PROCESSING AND PRODUCTS
Volatile Profiles and Lipid Oxidation of Irradiated Cooked Chicken Meat from
Laying Hens Fed Diets Containing Conjugated Linoleic Acid1
M. Du, D. U. Ahn,2 K. C. Nam, and J. L. Sell
Department of Animal Science, Iowa State University, Ames, Iowa 50011-3150
ABSTRACT The objective of this study was to determine the influence of dietary conjugated linoleic acid
(CLA) on lipid oxidation, volatile profiles, and sensory
characteristics of irradiated cooked chicken meat. Fortyeight 27-wk-old White Leghorn hens were fed a diet containing 0, 1.25, 2.5, or 5.0% CLA. After 12 wk of feeding
trial, hens were slaughtered, and boneless, skinless breast
and thigh muscles were separated. Meats of three birds
from a dietary treatment were pooled and ground together through a 9-mm and a 3-mm plate, and patties
were prepared. Patties were individually packaged and
cooked in a water bath at 85 C for 15 min. After cooling
to room temperature, patties were repackaged in oxygenpermeable or oxygen-impermeable bags, irradiated at 0
or 3 kiloGray (kGy) with an electron beam irradiator, and
analyzed for lipid oxidation, volatile profiles, and sensory
characteristics at 0 and 5 d of storage at 4 C.
Cooked meat patties from hens fed CLA diets had
lower TBA-reactive substances values and produced less
hexanal and pentanal than the control. The irradiated and
nonirradiated cooked chicken meat with aerobic packaging developed severe lipid oxidation during the 5-d storage at 4 C. Irradiation accelerated lipid oxidation in aerobic-packaged cooked chicken meat, but its effect was not
as significant as that of the packaging. No odor differences
were found among the cooked chicken meats from the
different dietary CLA treatments. The increased storage
stability of cooked meat from hens fed CLA diets was
caused by the increased saturated fatty acids and CLA
content in meat lipids. Tissue CLA was stable from oxidative changes and had minimal effect on volatile production in irradiated and nonirradiated cooked chicken meat
during storage.
(Key words: conjugated linoleic acid, cooked meat, lipid oxidation, volatiles, sensory characteristics)
2001 Poultry Science 80:235–241
cooking may not be sufficient to kill pathogens. Therefore,
some cooked meat products are not always safe to be
consumed directly by consumers. Irradiation of cooked
ready-to-eat meat products can significantly improve
safety and extend shelf life of those products. Low dose
(<10 kGy) irradiation is permitted for use with raw poultry and red meats to control pathogenic bacteria but not
with cooked meat. However, irradiation has huge potential to be used with cooked meat to improve the safety
of cooked meat products.
Ahn et al. (1998, 1999b) showed that ionizing radiation
influenced lipid oxidation, volatile production, and sensory characteristics of raw pork. Poultry meat contains
more PUFA than red meat, and the effect of irradiation
on cooked meat would be quite different from that on
raw meat because cooked meat is highly susceptible to
oxidative changes (Ahn et al., 1993). The objective of this
study was to determine the influence of dietary CLA on
lipid oxidation, volatile production, and sensory characteristics of irradiated and cooked chicken meat with different packaging.
INTRODUCTION
Dietary conjugated linoleic acid (CLA) is reported to
have anticarcinogenic and antiartherogenic effects and
modulates immune response in animals (Ip et al., 1995;
Belury et al., 1996). CLA fed to animals can easily be
incorporated into tissue, milk, and egg and produces
CLA-containing foods, which have beneficial effects on
human health. Du et al. (1999) reported that CLA feeding
reduced the amount of polyunsaturated fatty acid (PUFA)
in egg yolk lipids. Thus, CLA feeding may influence the
stability of lipids and change the volatile profiles of meat.
However, little information is available on the influence
of dietary CLA on volatiles and sensory characteristics
of irradiated, cooked, ready-to-eat meat products.
Cooked, ready-to-eat products are generally safe, but
microorganisms can be introduced during packaging. For
certain meat products, low temperature treatment in final
Received for publication April 6, 2000.
Accepted for publication September 26, 2000.
1
Journal paper Number J-18832 of the Iowa Agriculture and Home
Economics Experiment Station, Ames, Iowa 50011-3150. Project Number
3322, supported by the Hatch Act and CDFIN.
2
To whom correspondence should be addressed: duahn@iastate.edu.
Abbreviation Key: CLA = conjugated linoleic acid; kGy = kiloGray;
MS = mass spectrometry; PUFA = polyunsaturated fatty acids; TBARS
= 2-thiobarbituric acid reactive substances; TCA = trichloroacetic acid.
235
236
DU ET AL.
MATERIALS AND METHODS
Sample Preparation
Forty-eight, 27-wk-old White Leghorn hens kept in individual cages were assigned to one of the four diets that
contained 0, 1.25, 2.5, or 5% CLA. The energy balance
was maintained by substituting the CLA source with soybean oil on a weight:weight basis (Du et al., 1999). After
12 wk of receiving the CLA diets, hens were killed, and
breast and leg muscles were separated. Meats were vacuum-packaged and stored at −20 C for 5 mo before use.
Breast and leg muscles of three birds from each diet group
were pooled, ground twice through a 9-mm plate and a
3-mm plate, and used as a replication.
Approximately 50 g of ground meat was individually
sealed in bags and cooked in a water bath at 85 C for
15 min. After cooling to room temperature, patties were
removed from the cooking bags and vacuum-packaged
in oxygen-permeable or oxygen-impermeable bags (O2
permeability, 9.3 mL O2/m2 per 24 h at 0 C). The patties
were irradiated at 0 or 3 kGy with an electron beam
irradiator. Samples were stored at 4 C up to 5 d. Volatile
profile, lipid oxidation TBA-reactive substances (TBARS),
and sensory characteristics of meat were determined at
0 and 5 d of storage.
Volatile Analysis
Purge-and-trap dynamic headspace gas chromatography/mass spectrometry was used to identify and quantify the volatile compounds from meat. A 0.5-g cooked
meat sample was placed in a sample vial (40 mL), and
then one pack of oxygen absorber4 was added. The sample
vial was flushed with helium gas (99.999%) for 5 s at 40
psi; capped tightly with a Teflon-lined, open-mouth cap;
and placed in a refrigerated (4 C) sample tray. The maximum holding time for samples before volatile analysis
was less than 10 h to minimize oxidative changes during
the sample holding time (Ahn et al., 1999a).
Samples were purged with helium gas (40 mL/min)
for 15 min. Volatiles were trapped at 20 C using a Tenax/
Silica gel/Charcoal column5 and were desorbed for 2 min
at 220 C. The desorbed volatiles were concentrated at
−100 C with a cryofocusing unit and then were thermally
desorbed and injected (30 s) into a capillary gas chromatography column. We used a combined HP-Wax (7.5 m)
and HP-5 (30 m)6 column. Ramped oven temperature was
used. The initial oven temperature, 0 C, was held for 1.50
min. After that the oven temperature was increased to 20
C at 4 C per min, increased to 80 C at 10 C per min,
increased to 180 C at 20 C per min, and then kept at the
temperature for 4.5 min. The column pressure was 12 psi.
The ionization potential of the mass selective detector
(HP 59736) was 70 eV, and scan range was 33.1 to 450.
Identification of volatiles was achieved by comparing
mass spectrometry data of samples with those of the Wiley library.6 Standards, when available, were used to confirm the identification by the mass selective detector. The
area of each peak was integrated using ChemStation software,6 and the total ion counts × 104 were reported as an
indicator of volatiles generated from the meat samples.
TBARS Analysis
Five grams of cooked meat was placed into a 50-mL
test tube and homogenized with 15 mL deionized distilled
water by using a homogenizer7 for 10 s at highest speed.
One milliliter of meat homogenate was transferred to a
disposable test tube (3 × 100 mm), and butylated hydroxyanisole (50 µL, 7.2%) and TBA/trichloroacetic acid (TCA;
2 mL) were added. The mixture was vortexed and then
incubated in a boiling water bath for 15 min to develop
color. The sample then was cooled in cold water for 10
min, vortexed again, and centrifuged for 15 min at 2,000
× g. The absorbance of the resulting supernatant solution
was determined at 531 nm against a blank containing 1
mL deionized distilled water and 2 mL of TBA/TCA
solution. The amounts of TBARS were expressed as milligrams of malondialdehyde per kilogram of meat (Ahn et
al., 1999a).
Sensory Analysis
A 16-member trained sensory panel was used for sensory analysis. Four sample sets (vacuum and nonirradiated, aerobic and nonirradiated, vacuum and irradiated,
and aerobic and irradiated) were presented to panelists.
Two sample sets with aerobic packaging were presented
first at 30-min intervals for smell, with a sequence of
nonirradiated and irradiated samples. After 4 h of rest,
sensory panels were reorganized to finish the remaining
two sets with vacuum packaging. For evaluation of odor,
samples in capped scintillation vials (glass) were presented to each panelist in isolated booths. A 15-cm, linear
horizontal scale, anchored with the words ‘very weak’
and ‘very strong’ at opposite ends, was used to rate the
samples on the intensity of cooked chicken meat flavor,
irradiation odor, and rancidity. The responses from panelists were expressed to the nearest 0.5 cm, in numerical
values ranging from 0 (very weak) to 15 (very strong).
Sensory panels were asked to describe the odor characteristics, irradiation odor, and any other odor difference they
found among the four different samples in each set.
Statistical Analysis
4
Ageless type Z-100, Mitsubishi Gas Chemical America, Inc., New
York, NY 10022.
5
Tekmar-Dorham, Cincinnati, OH 45249.
6
Hewlett-Packard Co., Wilmington, DE 19808.
7
Type PT 10/35, Brinkman Instruments Inc., Westbury, NY 115900207.
The effects of dietary CLA on the volatiles, TBARS, and
sensory data of cooked meat were analyzed statistically
by ANOVA with SAS威 software (SAS Institute, 1985).
Student-Newman-Keuls multiple-range test was used to
compare differences among mean values (P < 0.05). Mean
237
CONJUGATED LINOLEIC ACID AND VOLATILES OF COOKED CHICKEN
TABLE 1. Fatty acid composition of chicken meat patties prepared from laying hens fed different levels
of conjugated linoleic acid (CLA)
Diet CLA level (%)
Fatty acid
composition1
Control
1.25%
Palmitic
Palmitoleic
Stearic
Oleic
Linoleic
Linolenic
CLA (cis 9, trans 11)
CLA (trans 10, cis 12)
CLA (trans 9, trans 11)
Other CLA
Arachidonic
Total SAFA
Total MUFA
Total PUFA
Total non-CLA PUFA
20.0b
1.2a
11.7d
33.1a
26.3a
1.4a
0.0d
0.0d
0.0d
0.0d
5.6a
31.7a
34.3a
33.8
33.8a
20.3b
0.8b
12.5c
30.0b
24.8a
1.1b
1.2c
1.1c
0.6c
0.9c
4.2b
32.8b
30.8b
33.9
30.1b
2.5%
5.0%
SEM
23.5a
0.4d
15.8a
24.3d
14.6c
0.9c
4.9a
5.1a
2.3a
2.6a
2.6c
39.3c
24.7d
33.1
19.2d
0.36
0.03
0.30
0.79
0.87
0.04
0.06
0.07
0.05
0.08
0.23
0.73
0.51
0.53
0.32
(% of total lipids)
22.8a
0.5c
14.3b
27.1c
20.6b
1.0c
2.1b
2.3b
1.2b
1.6b
4.0b
36.4c
27.7c
32.8
25.6c
Means within a row with no common superscript differ significantly (P < 0.05); n = 4.
SAFA = saturated fatty acids.
2
MUFA = monounsaturated fatty acids.
3
PUFA = polyunsaturated fatty acids.
a–d
1
values, SEM, and probabilities for treatment effects were
reported. Tukey grouping analysis was employed to compare combined effects of irradiation and packaging.
RESULTS AND DISCUSSION
The average moisture content for meat patties before
cooking was 79.1%; fat was 4.0%, and pH was 5.9, with
no significant differences among these chicken patties
from different dietary CLA treatments. However, there
were significant differences in fatty acid composition (Table 1). The control diet had the most linoleic, oleic, and
arachidonic acids, whereas the 5.0% CLA diet had the
least of these fatty acids. The amount of total saturated
fatty acid in meat increased, but that of total monounsa-
turated fatty acid (MUFA) and total non-CLA PUFA decreased with the increase of dietary CLA. The amount of
total PUFA was not influenced by the dietary CLA. Large
proportions of total PUFA (approximately one-eighth,
one-fourth, and one-half of total PUFA) in meats from
dietary CLA treatments were replaced by CLA isomers.
The decrease of non-CLA PUFA was expected to improve
the storage stability of cooked meat significantly, because
the conjugated form of CLA distributes electrons more
evenly than linoleic acid and makes CLA less susceptible
to free radical attack than linoleic acid. In fact, the CLA
isomers would behave like MUFA and reduce lipid oxidation by minimizing the initiation step of lipid oxidation.
At 0 d after cooking and irradiation, the meats from
hens fed the control diet had significantly higher TBARS
TABLE 2. Amount of 2-thiobarbituric acid reactive substances (mg/kg) in cooked chicken patties at Day 0
Nonirradiated
Diet
Aerobic
packaging
Control
1.25% CLA1
2.5% CLA
5.5% CLA
SEM
3.16a
2.70ab
2.40b
1.58c
0.17
Diet (D)
Irradiation (IR)
Packaging (P)
D × IR
D×P
IR × P
D × IR × P
Vacuum
packaging
Irradiated
Aerobic
packaging
(mg malondialdehyde/kg meat)
1.91a
4.33a
1.66ab
3.79a
1.49bc
4.12a
1.25c
2.52b
0.10
0.22
(P)
0.0001
0.0001
0.0001
0.09
0.0003
0.0001
0.5
Means within a row with no common superscript differ significantly (P < 0.05); n = 4.
CLA = conjugated linoleic acid.
a–c
1
Vacuum
packaging
1.58a
1.18b
1.14b
0.63c
0.06
238
DU ET AL.
TABLE 3. Amount of 2-thiobarbituric acid reactive substances (mg/kg) in cooked chicken patties
after 5 d of storage
Nonirradiated
Diet
Aerobic
packaging
Control
1.25% CLA1
2.5% CLA
5.0% CLA
SEM
10.45a
8.51b
8.48b
6.28c
0.25
Irradiated
Vacuum
packaging
Aerobic
packaging
Vacuum
packaging
(mg malondialdehyde/kg meat)
2.75a
7.37
2.26b
8.14
1.70c
7.65
1.05d
7.21
0.15
0.63
(P)
0.0001
0.007
0.0001
0.003
0.5
0.06
0.001
Diet (D)
Irradiation (IR)
Packaging (P)
D × IR
D×P
IR × P
D × IR × P
2.77a
1.85b
1.51c
1.03d
0.08
Means within a row with no common superscript differ significantly (P < 0.05); n = 4.
CLA = conjugated linoleic acid.
a–d
1
values than those fed CLA diets, and meat from the 5.0%
CLA diet had the lowest TBARS among the treatments.
The TBARS of meat during storage correlated well with
the amounts of total CLA in chicken meat (Tables 1 to
3). However, CLA did not act as an antioxidant but simply
was less susceptible to oxidation than linoleic acid. Irradiated cooked meats with aerobic packaging had higher
TBARS values, but those with vacuum packaging had
lower TBARS than the nonirradiated cooked meats (Table
2). The presence of oxygen has a significantly increased
lipid oxidation in meat. Ahn et al. (1992) found that vacuum packaging immediately after cooking significantly
reduced the oxidation of turkey meat patties. The interaction between irradiation and packaging showed that irradiation under vacuum effectively prevented lipid oxida-
tion. The TBARS values of aerobic-packaged cooked
chicken meats after 5 d of storage were higher than that
at Day 0 (Tables 2 and 3). Irradiation effects on the TBARS
of both vacuum- and aerobic-packaged cooked meats
were not as significant and consistent as that at Day 0,
indicating that irradiation had only a minor impact on
the oxidation of cooked meat lipids during storage. The
effect of dietary CLA on the storage stability of cooked
meat was significant in vacuum-packaged meats but was
low or not present in aerobic-packaged meats after 5 d
of storage (Table 3).
In nonirradiated cooked meat at Day 0, none of the
volatiles except for nonanal was influenced by the dietary
CLA under vacuum packaging. With aerobic packaging,
however, the contents of aldehydes (propanal, butanal,
TABLE 4. Volatile profiles of nonirradiated cooked chicken meat patties at Day 0
Volatile
compounds
Aerobic packaging
Control
1
1.25% CLA
2.5% CLA
Vacuum packaging
5.0% CLA
SEM
Control
1.25% CLA
(Total ion counts × 10 )
Acetaldehyde
Propanal
Octane
2-Propanone
1-Octene
2-Octene
Butanal
2-Butanone
Pentanal
3-Methylbutanone
Propanol
2,3-Dimethyldisulfide
Hexanal
Heptanal
1-Penten-3-ol
Nonanal
Hexanol
Total volatiles
856
226a
79
327
13b
9
176a
84
1,150a
120a
14
34
4,104a
14
18
8
6
7,238a
913
160a
62
335
12b
12
155ab
84
1,060ab
91ab
13
20
3,605ab
14
20
6
4
6,566b
392
216ab
89
274
18a
14
134ab
59
838b
56b
7
24
3,441ab
22
15
5
4
5,608c
699
86b
60
435
10b
12
94b
71
826b
86ab
12
21
3,319b
36
12
5
6
5,790c
1
5.0% CLA
SEM
3
127.0
26.3
10.7
60.0
1.3
1.5
16.8
12.2
77.0
12.3
2.1
4.8
181
5.6
3.3
0.9
0.8
213.5
154
137
143
737
84
48
76
256
586
87
15
130
3,739
22
25
6a
5
6,250
Means within a row with no common superscript differ significantly (P < 0.05); n = 4.
CLA = conjugated linoleic acid.
a–d
2.5% CLA
(Total ion counts × 10 )
4
86
115
86
732
107
37
72
250
696
131
20
105
3,548
15
23
7a
6
6,036
96
90
90
818
60
25
55
277
704
105
19
126
3,286
13
27
7a
5
5,803
116
62
84
752
76
28
71
278
638
83
13
100
2,846
16
21
3b
5
5,192
18.9
19.8
14.2
99.9
13.9
9.1
10.2
18.0
106
14.2
3.2
18.7
220
3.7
5.1
0.9
1.0
306.7
239
CONJUGATED LINOLEIC ACID AND VOLATILES OF COOKED CHICKEN
TABLE 5. Volatile profiles of irradiated at 3 kiloGray (kGy) cooked chicken meat patties at Day 0
Aerobic packaging
Volatile
compounds
Control
1.25% CLA1
Acetaldehyde
Propanal
Octane
2-Propanone
1-Octene
2-Octene
Butanal
2-Butanone
Pentanal
3-Methylbutanone
Propanol
2,3-Dimethyldisulfide
Hexanal
Haptanal
1-Penten-3-ol
Nonanal
Hexanol
Total volatiles
1,314ab
135ab
75ab
380b
13
9
82
92ab
962
56
9b
61
3,989a
14
22a
7b
20a
7,239a
1,636a
164a
87a
615a
17
10
110
123a
881
62
18a
83
3,136b
29
20ab
10a
17a
7,018a
2.5% CLA
Vacuum packaging
5.0% CLA
(Total ion counts × 104)
1,587a
1,114b
82ab
31b
54bc
31c
479ab
349b
18
22
11
12
85
70
ab
104
78b
786
901
62
75
10b
6b
70
63
2,725bc
2,545c
18
19
15ab
10b
6b
4b
6ab
11b
6,118b
5,340b
SEM
Control
1.25% CLA
120
27.6
8.3
53.0
4.0
1.7
12.7
10.5
74.7
9.1
2.1
8.1
169
3.9
2.7
0.9
2.6
230.0
222a
160a
133
357
53
33
42
133
499
72
29a
42
3,738a
14
24
5
6
5,562a
153b
145a
126
425
68
38
75
118
414
68
35a
49
3,688a
29
26
7
6
5,470a
2.5% CLA
5.0% CLA
(Total ion counts × 103)
89b
101b
168a
74b
117
99
412
405
61
57
37
31
54
51
166
123
415
326
89
84
13b
16b
21
30
2,963ab
2,614b
23
24
24
14
6
6
5
5
4,665ab
4,062b
SEM
17.2
12.7
21.2
39.3
8.2
7.5
10.2
19.4
97.4
9.7
3.7
7.4
226
4.2
4.5
1.0
0.5
277.2
Means within a row with no common superscript differ significantly (P < 0.05); n = 4.
CLA = conjugated linoleic acid.
a–d
1
pentanal, and hexanal) and total volatiles in cooked
chicken meat gradually decreased as the dietary content
of CLA increased (Table 4). The amounts of aldehydes
(propanal, butanal, pentanal, and hexanal) became significantly lower than the control when the dietary CLA
level increased to 2.5 or 5%, but all dietary CLA treatments
produced less total volatiles than the control. In irradiated
cooked meat at Day 0 (Table 5), only meat from hens
fed 5% dietary CLA had consistently less acetaldehyde,
propanal, propanol, hexanal, and hexanal contents than
the control.
In nonirradiated cooked meat after 5 d of storage, volatile profiles and content under vacuum packaging were
not much different from those of Day 0 (Table 6). With
aerobic packaging, however, the amount of aldehydes,
especially those of pentanal and hexanal, increased twoto threefold from Day 0, and total volatiles increased
twofold because of the two aldehydes. The amounts of
acetaldehyde decreased during the 5-d storage under aerobic packaging with no explainable reason. The effect of
all dietary CLA treatments in reducing aldehyde production in cooked chicken meat was significant after 5 d of
storage (Table 6). In irradiated cooked meat after 5 d of
storage, all dietary CLA treatments significantly reduced
volatiles, especially aldehydes, even with vacuum packaging (Table 7). With aerobic packaging, the amounts of
propanal, pentanal, and hexanal in cooked chicken meat
greatly increased during the 5-d storage. The amount of
propanal in cooked chicken meat from hens fed 5% CLA
was significantly lower than that of the control. Also, the
TABLE 6. Volatile profiles of nonirradiated cooked chicken meat patties after 5 d of storage
Volatile
compounds
Aerobic packaging
1
2.5% CLA
Vacuum packaging
Control
1.25% CLA
5.0% CLA
151
456
119
118
95
58
2,005a
45ab
10,591a
47
23
48
25
13
23
13,817a
(Total ion counts × 10 )
173
176
237
336
242
133
92
75
106
188
220
174
129
127
186
62
51
47
1,585b
1,576b
1,395c
20b
53a
22b
8,799b
8,909b
8,190b
45
63
95
26
57
53
31
34
36
23
38
29
11
9
13
22
13
19
11,542b
11,643b
10,735b
SEM
Control
1.25% CLA
346
114a
63a
157
85
109ab
522a
26
4,725a
19
49a
10ab
50a
7
7
6,289a
(Total ion counts × 10 )
144
186
165
143a
101a
41b
ab
b
55
43
48ab
200
247
241
85
102
122
103ab
121a
82b
434b
349c
313c
27
30
36
4,042ab
3,392bc
3,086c
14
30
19
20b
27b
22b
8b
7b
13a
50a
27b
11c
7
6
7
6
5
8
5,338b
4,673bc
4,214c
4
Acetaldehyde
Propanal
Octane
2-Propanone
Butanal
Ethylacetate
Pentanal
2,3-Dimethyldisulfide
Hexanal
Heptanal
Butanol
Pentanol
Hexanol
Nonanal
Octenol
Total volatiles
1
5.0% CLA
SEM
3
30.9
92.1
23.7
37.5
35.2
8.2
51.3
7.3
380
18.0
11.4
7.8
7.0
3.4
5.1
608.6
Means within a row with no common superscript differ significantly (P < 0.05); n = 4.
CLA = conjugated linoleic acid.
a–d
2.5% CLA
49.6
15.8
4.7
42.4
14.6
8.0
26.0
3.0
241
5.6
4.9
1.2
4.6
1.6
0.9
275.1
240
DU ET AL.
TABLE 7. Volatile profiles of irradiated at 3 kiloGray (kGy) cooked chicken patties after 5 d of storage
Aerobic packaging
Vacuum packaging
Volatile
compounds
Control
1.25% CLA1
Acetaldehyde
Propanal
Octane
2-Propanone
Butanal
Ethylactate
Pentanal
2,3-Dimethyldisulfide
Hexanal
Heptanal
Butanol
Pentanol
Hexanol
Nonanal
Octenol
Total volatiles
1,139
588a
107
366
135
44
2,738
134
10,730a
18
100
15
18
8
8
16,148a
(Total ion counts × 104)
1,385
1,294
1,073
497a
514a
238b
59
88
68
476
456
528
157
160
159
55
41
73
2,308
2,399
2,101
106
121
134
8,159b
8,742b
8,101b
41
38
44
118
115
95
19
23
19
18
28
30
10
6
8
7
9
9
13,413b
14,031b
12,678b
2.5% CLA
5.0% CLA
SEM
Control
1.25% CLA
148
60.2
15.0
101
18.9
19.4
158
25.1
509
9.6
21.8
5.0
5.6
1.6
1.2
646.0
155
255a
95a
261
68
57
899a
59
5,923a
20
44
10a
11
7
9
7,873a
127
130b
52ab
152
56
25
593b
47
3,803b
21
51
9a
6
10
6
5,088b
2.5% CLA
5.0% CLA
(Total ion counts × 103)
87
118
128b
59b
68ab
36b
144
156
86
94
66
73
703ab
522c
39
47
3,542bc
2,770c
30
20
51
24
b
6
9a
6
11
8
6
7
9
4,971b
3,954c
SEM
25.5
33.5
11.6
43.0
14.0
14.0
55.4
9.8
254
3.8
6.9
0.9
1.9
1.1
1.4
261.4
Means within a row with no common superscript differ significantly (P < 0.05); n = 4.
CLA = conjugated linoleic acid.
a–d
1
content of hexanal in cooked meat from hens fed 2.5 and
5.0% CLA was lower than that of the control. The amounts
of propanal, pentanal, hexanal, and total volatiles in aerobic-packaged cooked chicken meats were two- to threefold higher than those of the vacuum-packaged meat, and
that of acetaldehyde was 8- to 10-fold higher than those
of the vacuum-packaged meat (Table 7).
Aldehydes in irradiated cooked chicken composed approximately 75 to 80% of total volatiles in vacuum-packaged meat and 85 to 90% in aerobic-packaged meat at
Day 0. After 5 d of storage, the proportion of aldehydes
in both vacuum- and aerobic-packaged cooked chicken
meat increased to 90 to 95% of total volatiles. Because
hexanal and pentanal are suggested to be good indicators
of oxidation (Liu et al., 1992; Shahidi and Pegg, 1994), the
existence of large amounts of aldehydes indicates severe
lipid oxidation in aerobic-packaged cooked chicken meat
after 5 d of storage. For vacuum-packaged meat, there
were no changes in aldehydes and total volatiles contents
during the 5-d storage, indicating that even cooked meats
were stable under vacuum-packaged conditions. Meats
from hens fed CLA produced less aldehydes and total
volatiles than the control, but dietary effect was small
compared with packaging effect. Hexanal is the major
aldehyde produced in meat by lipid oxidation, and the
differences in hexanal content in meat could be related
to the changes in fatty acid composition of meat by the
dietary CLA (Table 1). Larick et al. (1992) reported that
pork with higher linoleic acid content produced more
aldehydes, especially hexanal and pentanal. As the dietary CLA increased, an increasing amount of linoleic
acid, suggested to be the major precursors of these aldehydes (Meynier et al., 1999), was replaced by conjugated
linoleic acid. Although cooked meat from hens fed high
levels of CLA had lower TBARS, aldehydes, and total
volatiles than the control, CLA itself did not prevent lipid
oxidation and volatile production in aerobic-packaged
meat. This result suggested that CLA was less susceptible
to oxidative changes but had no antioxidant effect in meat,
which was in agreement with Van den Berg et al. (1995)
who reported that CLA did not act as an efficient radical
scavenger and had no protective effects on lipid oxidation. The improved storage stability of cooked meat from
hens fed CLA was caused by the changes in fatty acid
composition in meat and the unique structural characteristics of CLA (diene conjugation), which make it less susceptible to free radical attack.
Dietary CLA treatments had no effect on the odor of
irradiated and nonirradiated cooked chicken meat. Dugan et al. (1999) showed that 2% dietary CLA has no effect
on the sensory characteristics of cooked pork. Irradiation
produced significant odor differences in cooked chicken
meat, but the irradiation effect was relatively small (Table
8). Hashim et al. (1995) reported that irradiating uncooked
chicken meat produced a characteristic bloody and sweet
TABLE 8. Off-odor1 of cooked chicken patties after 5 d of storage
Nonirradiated
Irradiated
Diet
Aerobic
packaging
Vacuum
packaging
Aerobic
packaging
Vacuum
packaging
Control
1.25% CLA2
2.5% CLA
5.0% CLA
SEM
5.7
7.6
7.8
7.2
0.77
7.7
7.5
7.5
5.8
0.72
7.4
6.5
5.9
6.7
0.80
6.1
6.0
6.4
5.2
0.79
Diet (D)
Irradiation (IR)
Packaging (P)
D × IR
D×P
IR × P
D × IR × P
(P)
0.6
0.03
0.4
0.5
0.4
0.3
0.3
a–d
Means within a row with no common superscript differ significantly (P < 0.05); n = 16.
1
Off-odor: 0 = very weak, 15 = very strong.
2
CLA = conjugated linoleic acid.
CONJUGATED LINOLEIC ACID AND VOLATILES OF COOKED CHICKEN
aroma that remained after the meat was cooked. Some
panelists noticed a metal-like odor or rancid vegetable
oil-like odor in aerobic-packaged cooked chicken meat.
Considering high TRARS values in aerobic-packaged
cooked meat after 5 d of storage, this off-odor should be
produced mainly by lipid oxidation rather than irradiation treatment.
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