the indicators of lipid oxidation, in oil emulsion did not increase by irradiation, and volatiles from lipids accounted for only a
small part of the off odor in irradiated meat.
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MS 20020125 Submitted 2/25/02, Revised 3/27/02, Accepted 5/12/02, Received
5/12/02
Journal paper nr JB 19758 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA 50011-3150. Project nr 6523, supported by the National Research Initiative
Competitive Grant/U.S. Dept. of Agriculture, Washington, D.C.
The authors are with the Dept. of Animal Science, Iowa State Univ., Ames,
Iowa 50011-3150. Direct inquiries to author Ahn (E-mail: duahn@
iastate.edu).
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75