Meat Science 56 (2000) 387±395 www.elsevier.com/locate/meatsci In¯uence of dietary conjugated linoleic acid on volatile pro®les, color and lipid oxidation of irradiated raw chicken meat M. Du, D.U. Ahn *, K.C. Nam, J.L. Sell Department of Animal Science, Iowa State University, Ames, IA 50011-3150, USA Abstract Forty-eight, 27-week-old White Leghorn hens were fed a diet containing 0, 1.25, 2.5 or 5.0% conjugated linoleic acid (CLA) for 12 weeks. At the end of the 12-week feeding trial, hens were slaughtered, and boneless, skinless breast and leg meats were separated from carcasses. Meats were ground through 9 and 3-mm plates, and patties were prepared. Patties prepared from each dietary treatment were divided into two groups and either vacuum- or aerobic-packaged. Patties were irradiated at 0 or 3.0 kGy using a linear accelerator and stored at 4 C. Samples were analyzed for thiobarbituric acid reactive substances, volatile pro®les, color and odor characteristics at 0 and 7 days of storage. Dietary CLA reduced the degree of lipid oxidation in raw chicken meat during storage. The content of hexanal and pentanal in raw chicken meat signi®cantly decreased as dietary CLA level increased. Irradiation accelerated lipid oxidation in meat with aerobic packaging, but irradiation eect was not as signi®cant as that of the packaging. Dietary CLA treatment improved the color stability of chicken patties. Color a*-value of irradiated raw chicken meat was higher than that of the nonirradiated meat. Dietary CLA decreased the content of polyunsaturated fatty acid and increased CLA in chicken muscles, which improved lipid and color stability and reduced volatile production in irradiated and nonirradiated raw chicken meat during storage. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Conjugated linoleic acid; Volatiles; Color; Raw chicken meat; Lipid oxidation; O-odor 1. Introduction Dietary conjugated linoleic acid (CLA) has been shown to have bene®cial eects on human health (Belury, Nickel, Bird & Wu, 1996; Ip, Scimeca & Thompson, 1995). CLA can be incorporated into meat, milk and egg by supplementing CLA sources in animal diets. CLA sources are prepared by alkali isomerization of linoleic acid-rich plant oils, and are available as free acid forms. Ip et al. (1999) indicated that CLA in butter from CLA-fed cows (triglyceride form) had higher tissue retention rates and had better anticancer eect than the equal amount of CLA sources (free acid forms) did in rats. However, dietary CLA may aect the sensory characteristics of meat, milk or egg. Loor and Herbein (1998) reported that CLA-fed cows produced milk with far less fat than controls. Ahn, Sell, Jo, Chamruspollert and Jeery, (1999) found that hard-boiled eggs from * Corresponding author. Tel.: +1-515-294-6595; fax: +1-515-2949143. E-mail address: duahn@iastate.edu (D.U. Ahn). hens fed CLA-enriched diet were rubbery and elastic, and were dicult to break using an Instron. In pork, CLA feeding improved the marbling of loin and reduced overall fat content (Dugan, Aalhus, Jeremiah, Kramer & Schaefer, 1999). However, the in¯uence of dietary CLA on the volatiles, color and odor characteristics of meat has not been studied. Irradiation treatment is the best method to control bacterial load in raw meat (Farkas, 1998). However, ionizing radiation generates free radicals that may induce lipid peroxidation and other chemical changes, and in¯uence the quality of foods (Branka, Branka & Dusan, 1992; Wong, Herald & Hachmeister, 1995). Poultry meat contains more polyunsaturated fatty acids (PUFA) than red meat and can be more susceptible to oxidative changes by irradiation. Dietary CLA is reported to reduce the content of PUFA in meat (Du, Ahn & Sell, 1999; Meynier, Genot & Gandemer, 1999). Therefore, meats from animals fed CLA will be less susceptible to lipid oxidation, color changes and volatile production than those from a control diet. Meat odor and color after irradiation are critical factor that can 0309-1740/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0309-1740(00)00067-X 388 M. Du et al. / Meat Science 56 (2000) 387±395 in¯uence consumer acceptance of the meat. The objective of this study was to determine the in¯uence of dietary CLA on lipid and color stability, volatile production and odor characteristics of raw chicken meat with different irradiation and packaging conditions. 2. Materials and methods 2.1. Sample preparation Forty-eight, 27-week-old White Leghorn hens kept in individual cages were assigned to one of the four diets containing 0, 1.25, 2.5 or 5% CLA source containing about 62% CLA isomers (Conlinco, Inc. Detroit Lakes, MN). The energy level was maintained by substituting the CLA source with soybean oil on a weight:weight basis (Du, et al., 1999). After 12 weeks of feeding with experimental diets, hens were sacri®ced, and breast and leg muscles were separated, vacuum-packaged and stored at ÿ20 C for 6 months. Meats of three birds from a dietary treatment were pooled and ground together through 9 and 3-mm plates, and used as a replication. Patties (40 g) prepared from each dietary treatment were divided into two groups and either vacuum- or aerobic-packaged. Patties were irradiated at 0 or 3.0 kGy using a linear accelerator. Samples were analyzed for thiobarbituric acid reactive substances (TBARS) and volatile pro®le at 0 and 7 days of storage at 4 C. Odor characteristics and color were analyzed at 7 days of storage. 2.2. Separation of lipid classes and fatty acid composition analysis Lipid separation and composition analyses were done as described in Du et al. (1999). Brie¯y, 2 g of meat patties was weighed into a test tube with 20 ml solvent (chloroform: methanol=2:1, vol.vol.) and homogenized. Twenty-®ve mg of butylated hydroxyanisole (BHA, 10%) dissolved in 98% ethanol was added to sample prior to homogenization. The homogenate was ®ltered into a 100-ml graduated cylinder and 5 ml of 0.88% NaCl solution was added. Then, the ®ltrate was mixed well. The contents were stored until the aqueous and organic layers clearly separated. The lower layer was dried at 50 C under nitrogen. One ml of methylating reagent (anhydrous methanolic-HCl-3N, Supelco, Bellefonte, PA) was added into extracted lipid and incubated in a water bath at 60 C for 40 min. Two ml of hexane was added to extract methylated fatty acids. The top hexane layer containing methylated fatty acids was used for gas chromatography±mass spectrometry (GC±MS) analysis. GC±MS conditions were the same as described in Du et al. (1999). 2.3. Volatile analysis A purge-and-trap apparatus connected to a gas chromatograph (GC) was used to analyze the volatiles from meat patties. Precept II and Purge-and-Trap Concentrator 3000 (Tekmer-Dohrman, Cincinnati, OH) were used to purge and collect volatiles. Two g of raw meat were placed in a sample vial (40 ml) and then added in one pack of oxygen absorber (Ageless Type Z-100, Mitsubishi Gas Chemical America, Inc., New York). The vials were ¯ushed with helium gas (99.999%) for 5 s at 40 psi and capped tightly with a Te¯on-lined, open-mouth cap. Vials were placed in a refrigerated (4 C) sample-tray. The maximum sample holding time in the sample tray before volatile analysis was less than 10 h to minimize oxidative changes during the sample holding time (Ahn, Jo & Olson, 1999b, 1999c). Meat samples were purged with helium gas (40 ml/min) for 15 min, and volatiles were trapped at 20 C using a Tenax/silica gel/charcoal column (TekmarDorham) and desorbed for 2 min at 220 C. The desorbed volatiles were concentrated at ÿ100 C using a cryofocusing unit before being thermally desorbed (220 C) and injected (30 s) into a capillary GC column. Ramped oven temperature was used. The initial oven temperature was 0 C and was held for 1.5 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 for 4.50 min. The column used was an HP-Wax (7.5 m) and HP-5 (30 m, Hewlett-Packard Co.) combined column, and the ¯ow pressure was set at 12 psi. A mass selective detector (MSD, HP 5973, Hewlett-Packard Co.) was used to determine volatile components. The ionization potential of the MS was 70 eV, and scan range was 33.1±450. Identi®cation of volatiles was achieved by comparing mass spectral data of samples with those of the Wiley library (Hewlett-Packard Co.) and also the standards. The area of each peak was integrated by using ChemStation software (HewlettPackard Co.), and total ion counts104 was reported as an indicator of volatiles generated from meat samples. 2.4. TBARS analysis Three g of meat was weighed into a 50-ml test tube and homogenized with 15 ml of deionized distilled water using a Polytron homogenizer (Type PT 10/35, Brinkman Instruments, Inc., Westbury, NY) for 10 s at highest speed. One ml of the meat homogenate was transferred to a disposable test tube (3100 mm), and butylated hydroxyanisole (50 ml, 7.2%) and thiobarbituric acid/trichloroacetic acid (TBA/TCA) (2 ml) were added. The mixture was vortexed and then incubated in a boiling water bath for 15 min to develop color. The M. Du et al. / Meat Science 56 (2000) 387±395 sample was then cooled in cold water for 10 min, vortexed again and centrifuged for 15 min at 2000 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. The amounts of TBARS were determined by comparing the standard curve of absorbance at 531 nm for series of malondialdehyde solutions analyzed by the same method, and expressed as milligrams of malondialdehyde per kilogram of meat (Ahn, Olson, Lee, Jo, Chen & Wu, 1998). 2.5. Color measurement The color of meat patties was measured in package using a Hunter LabScan Colorimeter (Hunter Laboratory, Inc., Reston, VA) and expressed as color L (lightness), a* (redness) and b* (yellowness) values. The same package materials were used to cover a white standard plate in order to eliminate the in¯uence of packaging materials on meat color. 2.6. Sensory analysis The intensity and descriptive characteristics of odor of raw chicken meat were determined using a 16-member trained sensory panel. Training sessions were conducted to familiarize panelists with the irradiation odor, the scale to be used and with the range of attribute intensities likely to be encountered during the study. Four sample sets using packaging and irradiation combinations (vacuum-packaged/nonirradiated, aerobicpackaged/nonirradiated, vacuum-packaged/irradiated and aerobic-packaged/irradiated) were prepared. Two sets of samples were presented to sensory panelists with a 30-min interval. For evaluation of odor, samples in coded, capped scintillation vials (glass) were presented to each panelist in isolated booths. A 15-cm linear horizontal scale, anchored with the words `no o-odor' and `very strong o-odor' at opposite ends, was used to rate the samples on the intensity of o-odor. The responses from the panelists were expressed in numerical values ranging from 0 (no o-odor) to 15 (strong o-odor) to the nearest 0.5 cm. Sensory panelists were also asked to characterize the odor that best described it. 3. Results and discussion 3.1. TBARS values The basic chemical composition indicated that there were no dierences in total fat and water content, and pH values in raw chicken meat from hens fed dierent dietary CLA. The fatty acid composition analysis, however, showed that dietary CLA reduced the content of monounsaturated fatty acids and non-CLA PUFA, but increased saturated fatty acids content in chicken meat. Increasing amounts of linoleic acid in raw chicken meat were replaced by CLA as dietary levels of CLA increased (Table 1). TBARS results at day 0 indicated that meats from hens fed CLA had lower TBARS than controls under all packaging and irradiation conditions (Table 2). As dietary CLA levels increased, TBARS in both irradiated and nonirradiated meat decreased, and maximum decrease of TBARS value was observed at 5.0% dietary CLA treatment. The increased storage stability in meats from hens fed CLA should be caused by the increase in saturated fatty acid (SAFA) and decreased non-CLA PUFA in meat. Although CLA itself did not act as an antioxidant, its conjugated structure made the fatty acid less susceptible to free radical attacks. Irradiation and packaging also had signi®cant in¯uence on the TBARS values (Table 2). Signi®cant interaction between diet and irradiation, and diet and packaging indicated that dietary CLA improved the stability of meat lipids during irradiation and storage under aerobic conditions. After 7 days of storage, the TBARS values of aerobicpackaged raw chicken meat were much higher than that of the 0 day, indicating signi®cant development of lipid oxidation in those meats during the storage. The TBARS values of meat from hens fed CLA produced less TBARS than the control, and a 2.5% or higher level of CLA treatment was better than 1.25% in reducing lipid oxidation in aerobic-packaged chicken meat Table 1 Fatty acid composition of chicken meat patties prepared from laying hens fed dierent levels of CLAa Fatty acid composition Level of CLA (% of total fatty acids) 2.7. Statistical analysis The eect of dietary CLA on the volatiles, TBARS and sensory data of cooked meat was analyzed statistically by GLM using SAS1 software (SAS Institute, 1985). Student±Newman±Keuls multiple range test was used to compare dierences among mean values (P<0.05). Mean values and SEM were reported. Tukey grouping analysis was employed to analyze the possibilities of diet, irradiation and packaging eects. 389 Linoleic acid Arachidonic acid Total CLA Total SAFA Total MUFA Total PUFA Total non-CLA PUFA Control 1.25% CLA 2.5% CLA 5.0% CLA SEM 26.3a 5.6a 0.0d 31.7a 34.3a 33.8 33.8a 24.8a 4.2b 3.8c 32.8b 30.8b 33.9 30.1b 20.6b 4.0b 7.2b 36.4c 27.7c 32.8 25.6c 14.6c 2.6c 13.9a 39.3c 24.7d 33.1 19.2d 0.87 0.23 0.26 0.73 0.51 0.53 0.32 a Means within a row with no common letter dier signi®cantly (p<0.05); n=4. 390 M. Du et al. / Meat Science 56 (2000) 387±395 lower aldehyde contents in meats from hens fed CLA diets than the control could be related to the decrease in non-CLA n-6 fatty acids (linoleic and arachidonic acids) in those meats (Table 1). The contents of hexanal and pentanal detected in this study showed that they were positively related to linoleic acid but negatively related to CLA content in meat (Table 1). This indicated that CLA in meat did not participate in lipid oxidation and suggested that CLA was not susceptible to oxidative change. N-6 PUFA, such as linoleic acid and arachidonic acid, are suggested to be the precursors for hexanal (Meynier et al., 1999). With vacuum packaging, the content of 2-propanone in both irradiated and nonirradiated raw meat from 5% dietary CLA treatment was higher, but that of the hexanal was lower than other dietary treatments (Table 3). The reason for the increased 2-propanone and decreased hexanal levels in raw meat from high dietary CLA (5%) was not clear, but should not be related to lipid oxidation in the meat, especially with vacuum packaging. After 7 days of storage, the amount of total volatiles in aerobic-packaged raw chicken meat increased from 0 during storage. However, irradiation had no signi®cant eect on the oxidation of raw chicken meat during the 7-day storage (Table 2). No increase in TBARS was observed in raw chicken meat with vacuum packaging. 3.2. Volatile pro®les At day 0 with aerobic packaging, nonirradiated raw meat from hens fed CLA produced signi®cantly lower amounts of acetaldehyde, propanal, pentanal, hexanal and total volatiles than the control. The content of 2propanone in meat from hens fed 5% CLA was higher than that of the other dietary treatments (Table 3). Volatile pro®les and the eect of CLA on the content of aldehydes, 2-propanone and total volatiles in irradiated raw chicken meat also had similar trends as in aerobicpackaged. However, the content of hexanal in irradiated raw chicken meat was several folds higher than that of the nonirradiated meat, and dietary CLA signi®cantly reduced it (Table 3). The amounts of aldehydes and total volatiles in both irradiated and nonirradiated meat were in good agreement with TBARS (Table 2). The Table 2 TBARS values (mg/kg) of raw chicken patties after 0 and 7 days of storagea Nonirradiated Diet Aerobic packaging Day 0 Control 1.25% CLA 2.5% CLA 5.0% CLA S.E.M. Irradiated Vacuum packaging 2.88a 1.58b 1.22c 0.73d 0.04 1.07a 0.68b 0.56c 0.52c 0.04 3.61a 1.61b 1.46b 0.77c 0.15 1.28a 0.86b 0.72c 0.56d 0.03 10.55a 7.46b 4.50c 4.20c 0.26 1.22a 0.90b 0.74b 0.56c 0.05 Probability 0.0001 0.0002 0.0001 0.004 0.0001 0.0001 0.04 9.39a 7.21b 6.29b 3.45c 0.304 0.65a 0.58b 0.54b 0.47c 0.02 Diet (D) Irradiation (IR) Packaging (P) DIR DP IRP DIRP a Vacuum packaging (TBARS values (mg/kg)) Diet (D) Irradiation (IR) Packaging (P) DIR DP IRP DIRP Day 7 Control 1.25% CLA 2.5% CLA 5.0% CLA S.E.M. Aerobic packaging Means within a row with no common letters dier signi®cantly (p<0.05); n=4. Probability 0.0001 0.06 0.0001 0.0001 0.0001 0.3 0.0001 M. Du et al. / Meat Science 56 (2000) 387±395 391 Table 3 Volatile pro®les of nonirradiated and irradiated raw chicken patties after 0 day of storagea Volatile compounds Aerobic packaging (total ion counts104) Vacuum packaging (total ion counts104) Control 1.25% 2.5% 5.0% S.E.M. Non irritated Acetaldehyde 1-Heptene Propanal Octane 2-Propanone 1-Octene Pentanal Hexanal Butanol 1-Penten-3-ol Total volatiles 49a 34 133a 36 559b 0 304a 3708a 17 8a 4848a 19b 14 42b 28 791b 0 213ab 1391b 16 4b 2518b 13b 17 28b 48 656b 0 119ab 1614b 12 5ab 2512b 13b 10 16b 34 1112a 0 73b 786b 12 6ab 2062b Irradiated at 3.0 kGy Acetaldehyde 1-Heptene Propanal Octane 2-Propanone 1-Octene Pentanal Hexanal Butanol 1-Penten-3-ol Total volatiles 186a 37 63a 53 770b 24 343a 4029a 26 15 5546a 119b 27 30b 50 988ab 26 256ab 2663b 19 7 4185b 122b 23 25b 65 955ab 38 188b 1301c 14 6 2737c 103b 18 18b 55 1351a 16 158b 552d 17 6 2294c a Control 1.25% 2.5% 5.0% S.E.M. 7.7 10.5 17.8 5.6 90.2 0.0 49.1 442 2.9 0.9 346.0 4 0 0 0 732b 21 0 29 7 0 793b 3 0 0 0 613b 20 0 18 9 0 663b 4 0 0 0 645b 17 0 17 6 0 689b 5 0 0 0 1047a 20 0 28 11 0 1111a 1.0 0.0 0.0 0.0 68.2 2.1 0.0 4.4 1.9 0.0 73.7 14.5 11.3 18.4 7.9 138 7.4 40.6 233 2.8 2.8 469.9 11 19 0 0 900b 31 27 505a 7 0 1500 13 16 0 0 1041b 28 33 131b 7 0 1269 13 17 0 0 889b 26 25 98b 10 0 1078 13 14 0 0 1269a 23 24 72b 12 0 1427 2.4 4.9 0.0 0.0 66.9 4.7 4.3 36.7 1.4 0.0 101.7 Means within a row with no common letter dier signi®cantly (p<0.05); n=4. day by ®ve- to six-fold. Among the volatiles, the increases of propanal, pentanal and hexanal were the most signi®cant in both irradiated and nonirradiated raw chicken meats (Table 4). A few other aldehydes such as butanal, 3-methyl butanal, heptanal and octanal, not found in raw chicken meat at day 0, also were detected. The amounts of aldehydes in chicken meats from hens fed CLA were lower than the control, but the proportional increase of aldehydes in CLA meat during the 7-day storage was higher than the control. This indicated that CLA itself has no antioxidant eect in meat. This is in agreement with Van den Berg, Cook and Tribble (1995), who reported that CLA was not an ecient radical scavenger and had no protective eects on lipid oxidation. CLA is a mixture of linoleic acid isomers, but CLA is much more stable to oxidative changes than linoleic acid because of the conjugated arrangement of double bonds in CLA. Hexanal and pentanal contents in volatiles were suggested to be good indicators of oxidation ( Ahn et al., 1998; Liu, Booren, Gray & Crackel, 1992; Shahidi & Pegg, 1994). In this study, we also found that there were positive relationships between aldehydes and TBARS values. Aldehydes composed over 90% of total volatiles in both irradiated and nonirradiated raw chicken meat after 7 days of storage, indicating severe lipid oxidation under aerobic conditions. With vacuum packaging, however, there was not much change in volatiles content in both irradiated and nonirradiated raw chicken meats during the 7-day storage (Table 4). Thus, raw meats were stable under vacuum regardless of irradiation conditions. Production of aldehydes in raw meat was strongly in¯uenced by packaging, but diet and irradiation had only limited impact on those volatiles (Table 5). The irradiation eect on the volatiles indicated that 3 kGy irradiation had no in¯uence on the volatile composition of raw chicken meat after 7 days of storage. Packaging had signi®cant eects on almost all volatiles in raw chicken meat, showing the importance of oxygen exclusion in minimizing oxidative changes of meat (Tables 4 and 5). 3.3. Color change Dietary CLA treatment in¯uenced meat color (Table 6). After 7 days of storage, meat from hens fed a 5.0% CLA diet had lower L- and b*values, and higher a* than the control. Visually, the color of meat from the control diet appeared a little lighter and grayer than that of the 5.0% CLA diet. This indicated that CLA improved meat color after 7 days of storage, which may be related to the improved oxidative stability of meat from CLA feeding groups. Packaging and irradiation had signi®cant eects on all L, a* and b* values (Table 6). Irradiation increased the a* value of raw chicken 392 M. Du et al. / Meat Science 56 (2000) 387±395 Table 4 Volatile pro®les of nonirradiated and irradiated raw chicken patties after 7 days of storagea Aerobic packaging (total ion counts104) Vacuum packaging (total ion counts104) Volatile compounds Control 1.25% 2.5% 5.0% S.E.M. Nonirradiated Acetaldehyde 1-Heptene Propanal Octane 2-Propanone 1-Octene Butanal 2-Butanone 3-Methylbutanal Pentanal 2-Methylpentanal Hexanal Heptanal 1-Penten-3-ol Octanal Hexanol Total volatiles 142a 27 975a 133a 992a 15 147 162a 50a 2168a 267a 21 774a 102 335a 43 39 27 371a 81ab 35 640ab 80b 896ab 9 142 130ab 27b 1784ab 177ab 14 485b 136 207b 33 48 18 910b 34b 46 248b 53b 438b 10 51 81c 20b 500c 46b 14 947d 59 67c 19 25 16 644bc 91ab 30 173b 57b 871ab 15 92 104bc 19b 1110bc 34b 8789c 97 54c 28 21 11 566c 21.0 6.3 178 8.7 130 3.4 24.8 12.0 4.4 247 41.6 1166 19.3 24.6 9.4 6.8 1636 21 18 0 30 438 31 0 25 0 0 0 34 0 0 0 5 602 Irradiated at 3.0 kGy Acetaldehyde 1-Heptene Propanal Octane 2-Propanone 1-Octene Butanal 2-Butanone 3-Methylbutanal Pentanal 2-Methylpentanal Hexanal Heptanal 1-Penten-3-ol Octanal Hexanol Total volatiles 390 39 1645a 186 1241 30 122 148 114 1630 214a 23 397a 364 245a 45a 26 29 836a 235 28 764b 101 1253 22 150 152 58 1694 145ab 12 464b 138 142b 28ab 18 17 391b 196 42 753b 138 1123 19 158 160 96 2119 87ab 13 547b 164 110b 22ab 19 18 753b 147 48 122c 102 1216 16 152 118 65 1683 56b 9143b 164 47b 14b 18 13 111b 60.4 18.7 135 0.6 174.2 3.4 33.2 26.1 26.0 378 35.6 1800 130 29.8 6.6 5.4 2402 9 19 0 36 477 10 0 50 27 126a 0 78 0 0 0 9a 841 a Control 1.25% 2.5% 5.0% S.E.M. 8 27 0 32 611 22 0 25 0 0 0 27 0 0 0 10 762 11 34 0 32 581 25 0 23 0 0 0 13 0 0 0 9 728 7 28 0 26 629 29 0 22 0 0 0 18 0 0 0 8 767 3.3 11.8 0.0 7.1 71.8 7.4 0.0 3.0 0.0 0.0 0.0 7.7 0.0 0.0 0.0 1.5 92.0 11 26 0 48 497 19 0 36 18 100a 0 32 0 0 0 8ab 795 11 35 0 37 541 22 0 47 15 37b 0 36 0 0 0 6ab 787 13 31 0 34 720 10 0 52 15 19b 0 22 0 0 0 4b 920 2.1 7.7 0.0 6.0 71.6 5.3 0.0 12.0 3.4 16.2 0.0 13.6 0.0 0.0 0.0 0.9 100.9 Means within a row with no common letter dier signi®cantly (p<0.05); n=4. meat, and irradiated meat appeared redder than the nonirradiated. This result agrees with that of Nanke, Sebranek and Olson (1998). Large dierences in all L-, a*- and b* values between vacuum- and aerobic-packaging indicated that vacuum packaging was helpful in preserving meat color. Luchsinger et al. (1996) reported that irradiated vacuum-packaged pork chops appeared redder and were more stable during storage. 3.4. Sensory analysis No dierence in o-odor among raw chicken meats from dierent CLA diets and between packaging within an irradiation treatment was found (Table 7). However, irradiation had a signi®cant eect on the o-odor of raw chicken meat (P<0.0001) after 7 days of storage. Approximately two-fold higher o-odor scores in vacuum-packaged irradiated raw chicken meat compared with nonirradiated meat indicated that the dierence in o-odor between irradiated and nonirradiated meat is not oxidation-related. Heath et al. (1990) and Hashim, Resurreccion and McWatters (1995) showed that irradiating raw chicken meat produced a characteristic bloody and sweet aroma. Ahn, Jo and Olson (1999a) reported that sulfur-containing volatiles, not lipid oxidation-dependent volatiles, were responsible for the o-odor in irradiated pork. Irradiation-dependent production of sulfur compounds was not dose-dependent at <10 kGy, but was related to radiolytic degradation of amino acids. Batzer and Doty (1955) found that methyl mercaptan and hydrogen sul®de were important to irradiation odor. Angelini, Merritt, Mendelshon M. Du et al. / Meat Science 56 (2000) 387±395 393 occur following irradiation are distinctly dierent from those of warmed-over ¯avor in oxidized meat. These results indicated that sulfur-containing compounds could be the major volatile components responsible for irradiation odor in meat. Our recent tests with dierent column combinations, which detected large amounts of sulfur-containing compounds in irradiated meats supported this concept (unpublished data). However, no and King (1975) reported that most sulfur compounds had low odor thresholds and were considered as important to irradiation odor. Patterson and Stevenson (1995) found that dimethyl trisul®de is the most potent oodor compound, followed by cis-3- and trans-6-nonenals, oct-1-en-3-one and bis(methylthio-)methane in irradiated chicken meat. These studies also provided evidence to support the concept that the changes that Table 5 The probabilities of diet, irradiation and package eects on the volatile composition of raw meat patties after 7 days of storagea Probability Volatiles Diet Irradiation Packaging Acetaldehyde 1-Heptene Propanal Octane 2-Propanone Octene Butanal 2-Butanone 3-Methylbutanal Pentanal 2-Methylpentanal Hexanal Heptanal 1-Penten-3-ol Octanal Hexanol 0.004 0.4 0.0001 0.0001 0.2 0.7 0.5 0.2 0.07 0.2 0.0001 0.0001 0.6 0.0001 0.02 0.04 0.0001 0.6 0.007 0.0001 0.002 0.6 0.08 0.004 0.0001 0.04 0.98 0.05 0.1 0.1 0.5 0.002 0.0001 0.1 0.0001 0.0001 0.0001 0.1 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 Total volatiles 0.0001 0.1 0.0001 a Probabilities less than 0.05 were considered as signi®cantly dierent (n=8). Table 6 Color values of raw chicken patties after 7 days of storagea Diet Nonirradiated Irradiated Aerobic packaging Control 1.25% 2.5% 5.0% S.E.M. Vacuum packaging Aerobic packaging L a* b* L a* b* L a* b* L a* b* 58.5a 56.6b 55.6b 53.2c 0.65 12.9ab 13.3a 12.3b 13.6a 0.23 23.0a 23.0a 20.9b 21.7ab 0.40 53.6a 51.0b 50.1b 49.8b 0.69 18.3 18.9 18.4 17.9 0.28 17.6a 16.8a 15.9b 14.9c 0.30 58.8 57.2 56.3 56.6 0.73 13.8 14.1 14.3 13.6 0.37 22.7a 22.1ab 21.5b 20.1c 0.33 53.7 52.7 51.7 53.6 0.65 20.1 20.2 20.0 19.1 0.36 18.1 17.7 17.0 17.6 0.27 Probability Treatment L a* b* Diet (D) Irradiation (IR) Packaging (P) DIR DP IRP DIRP 0.0001 0.0001 0.0001 0.004 0.0003 0.4 0.9 0.1 0.0001 0.0001 0.03 0.08 0.06 0.3 0.0001 0.03 0.0001 0.2 0.2 0.0001 0.0002 a Vacuum packaging Means within a row with no common letter dier signi®cantly (p<0.05), n=4. 394 M. Du et al. / Meat Science 56 (2000) 387±395 Table 7 The o-odora of raw chicken patties after 7 days of storageb Nonirradiated Irradiated Diet Aerobic packaginga Vacuum packaginga Aerobic packaging Vacuum packaging Control 1.25% 2.5% 5.0% S.E.M. 5.7 5.2 6.2 4.3 0.76 3.6 3.8 4.1 5.6 0.76 6.0 6.7 6.1 4.5 0.70 7.5 7.8 6.5 8.3 0.80 Probability Diet (D) Irradiation (IR) Packaging (P) DIR DP IRP DIRP a b 0.99 0.0001 0.4 0.5 0.007 0.0003 0.9 O-odor: 0=no o-odor, 15=strong o-odor. Means within a row with no common letter dier signi®cantly (p<0.05), n=16. signi®cant amounts of sulfur compounds were detected in raw chicken meat under the conditions used in this study. 4. Conclusion Results showed that the TBARS values of meat patties decreased as the dietary CLA level increased. The volatile composition of raw chicken meat was signi®cantly in¯uenced by dietary CLA levels and irradiation. Dietary CLA improved color stability, and sensory panelists could not dierentiate the odor of meat patties from four dierent CLA treatments. Vacuum packaging virtually protected lipid oxidation and volatile production in both irradiated and nonirradiated raw chicken meat during storage. Acknowledgements Paper No. J-18831 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA 50011-3150. Project No. 3322. This research has been supported by the Hatch Act and CDFIN. References Ahn, D. U., Jo, C., & Olson, D. G. (1999a). Analysis of volatile components and the sensory characteristics of irradiated raw pork. Meat Science, 54, 209±215. Ahn, D. U., Olson, D. G., Lee, J. I., Jo, C., Chen, X., & Wu, C. (1998). Packaging and irradiation eects on lipid oxidation and volatiles in pork patties. Journal of Food Science, 63, 15±19. Ahn, D. U., Jo, C., & Olson, D. G. (1999b). Headspace oxygen in sample vials aects volatiles production of meat during the automated purge-and-trap/GC analyses. Journal of Agricultural and Food Chemistry, 47, 2776±2781. Ahn, D. U., Jo, C., & Olson, D. G. (1999c). Volatile pro®les of raw and cooked turkey thigh as aected by purge temperature and holding time before purge. Journal of Food Science, 64, 230± 233. Ahn, D. U., Sell, J. L., Jo, C., Chamruspollert, M., & Jeery, M. (1999b). Eect of dietary conjugated linoleic acid on the quality characteristics of chicken eggs during refrigerated storage. Poultry Science, 78, 922±928. Angelini, P., Merritt Jr, C., Mendelshon, J. M., & King, F. J. (1975). Eect of irradiation on volatile constituents of stored haddok ¯esh. Journal of Food Science, 40, 197±199. Batzer, O. F., & Doty, D. M. (1955). Nature of undesirable odors formed by gamma irradiation of beef. Journal of Agricultural and Food Chemistry, 3, 64±69. Belury, M. A., Nickel, K. P., Bird, C. E., & Wu, Y. (1996). Dietary conjugated linoleic acid modulation of phorbol ester skin tumor promotion. Nutrition and Cancer, 26, 49±157. Branka, K., Branka, M., & Dusan, R. (1992). Radiation-induced oxidative chemical changes in dehydrated egg products. Journal of Agricultural and Food Chemistry, 40, 662±666. Du, M., Ahn, D. U., & Sell, J. L. (1999). Eect of dietary conjugated linoleic acid on the composition of egg yolk lipids. Poultry Science, 78, 1639±1645. Dugan, M. E. R., Aalhus, J. L., Jeremiah, L. E., Kramer, J. K. G., & Schaefer, A. L. (1999). The eects of feeding conjugated linoleic acid on subsequent pork quality. Canadian Journal of Animal Science, 79, 45±51. Farkas, J. (1998). Irradiation as a method for decontaminating food. International Journal of Food Microbiology, 44, 189±204. Hashim, I. B., Resurreccion, A. V. A., & McWatters, K. H. (1995). Disruptive sensory analysis of irradiated frozen or refrigerated chicken. Journal of Food Science, 60, 664±666. Heath, J. A., Owens, S. L., Tesch, S., & Hannah, K. W. (1990). Eect of high-energy electron irradiation of chicken on thiobarbituric acid values, shear values oder and evoked yield. Poultry Science, 69, 313± 319. M. Du et al. / Meat Science 56 (2000) 387±395 Ip, C., Banni, S., Angioni, E., Carta, G., McGinley, J., Thopson, H. J., Barbano, D., & Bauman, D. (1999). Conjugated linoleic acid-enriched butter fat alters mammary gland morphogenesis and reduces cancer risk in rats. Journal of Nutrition, 129, 2135±2142. Ip, C., Scimeca, J. A., & Thompson, H. (1995). Eect of timing and duration of dietary conjugated linoleic acid on mammary cancer prevention. Nutrition and Cancer, 24, 241±247. Liu, H. F., Booren, A. M., Gray, J. I., & Crackel, J. I. (1992). Antioxidant ecacy of oleoresin rosemary and sodium tripolyphosphate in restructured pork steak. Journal of Food Science, 57, 803±806. Loor, J. J., & Herbein, J. H. (1998). Exogenous conjugated linoleic acid isomers reduce bovine milk concentration and yield by inhibiting de novo fatty acid synthesis. Journal of Nutrition, 128, 2411±2419. Luchsinger, S. E., Kropf, D. H., Garacia-Zepeda, C. M., Hunt, M. C., Marsden, J. L., Rubio-Canas, E. J., Kastner, C. L., Kuecker, W. G., & Mata, T. (1996). Color and oxidative rancidity of gamma and electron beam-irradiated boneless pork chops. Journal of Food Science, 61, 1000±1005, 1093. Meynier, A., Genot, C., & Gandemer, G. (1999). Oxidation of muscle 395 phospholipids in relation to their fatty acid composition with emphasis on volatile compounds. Journal of the Science of Food and Agriculture, 79, 797±804. Nanke, K. E., Sebranek, J. G., & Olson, D. G. (1998). Color characteristics of irradiated vacuum-packaged pork, beef, and turkey. Journal of Food Science, 63, 1001±1006. Patterson, R. L. S., & Stevenson, M. H. (1995). Irradiation-induced o-odor in chicken and its possible control. British Poultry Science, 36, 425±441. S. A. S., Institute (1985). SAS user's guide. SAS Institute. NC: Cary Inc. Shahidi, F., & Pegg, R. B. (1994). Hexanal as an indicator of meat ¯avor deterioration. Journal of Food Lipids, 1, 177±186. Van den Berg, J. J., Cook, N. E., & Tribble, D. L. (1995). Reinvestigation of the antioxidant properties of conjugated linoleic acid. Lipids, 30, 599±605. Wong, Y. C., Herald, T. J., & Hachmeister, K. A. (1995). Comparison between irradiation and thermally pasteurized liquid egg white on functional, physical, and microbiological properties. Poultry Science, 75, 803±808.