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Volatile Profiles and Lipid Oxidation of Irradiated Cooked Chicken Meat from Laying Hens Fed Diets Containing Conjugated Linoleic Acid1
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: [email protected].
Abbreviation Key: CLA = conjugated linoleic acid; kGy = kiloGray; MS = mass spectrometry; PUFA = polyunsaturated fatty acids; TBARS = 2-thiobarbituric acid reactive substances; TCA = trichloroacetic acid.
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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
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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
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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
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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
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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.
REFERENCES Ahn, D. U., A. Ajuyah, F. H. Wolfe, and J. S. Sim, 1993. Oxygen availability affects prooxidant catalyzed lipid oxidation of cooked turkey patties. J. Food Sci. 58:278–282. Ahn, D. U., C. Jo, and D. G. Olson, 1999a. Headspace oxygen in sample vials affects volatile production of meat during the automated purge-and-trap/GC analyses. J. Agric. Food Chem. 47:2776–2781. Ahn, D. U., D. G. Olson, C. Jo, J. Love, and S. K. Jin, 1999b. Volatile production and lipid oxidation in irradiated cooked sausage as related to packaging and storage. J. Food Sci. 64:226–229. Ahn, D. U., D. G. Olson, J. I. Lee, C. Jo, X. Chen, and C. Wu, 1998. Packaging and irradiation effects on lipid oxidation and volatiles in pork patties. J. Food Sci. 63:15–19. Ahn, D. U., F. H. Wolfe, J. S. Sim, and D. H. Kim, 1992. Packaging cooked turkey meat patties while hot reduced lipid oxidation. J. Food Sci. 57:1075–1077, 1115. Belury, M. A., K. P. Nickel, C. E. Bird, and Y. Wu, 1996. Dietary conjugated linoleic acid modulation of phorbol ester skin tumor promotion. Nutr. Cancer 26:149–157.
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Du, M., D. U. Ahn, and J. L. Sell, 1999. Effect of dietary conjugated linoleic acid on the composition of egg yolk lipids. Poultry Sci. 78:1639–1645. Dugan, M.E.R., J. L. Aalhus, L. E. Jeremiah, J.K.G. Kramer, and A. L. Schaefer, 1999. The effects of feeding conjugated linoleic acid on subsequent pork quality. Can. J. Anim. Sci. 79:45–51. Hashim, I. B., A.V.A. Resurreccion, and K. H. McWatters, 1995. Disruptive sensory analysis of irradiated frozen or refrigerated chicken. J. Food Sci. 60:664–666. Ip, C., J. A. Scimeca, and H. Thompson, 1995. Effect of timing and duration of dietary conjugated linoleic acid on mammary cancer prevention. Nutr. Cancer 24:241–247. Larick, D. K., B. E. Turner, W. D. Schoenherr, M. T. Coffey, and D. H. Pilkington, 1992. Volatile compound content and fatty acid composition of pork as influenced by linoleic acid content of the diet. J. Anim. Sci. 70:1397–1403. Liu, H. F., A. M. Booren, J. I. Gray, and J. I. Crackel, 1992. Antioxidant efficacy of oleoresin rosemary and sodium tripolyphosphate in restructured pork steak. J. Food Sci. 57:803–806. Meynier, A., C. Genot, and G. Gandemer, 1999. Oxidation of muscle phospholipids in relation to their fatty acid composition with emphasis on volatile compounds. J. Sci. Food Agric. 79:797–804. SAS威 Institute, 1985. SAS User’s Guide. SAS Institute, Inc., Cary, NC. Shahidi, F., and R. B. Pegg, 1994. Hexanal as an indicator of meat flavor deterioration. J. Food Lipids 1:177–186. Van den Berg, J. J., N. E. Cook, and D. L. Tribble, 1995. Reinvestigation of the antioxidant properties of conjugated linoleic acid. Lipids 30:599–605.
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: [email protected].
Abbreviation Key: CLA = conjugated linoleic acid; kGy = kiloGray; MS = mass spectrometry; PUFA = polyunsaturated fatty acids; TBARS = 2-thiobarbituric acid reactive substances; TCA = trichloroacetic acid.
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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
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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
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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
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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
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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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