Production of Off-Odor Volatiles from Liposome-Containing Amino Acid Homopolymers by Irradiation
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JFS: Food Chemistry and Toxicology Production of Off-Odor Volatiles from Liposome-Containing Amino Acid Homopolymers by Irradiation ABSTRACT: Irradiation not only generated many new volatiles but also destroyed some volatiles already present in nonirradiated amino acid homopolymer-in-liposome systems. The amounts of some volatiles greatly increased, but others significantly decreased after irradiation. The majority of newly generated and increased volatiles by irradiation were sulfur compounds, indicating that sulfur amino acids are the most susceptible to changes by irradiation. More than one site in amino acid side chains was labile to free radical attack, and many volatiles were produced by the secondary chemical reactions after the primary radiolytic degradation of side chains. Although nonirradiated samples also produced some sulfury notes, irradiated samples produced much a stronger and astringent sulfury odor than nonirradiated samples. Keywords: amino acid homopolymer, secondary chemical reactions, irradiation, volatiles, sulfur compounds Introduction I RRADIATION OF AMINO ACIDS PRODUCED DISTINCT VOLATILE COM- pounds via radiolytic degradation, but the resulting odor was much stronger and astringent than that from irradiated meat (Jo and Ahn 2000). Patterson and Stevenson (1995) found that dimethyl trisulfide is the most potent off-odor compound in irradiated chicken meat, followed by cis-3- and trans-6-nonenals, oct-1en-3-one, and bis(methylthio-)methane. Ahn and others (1999, 2000) found many more sulfur compounds were produced from irradiated meat. Irradiation produced many new volatile compounds in oil emulsion prepared from fatty acids, but had little effect on the sensory characteristics of oil emulsion (Lee and Ahn 2002). The amounts of aldehydes, the indicators of lipid oxidation, in oil emulsions did not increase by irradiation (Lee and Ahn 2002), and volatiles from lipids accounted for only a small part of the off-odor in irradiated meat (Ahn and others 1997, 1998a, 1999). The odor of irradiated meat was characterized as “bloody and sweet,” “barbecued corn-like”, “hot fat,” “burned oil,” or “burned feathers” (Heath and others 1990; Hashim and others 1995; Ahn and others 2000). Merritt and others (1975, 1978) observed that the odor found in a lipid or a component of meat irradiated separately was different from that of the meat. Diehl (1995) indicated that there was a substantial difference between the radiation chemistry of pure substances and that of the same substances when they were components of complex food systems. Many new volatiles were generated, and the amount of volatiles produced from amino acid homopolymers were changed after irradiation (Ahn 2001). More than 1 site of amino acid side chains was susceptible to free radical attack, and many volatiles were produced by the secondary chemical reactions after the primary radiolytic degradation of side chains. Only sulfur-containing volatiles, however, produced strong odor that was similar to or close to irradiation odor, and methionine was the most important amino acid in producing irradiation odor (Ahn 2001). The perception of odor from samples containing sulfur volatiles, however, changed greatly depending on the composition and amounts © 2002 Institute of Food Technologists jfsv67n7p2659-2665ms20020084-MO.p65 2659 present in the sample (Ahn 2001). This suggests that irradiation odor is either a composite of the volatiles generated in the protein and lipid portion of meat or a result of products formed by the interactions between fat and proteins during and after irradiation. The objective of this study was to determine the volatile compounds produced from a mixture of liposome containing amino acid homopolymers by irradiation and the interactions of the volatile compounds on odor characteristics of the irradiated liposome/amino acid system. Materials and Methods Sample preparation Phosphatidylcholine, phosphatidic acid, amino acid homopolymers, glutathione, and Met-Gly-Met-Met were purchased from Sigma (St. Louis, Mo., U.S.A.). A phospholipid liposome system prepared with phosphatidylcholine and phosphatidic acid was used because it represents cell membranes of meat. Phosphatidylcholine (100 mg dissolved in chloroform) was evaporated from chloroform to thin film on the wall of a 40-mL sample vial. The vial was placed under a nitrogen stream to remove any chloroform. All 7 amino acid polymer groups were used in this study: aliphatic (poly-L-alanine, poly-Lglycine, poly-L-leucine), aliphatic hydroxyl (poly-L-threonine), basic (poly-L-histidine, poly-L-lysine), acidic (poly-L-aspartic acid, poly-L-glutamic acid), aromatic (poly-L-tyrosine), amide (poly-L-asparagine), and sulfur-containing (Met-Gly-Met-Met, glutathione) side chain groups. Each combination of selected amino acid polymer groups was weighed into a vial coated with phospholipids and was hydrated with 20 mL citrate-phosphate buffer (100 mM, pH 6.0) by gently shaking the solution for 15 min. The milky suspension was then vortexed to disperse the phospholipids before use. The concentration of each amino acid homopolymer was 2 mg/mL buffer, and each amino acid polymer group was treated like a single compound. A liposome containing all 7 amino acid polymer groups (used as a “reference”) and 7 liposome solutions containing 6 amino acid homopolymer groups Vol. 67, Nr. 7, 2002—JOURNAL OF FOOD SCIENCE 9/20/2002, 10:36 AM 2659 Food Chemistry and Toxicology D.U. AHN AND E.J. LEE Volatiles by protein-lipid interactions . . . Table 1—Volatiles and odor characteristics of an amino acid homopolymer mixture containing all amino acid groups after irradiation* Total ion counts × 104 Food Chemistry and Toxicology Volatiles 0 kGy 5 kGy SEM Sulfur dioxide 1-Butene 1,1'-Oxybis ethane 2-Propanone Dimethyl sulfide Carbon disulfide Methyl thiirane 2-Methyl-2-propanol 1,1-Dimethylethyl hydroperoxide 2-Ethoxy butane 2-Methyl propanal Hexane Butanal 2-Pentene Methylthio ethane Ethyl acetate Benzene 3-Methyl-butanal 1,4-Dioxane 3,3-Dimethyl-2-butanone Dimethyl disulfide Toluene Ethyl-benzene 1,3-Dimethyl-benzene Methyl ethyl disulfide Xylene Dimethyl trisulfide 0b 386a 875a 37459 a 0b 454b 210a 0b 517a 1210a 211b 299b 357b 223a 2421a 0b 324a 0b 90 11 24 340 7 112 5 15 8 292a 0b 757a 261a 259a 0b 67a 0b 108a 304a 107a 57b 251a 48b 160a 0b 0b 0b 0b 115a 76a 107b 0b 54a 0b 5083a 139a 0b 0b 34490 a 0b 1228a 43b 66a 145a 7010a 4 4 326 26 5 5 3 166 10 4 5 692 3 34 3 1 3 400 Odor characteristics of irradiated samples—Hard-boiled egg, boiled sweet a,b Means with no corn, sweet and sulfury, steamed vegetable common superscript differ significantly (p < 0.05), n = 4. *Contains acidic, aliphatic, aliphatic hydroxyl, amide, aromatic, basic, and sulfur amino acid groups. SEM = standard error of the mean. were prepared. Four 5-mL portions of samples were transferred to scintillation vials and irradiated at 0 or 5 kGy using a Linear Accelerator (Circe IIIR; Thomson CSF Linac, Saint-Aubin, France). The energy and power level used were 10 MeV and 10 kW, respectively, and the average dose rate was 92.8 kGy/min. The max/min ratio was approximately 1.09. To confirm the target dose, 2 alanine dosimeters per cart were attached to the top and bottom surfaces of a sample vial. The alanine dosimeter was read using a 104 Electron Paramagnetic Resonance Instrument (Bruker Instruments Inc., Billerica, Mass., U.S.A.). Samples were used to determine volatile profiles and odor characteristics before and after irradiation. The volatiles and odor characteristics between the reference and other liposomes containing 6 amino acid groups were compared. Volatile analysis A purge-and-trap apparatus (Precept II and Purge & Trap Concentrator 3000; Tekmar-Dohrmann, Cincinnati, Ohio, U.S.A.) connected to a gas chromatograph/mass spectrometer (GC/MS; Hewlett-Packard Co., Wilmington, Del., U.S.A.) was used to analyze the volatiles produced (Ahn and others 2001). Sample solution (1 mL) was placed in a 40-mL sample vial, and the vials were flushed with helium (40 psi) for 5 s. Samples were held in a refrigerated (4 °C) sample-holding tray before analysis. The maximum holding time was less than 6 h to minimize oxidative changes (Ahn and others 1999). The sample was purged with helium (40 mL/min) for 13 min at 40 °C. Volatiles were trapped using a 2660 Table 2—Volatiles and odor characteristics of the liposome “without acidic amino acid group” after irradiation* Total ion counts × 104 Volatiles Sulfur dioxide 1-Butene Methanethiol 1,1'-Oxybis-ethane 2-Propanone Dimethyl sulfide Carbon disulfide 1,1-Dimethylethyl hydroperoxide 2-Methyl propanal Hexane Butanal 2-Pentene Methylthio ethane 1-Methoxy-1-propene Benzene 3-Methyl butanal 1,4-Dioxane 3,3-Dimethyl-2-butanone Dimethyl disulfide Toluene Ethyl benzene 1,3-Dimethyl benzene Methyl ethyl disulfide Xylene Dimethyl trisulfide 0 kGy 5 kGy SEM 0b 834a 41a 0b 220a 35977 a 0b 436b 61a 36b 9601a 175b 18682 b 457a 1184a 0b 176 2 482 11 2667 18 79 6 0b 0b 268 195a 0b 0b 0b 290b 215 60a 78b 346a 46b 141a 0b 0b 0b 58a 108a 249 0b 105a 109a 6300a 454a 123 0b 34734 a 0b 1199a 48b 69a 170a 7891a 8 33 16 4 2 26 205 18 56 5 907 4 50 7 5 9 1230 Odor characteristics of irradiated samples—Hard-boiled egg, boiled sweet corn, boiled vegetable, solvent/burned plastic a,b Means with no common superscript differ significantly (p < 0.05), n = 4 *Contains aliphatic, aliphatic hydroxyl, amide, aromatic, basic, sulfur amino acid groups SEM = standard error of the mean. Tenax column (Tekmar-Dohrmann) and desorbed for 2 min at 225 °C, focused in a cryofocusing module (–90 °C) and then thermally desorbed into a column for 30 s at 225 °C. An HP-624 column (7.5 m x 0.25 mm internal dia., 1.4 çm nominal), an HP-1 column (52.5 m x 0.25 mm internal dia., 0.25 çm nominal; Hewlett-Packard Co.) and an HP-Wax column (7.5 m x 0.25 mm i.d., 0.25 ?m nominal) were connected using zero deadvolume column connectors ( J &W Scientific, Folsom, Calif., U.S.A.). Ramped oven temperature was used to improve volatile separation. The initial oven temperature of 0 °C was held for 2.5 min. After that, the oven temperature was increased to 15 °C at 2.5 °C/min, increased to 45 °C at 5 °C/min, increased to 110 °C at 20 °C/min, increased to 210 °C at 10 °C/min, and then was held for 4.5 min at that temperature. Constant column pressure at 20.5 psi was maintained. The ionization potential of mass-selective detector (Model 5973; Hewlett-Packard Co.) was 70 eV, and the scan range was 18.1 - 300 m/z. Identification of volatiles was achieved by comparing mass spectral data of samples with those of the Wiley library (Hewlett-Packard Co.). Standards, when available, were used to confirm the identification by the mass selective detector. The area of each peak was integrated using the ChemStation (Hewlett-Packard Co.), and the total peak area (pA*s x 10 4) was reported as an indicator of volatiles generated from the sample. Odor characteristics Ten trained sensory panelists were used to characterize odor in samples. Panelists were selected based on interest, availability, and performance in screening tests conducted with samples JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 7, 2002 jfsv67n7p2659-2665ms20020084-MO.p65 2660 9/20/2002, 10:36 AM Volatiles by protein-lipid interactions . . . Total ion counts × 104 Volatiles 0 kGy 5 kGy SEM Sulfur dioxide 1-Butene Methanethiol 1,1'-Oxybis ethane 2-Propanone Dimethyl sulfide Carbon disulfide Methyl thiirane 2-Methyl-2-propanol 1,1-Dimethylethyl hydroperoxide 2-Ethoxy butane 2-Methyl propanal Hexane Butanal Methylthio ethane Dimethyl tetrasulfide 1,4-Dioxane 3,3-Dimethyl-2-butanone Dimethyl disulfide Toluene Ethyl-benzene 1,3-Dimethyl benzene Methyl ethyl disulfide Dimethyl trisulfide 0b 309a 0b 306a 36575 a 0b 482b 229a 0b 437a 1394a 157b 614a 77b 2131b 299a 2048a 0b 374a 0b 86 31 37 15 1328 14 108 7 3 10 352a 0b 0b 230a 0b 0b 264a 98a 81b 48a 0b 0 0b 0b 0b 96a 216a 0 91a 63a 0b 0b 43671 a 0b 1400a 13 59a 15330 a 3 1 50 16 4 18 41 11 1459 1 23 12 2 367 Odor characteristics of irradiated samples—Hard-boiled egg, fermented vegetable, rotten vegetable a,b Means with no common superscript differ significantly (p < 0.05), n = 4 *Contains acidic, aliphatic hydroxyl, amide, aromatic, basic, and sulfur amino acid groups. similar to those to be tested. During training, a lexicon of aroma terms to be used on the ballot was developed. Samples were placed in glass containers, and the sample temperature was brought to 25 °C before samples were tested. All treatments were presented to each panelist, and the order of presentation was randomized. Panelists characterized overall odor of each sample, and rated the intensity of the selected attributes on 15 unit linear scales. Statistical analysis Four replicated analyses were done for the volatiles of samples. Data were analyzed using the generalized linear model procedure of SAS software (SAS 1989): Student’s t-test was used to compare differences between irradiated and nonirradiated means. Mean values and standard error of the means (SEM) were reported. Significance was defined at p < 0.05. Results and Discussion I RRADIATION GREATLY INFLUENCED THE AMOUNTS AND PROFILES OF volatiles in amino acid homopolymer-in-liposome systems (Tables 1 through 8). Table 1 shows the volatiles produced from the reference sample (the amino acid homopolymer-in-liposome system that contains all 7 amino acid homopolymer groups) before and after irradiation. Many new volatiles including sulfur dioxide, dimethyl sulfide, 2-methyl-2-propanol, 2-methyl-propanal, methylthio ethane, benzene, methyl ethyl disulfide, and dimethyl trisulfide were generated, and the amounts of carbon sulfide, dimethyl disulfide, and ethyl benzene increased greatly after irradiation. On the other hand, methyl thiirane, 1,1-dimethylethyl hydroperoxide, 2-ethoxy butane, 2-pentene, ethyl acetate, 1,4-dioxane, 3,3-dimethyl-2-butanone, and toluene disap- Table 4—Volatiles and odor characteristics of the liposome “without aliphatic hydroxyl amino acid group” by irradiation Total ion counts × 104 Volatiles 0 kGy 5 kGy SEM Sulfur dioxide 1-Butene 1-Methyl pyrrole 2-Propanone Dimethyl sulfide Carbon disulfide Methyl thiirane 2-Methyl-2-propanol 1,1-Dimethylethyl hydroperoxide 2-Ethoxy butane 2-Methyl propanal Hexane Butanal 2-Pentene 4-Methyl-3-hexanol Ethyl acetate Benzene 3-Methyl butanal 1,4-Dioxane 3,3-Dimethyl-2-butanone Dimethyl disulfide Toluene Ethyl benzene 1,3-Dimethyl benzene Xylene Dimethyl trisulfide 0b 280a 124a 38767 a 0b 490b 239a 0b 504a 1814a 199b 0b 422b 231a 2793a 0b 373a 0b 130 21 5 452 5 123 5 2 4 390a 0b 0b 265a 244a 71a 95a 0b 190b 306a 102a 62b 327a 44b 124a 0b 0b 0b 135a 106a 119b 0b 0b 0b 5779a 272a 0b 0b 31460 a 0b 1405a 48b 160a 10287 a 11 4 29 16 7 3 3 106 11 15 4 1731 16 20 9 3 124 Odor characteristics of irradiated samples—Hard-boiled egg, sulfury, boiled sweet corn, boiled cabbage a,b Means with no common superscript differ significantly (p < 0.05), n = 4 Contains acidic, aliphatic, amide, aromatic, basic, and sulfur amino acid groups. peared, and the amounts of 1-butene, 1,1-oxybis ethane, 2-propanone, hexane, butanal, and 1,3-dimethyl benzene significantly decreased after irradiation. The majority of newly generated and increased volatiles by irradiation were sulfur compounds indicating that sulfur-containing amino acids are among the most susceptible amino acid groups to irradiation. Sensory panelists described the odor of irradiated amino acid homopolymers-in-liposome as “hard-boiled egg,” “boiled sweet corn,” “sweet and sulfury,” or “steamed vegetable,” typical odor characteristics of sulfur volatile-containing samples, indicating that sulfur volatiles played the major role in the odor of the irradiated meat sample. Although nonirradiated samples also produced some sulfury notes, irradiated samples produced much stronger and astringent sulfury odor than nonirradiated ones. Hashim and others (1995) described the characteristic of irradiation odor as a “bloody and sweet” aroma, and Ahn and others (2000) described it as a “barbecued corn-like” odor. All liposome groups containing the “sulfur amino acids” group produced similar odor characteristics, indicating that sulfur amino acids are mainly responsible for irradiation odor as suggested by Ahn (2001). The volatile profiles of all 6 liposome systems that contain 6 amino acid homopolymer groups (Table 2 to 7), except for the 1 without sulfur amino acid group (Table 8), were similar to that of the reference (Table 1). Most of the volatiles detected in the nonirradiated reference were also found in the liposome that contains the “all amino acid homopolymers but acidic group” (“without acidic group”), except that methyl thiirane, 2-ethoxy butane, hexane, and ethyl acetate found in the reference were not detected (Tables 1 and 2). After irradiation, methanethiol and 1Vol. 67, Nr. 7, 2002—JOURNAL OF FOOD SCIENCE jfsv67n7p2659-2665ms20020084-MO.p65 2661 9/20/2002, 10:36 AM 2661 Food Chemistry and Toxicology Table 3—Volatiles and odor characteristics of the liposome “without aliphatic amino acid group” after irradiation* Volatiles by protein-lipid interactions . . . Table 5—Volatiles and odor characteristics of the liposome “without amide amino acid group” after irradiation* Table 6—Volatiles and odor characteristics of the liposome “without aromatic amino acid group” after irradiation* Total ion counts × 104 Volatiles Food Chemistry and Toxicology Sulfur dioxide 1-Butene 1-Methyl pyrrole 1,1'-Oxybis ethane 2-Propanone Dimethyl sulfide Carbon disulfide Methyl thiirane 2-Methyl-2-propanol 1,1-Dimethylethyl hydroperoxide 2-Ethoxy butane 2-Methyl propanal Hexane Butanal 2-Pentene Methylthio ethane Ethyl acetate Benzene Dimethyl tetrasulfide 3-Methyl butanal 1,4-Dioxane 3,3-Dimethyl-2-butanone Dimethyl disulfide Toluene Ethyl benzene 1,3-Dimethyl benzene Methyl ethyl disulfide Xylene Dimethyl trisulfide 0 kGy 5 kGy 0b 2365a SEM 307a 56a 1667a 39351 a 0b 534b 236a 0b 503a 236b 0b 648b 382b 239a 3649a 0b 394a 0b 52 14 1 28 225 1 61 3 3 5 345a 0b 45 0b 239a 0b 327a 0b 0b 271 216b 83a 72b 501a 69b 224a 0b 0b 0b 0b 165a 56 115a 0b 75a 0b 5875a 86a 244 277a 0b 31407 a 0b 508a 47b 63a 244a 10226 a 5 2 4 3 4 1 4 31 13 10 10 3 1141 12 4 7 1 52 540 Total ion counts × 104 Volatiles Sulfur dioxide 1-Butene 1,1'-Oxybis ethane 2-Propanone Dimethyl sulfide Carbon disulfide Methyl thiirane 2-Methyl-2-propanol 1,1-Dimethylethyl hydroperoxide 2-Ethoxy butane 2-Methyl propanal Hexane Butanal 2-Pentene Methylthio ethane 4-Methyl-3-hexanol Ethyl acetate Benzene Dimethyl tetrasulfide 3-Methyl butanal 1,4-Dioxane 3,3-Dimethyl-2-butanone Dimethyl disulfide Toluene Ethyl benzene 1,3-Dimethyl benzene Methyl ethyl disulfide Xylene Dimethyl trisulfide 0 kGy 5 kGy SEM 0b 2610a 299a 51 a 38847 a 0b 406b 191a 0b 456a 219b 207b 416b 267a 3542a 0b 365a 0b 133 14 4 513 5 47 0 4 37 322a 0b 0a 234a 240a 0b 61a 85a 0b 0b 161b 446a 94a 67b 309a 50b 152a 0b 0b 0b 0b 146a 149a 105b 0b 86a 0b 0b 6353a 91a 195a 0b 0b 41409 a 0b 1380a 44b 67a 141a 12058 a 1 1 57 3 3 4 1 1 109 7 5 24 2 1264 3 21 4 2 3 175 Odor characteristics of irradiated samples—Hard-boiled egg, boiled sweet corn, sweet and sulfury, steamed vegetablea,bMeans with no common superscript differ significantly (p < 0.05), n = 4 *Contains acidic, aliphatic, aliphatic hydroxyl, aromatic, basic, and sulfur amino acid groups. Odor characteristics of irradiated samples—Cooked/spoiled cabbage, sulfury, soy sauce, sewage a,b Means with no common superscript differ significantly (p < 0.05), n = 4. *Contains acidic, aliphatic, aliphatic hydroxyl, amide, basic and sulfur amino acid groups. methoxy-1-propene, not found in the reference, were produced from the “without acidic group” sample. The amount of 2-propanone in the “without acidic group” decreased by about 50% after irradiation, but was much higher than that in the irradiated reference. Methyl thiirane was not detected in the “liposome without acidic group”, suggesting that methyl thiirane was formed from the secondary reaction between sulfur compounds and acidic amino acid side chain products. 2-Methyl-2-propanal was not detected in the irradiated “liposome without acidic group”, indicating that a product of the acidic group side chain is needed to form this compound. Toluene was detected in all nonirradiated liposomes, but disappeared after irradiation. It is difficult to pinpoint the main reason for this change. The new production of benzene and the disappearance of toluene by irradiation, however, suggest that the radiolytic cleavage of the methyl group from toluene may be the mechanism behind the benzene formation. Du and others (2001a,b) found benzene in both irradiated and nonirradiated broiler meats, indicating that benzene and toluene could be produced from the components naturally present in meat even without irradiation. The odor characteristics of the liposome in the “without acidic group” were similar to those of the reference, but some sensory panels received a “solvent/burned plastic odor” note from the irradiated liposome “without acidic group” sample (Tables 1 and 2). 2-Pentene, ethyl acetate and 3-methyl butanal detected in the nonirradiated reference were not found in the nonirradiated liposome that contained the “all amino acid homopolymers but aliphatic group” (“without aliphatic group”, Tables 1 and 3). After irradiation, methanethiol and dimethyl tetrasulfide, not found in the reference, were detected in the “without aliphatic group” sample. Butanal, benzene, and 3-methyl butanal detected in the reference, however, were not found. This indicates that 2-pentene, benzene, 3-methyl butanal, and xylene are derived from the aliphatic amino acid side chain. Methanethiol could be formed in the presence of the acidic and aliphatic side chain groups. Benzene was not detected in liposomes when the aliphatic amino acid group was not present, indicating that benzene was derived from aliphatic amino acid side chains. The odor intensity and characteristics of the “liposome without aliphatic group” sample were similar to those of the reference ( Tables 1 and 3). From the nonirradiated liposome that contained the “all amino acid homopolymer but aliphatic hydroxyl group” (“without aliphatic hydroxyl group”), 2 volatiles, namely, 1-methyl pyrrole, and 4-methyl-3-hexanol, not found in the nonirradiated reference sample, were detected (Tables 1 and 4). After irradiation, methylthio ethane and methyl ethyl disulfide that were found in the reference were not found in the “liposome without aliphatic hydroxyl” group. 1,1-Oxybis ethane found in the reference was not present in both the irradiated and nonirradiated “liposome without aliphatic hydroxyl” groups. An aliphatic hydroxyl group side chain is needed for 1,1-oxybis ethane, methylthio ethane, and methyl ethyl disulfide formation. The odor characteristics of the “liposome without aliphatic hydroxyl” group sample were 2662 JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 7, 2002 jfsv67n7p2659-2665ms20020084-MO.p65 2662 9/20/2002, 10:36 AM Volatiles by protein-lipid interactions . . . Table 8—Volatiles and odor characteristics of the liposome “without sulfur amino acid group” by irradiation Total ion counts × 104 Volatiles 0 kGy 5 kGy SEM sulfur dioxide 1-Butene 1,1'-Oxybis ethane 2-Propanone Dimethyl sulfide Carbon disulfide Methyl thiirane 2-Methyl-2-propanol 1,1-Dimethylethyl hydroperoxide 2-Ethoxy butane 2-Methyl propanal Hexane Butanal 2-Pentene Methylthio ethane 4-Methyl-3-hexanol Ethyl acetate Benzene 3-Methyl butanal 3,3-Dimethyl-2-butanone Dimethyl disulfide Toluene Ethyl-benzene 1,3-Dimethyl benzene Methyl ethyl disulfide Xylene Dimethyl trisulfide 0b 370a 1248a 37517 a 0b 472b 232a 0b 519a 2512a 179b 242b 358b 219a 3502a 0b 363a 0b 183 16 30 607 1 209 7 4 9 373a 0b 60b 244a 274a 0b 68a 69a 0b 151b 100a 69b 316a 49b 147a 0b 0b 0b 0b 119a 314a 191b 0b 79a 0b 0b 5295a 217a 0b 52027 a 0b 1232a 42b 85a 161a 7845a 9 2 56 56 10 1 2 2 21 8 6 4210 5 8 6 5 3 782 Odor characteristics of irradiated samples—Strong hard-boiled egg, fermentation odor, sulfury, hospital odor a,b Means with no common superscript differ significantly (p 0.05), n = 4 *Contains acidic, aliphatic, aliphatic hydroxyl, amide, aromatic, and sulfur amino acid groups. similar to those of the reference (Tables 1 and 4). Among the volatiles of nonirradiated liposome that contained the “all amino acid homopolymers but amide group” (“without amide group”), 1-methyl pyrrole was the only volatile not found in the reference. Dimethyl tetrasulfide was the only new volatile that was not detected in irradiated reference, and 1,4-dioxane was still remaining in the liposome “without amide group” after irradiation (Tables 1 and 5). The odor characteristics of liposome “without amide group” sample were similar to those of the reference (Tables 1 and 5). The difference between the volatiles of the reference and the volatiles of “liposome that contained the all amino acid homopolymers but aromatic group” (“without aromatic group”) was very small. In nonirradiated samples, hexane and 4-methyl-3hexanol were the 2 volatiles found only in 1 of the 2 samples (the reference and the “without aromatic group”), and dimethyl tetrasulfide was the only volatile not detected in the irradiated reference (Tables 1 and 6). Dimethyl tetrasulfide could be formed in irradiated liposomes only when no aromatic and amide but all other amino acid side chain groups were present. The irradiated “liposome without aromatic group” also had odor characteristics similar to those of the reference (Tables 1 and 6). The difference between the volatiles of the reference and those of liposome that contained “all amino acid homopolymer but basic group” (“without basic group”) was also very small. In nonirradiated samples, 4-methyl-3-hexanol and 1,4-dioxane were the 2 volatiles found only in the reference and “without basic group”, but little difference in volatile profiles and odor char- Total ion counts × 104 Volatiles 1-Butene 1,1-Dimethyl cyclopropane Pentane 2-Propanone 2-Methyl-2-propanol 1,1-Dimethylethyl hydroperoxide 2-Ethoxy butane 2-Methyl propanal Hexane 3-Methylfuran Butanal 2-Pentene 4-Methyl-3-hexanol Benzene 3-Methyl butanal 1,4-Dioxane 3,3-Dimethyl-2-butanone Toluene Ethyl benzene 1,3-Dimethyl benzene Octane Xylene 0 kGy 5 kGy SEM 295a 132b 0b 0b 39551 a 0b 455a 65a 183a 377b 297a 0b 19 1 1 785 5 4 342a 0b 48b 0b 0b 432a 244a 0b 377a 181a 88a 358a 57b 188a 0b 0b 0b 146a 122a 79a 156a 0b 0b 9948a 268b 0b 0b 0b 892a 0b 79a 79a 9 3 13 2 6 11 8 271 14 10 8 11 23 10 2 3 Odor characteristics of irradiated samples—Hospital odor, alcohol, solvent, wet dog a,b Means with no common superscript differ significantly (p < 0.05), n = 4. *Contains acidic, aliphatic, aliphatic hydroxyl, amide, aromatic, and basic amino acid groups. acteristics were detected in irradiated samples (Tables 1 and 7). This shows that 1,4-dioxane is derived from the basic amino acid side chain group. The volatile profiles and odor characteristics between the reference sample and the “all amino acid homopolymers but sulfur amino acid group” (“without sulfur group”) were totally different. Both the irradiated and nonirradiated “liposomes without sulfur group” produced no sulfur volatiles, and odor was characterized as a “hospital odor,” “alcohol,” “solvent,” or “wet dog,” and the intensity (data not shown) was very weak compared to that of other sulfur amino acid-containing liposomes (Table 8). In the nonirradiated “liposome without sulfur group”, 4-methyl –3-hexanol not found in the reference was detected, but butanal and ethyl acetate found in the reference were not (Tables 1 and 8). In the irradiated “liposome without sulfur group”, 1,1-dimethyl cyclopropane, pentane, and butanal were the new volatiles not found in the irradiated reference. Within the “liposomes without sulfur group”, irradiation newly produced 1,1-dimethyl cyclopropane, pentane, 2-methyl propanal, 3-methyl furan, butanal, benzene, octane, and xylene. However, many volatiles present in nonirradiated samples such as 1,1-dimethylethyl hydroperoxide, 2ethoxy butane, 2-pentene, 4-methoxy-3-hexanol, 1,4-dioxane, 3,3-dimethyl-2-butanone, toluene, and 1,3-dimethyl benzene disappeared after irradiation (Table 8). The volatile profiles of “liposomes with amino acid homopolymers” (Tables 1 through 8) clearly show that many aldehydes and hydrocarbons could be produced from amino acid side chains, and sensory characteristics of liposomes explained why irradiation odor in meat was different from lipid oxidation odor (Ahn and others 1997, 1998b, 1999). Patterson and Stevenson (1995) identified dimethyl trisulfide and bis(methylthio-)methane as the most potent off-odor sulfur compounds in irradiated Vol. 67, Nr. 7, 2002—JOURNAL OF FOOD SCIENCE jfsv67n7p2659-2665ms20020084-MO.p65 2663 9/20/2002, 10:36 AM 2663 Food Chemistry and Toxicology Table 7—Volatiles and odor characteristics of the liposome “without basic amino acid group” after irradiation* Volatiles by protein-lipid interactions . . . Table 9—Summary of volatile changes in liposome containing amino acid homopolymers by irradiation Volatile compounds All B* Food Chemistry and Toxicology Sulfur dioxide 1-Butene 1,1-Dimethyl cyclopropane Methane thiol Pentane 1-Methyl pyrrole 1,1'-Oxybis ethane 2-Propanone Dimethyl sulfide Carbon disulfide Methyl thiirane 2-Methyl-2-propanol 1,1-Dimethylethyl hydroperoxide 2-Ethoxy butane 2-Methyl propanal Hexane Methylfuran Butanal 2-Pentene Methylthio ethane Dimethyl tetrasulfide 1-Methoxy-1-propene 4-Methyl-3-hexanol Ethyl acetate Benzene 3-Methyl-butanal 1,4-Dioxane 3,3-Dimethyl-2-butanone Dimethyl disulfide Toluene Ethyl benzene 1,3-Dimethyl-benzene Methyl ethyl disulfide Octane Xylene Dimethyl trisulfide No acidic A** C*** A A C No aliphatic OH B A C No amide B A No aromatic C B Nd1 D2 N3 Nd D D D Ô4 D D Nd Nd –5 Nd Nd N Ô – Nd D N D D Ô Nd Nd – Nd D N Nd D N Nd D N D D Ô D D Ô D D Ô Nd Nd – Nd Nd – Nd Nd – Nd Nd Nd D D Nd D D Nd D Nd – Nd – Nd – D Ô D Ô D N D Ó6 Nd 07 D N Nd 0 Nd Nd Nd D D Nd D Nd Nd D D Nd Nd D D D D Nd Nd Nd N – – Ô Ô N Ó – – 0 Nd Nd Nd D D Nd D D Nd D D Nd Nd D D D D Nd D Nd N – – Ô Ô N Ó 0 N 0 Nd Nd D Nd D Nd D D Nd D Nd Nd Nd Nd D D D Nd D Nd – – 0 – Ô N Ó 0 N 0 Nd Nd D D D Nd D D Nd D Nd – Nd Nd – Nd Nd 0 Nd D Ô D D Ô D D N Nd D Ó D Nd 0 D D N Nd Nd 0 D Nd Nd Nd D D D D Nd D Nd D Nd D Nd D D Nd Nd Nd Nd D Nd D D D D D D D Nd Nd Nd Nd Nd D D Nd D Nd D Nd Nd Nd Nd D D Nd Nd D Nd D D D Nd D D Nd Nd Nd Nd D D Nd Nd Nd Nd Nd Nd D D D D D D D Nd Nd Nd Nd Nd D D Nd D Nd D Nd D Nd Nd D D D Nd D Nd D D D Nd D D – N N – – 0 N – N – – N Ó – 0 Ó 0 Ó Ô N – N N D Nd Nd Nd D Nd Nd Nd Nd Nd Nd Nd Nd D D D D Nd Nd Nd Nd Nd Nd Nd D D Nd Nd Nd D D Nd Nd Nd Nd Nd Nd Nd D Nd D D D Nd Nd D 0 N N – 0 – N N – – – – – 0 0 Ó 0 N N N – – N D Nd Nd Nd D D Nd Nd Nd D D Nd D D D D D D D Nd Nd Nd Nd Nd D D Nd D Nd Nd Nd Nd Nd Nd D D Nd Nd D Nd D D Nd Nd D D 0 N N – Ô 0 – – – 0 0 N Ó 0 0 Ó 0 Ó Ô – – N N D Nd D Nd Nd D Nd Nd Nd Nd D Nd D D D D D D D Nd Nd Nd Nd Nd 0 D D N Nd D – Nd Nd – Nd D N D Nd 0 D D N Nd D N Nd Nd – Nd Nd – D Nd 0 D D N Nd D – D D Ó D Nd 0 D D - D Nd 0 D D Ó D D Ô D D N Nd Nd – Nd D N Nd D N Nd Nd D D Nd D Nd D D Nd Nd Nd D D Nd Nd D Nd D D D Nd D D 0 N Ô – Ô 0 N – – – 0 N – 0 0 Ó 0 Ó Ô N – N N B No aliphatic C B A C No basic B A No sulfur C B A C Nd D D D Nd Nd N Ô – Nd Nd D D Nd D – Ô N – – – Ô Ô N Ó 0 N 0 Nd Nd Nd D D Nd D D Nd D Nd Nd Nd D D D D Nd D Nd – – – Ô Ô N Ó 0 N 0 Nd Nd Nd D D Nd Nd Nd Nd D Nd D Nd D D Nd Nd Nd D Nd – N – Ô Ô – – – N 0 0 N N – Ô 0 N N – 0 0 N Ó 0 0 Ó 0 Ó Ô N – N N D Nd D Nd D D Nd Nd Nd D D Nd D Nd D D D D D Nd Nd Nd Nd Nd D D Nd D Nd D Nd Nd Nd Nd D D Nd Nd D Nd D D D Nd D D 0 N Ó – Ô 0 N – – 0 0 N Ó – 0 Ó 0 Ó Ô N – N N D Nd D Nd Nd D Nd Nd Nd D Nd Nd D D D Nd D D D Nd Nd Nd Nd Nd D D D D Nd Nd Nd Nd Nd Nd D D Nd Nd Nd Nd D Nd Nd D D Nd 0 N Ó N N 0 – – – 0 – N Ô 0 0 – 0 Ó 0 – N N – *B: before irradiation, **A: after irradiation, ***C: change of volatiles by irradiation 1 Nd: not detected, 2 D: detected, 3 N: newly generated, 4 Ô : decreased, 5 –: no change, 6 Ó : increased, 7 0: disappeared. chicken meat, but our data indicated that many other sulfur compounds could be produced from methionine and cysteine (Table 7). Table 9 summarizes changes in volatiles of liposomes containing various amino acid homopolymer mixtures. Detailed explanations on the changes of volatiles in each amino acid homopolymer mixture can be found in the text. Conclusions T HE PRODUCTION OF MANY NEW VOLATILES FROM AMINO ACIDS BY application of irradiation indicated that more than 1 site in amino acid side chains was susceptible to free radical attack, and many volatiles can apparently be produced by secondary chemical reactions after the primary radiolytic degradation of side chains. Only sulfur-containing volatiles, however, produced strong off-odor that was similar to irradiation odor of meat. The perception of odor from samples containing sulfur volatiles changed somewhat depending on the composition of other volatiles in the sample. Although some volatiles produced from nonsulfur amino acid homopolymers interacted with sulfur com2664 pounds, their roles in the odor characteristics of irradiated liposomes can be considered minor. References Ahn DU. 2002. Production of volatiles from amino acid homopolymers by irradiation. J Food Sci. Forthcoming. Ahn DU, Sell JL, Jeffery M, Jo C, Chen X, Wu C, Lee JI. 1997. Dietary vitamin E affects lipid oxidation and total volatiles of irradiated raw turkey meat. J Food Sci 62(5):954-959. Ahn DU, Olson DG, Lee JI, Jo C, Wu C, Chen X. 1998a. Packaging and irradiation effects on lipid oxidation and volatiles in pork patties. J Food Sci 63(1):15-19. Ahn DU, Olson DG, Jo C, Chen X, Wu C, Lee JI. 1998b. Effect of muscle type, packaging, and irradiation on lipid oxidation, volatile production and color in raw pork patties. Meat Sci 49(1):27-39. Ahn DU, Olson DG, Jo C, Love J, Jin SK. 1999. Volatiles production and lipid oxidation of irradiated cooked sausage with different packaging during storage. J Food Sci 64(2):226-229. Ahn DU, Jo C, Olson DG. 2000. Analysis of volatile components and the sensory characteristics of irradiated raw pork. Meat Sci 54:209-215. Ahn DU, Nam KC, Du M, Jo C. 2001. Volatile production of irradiated normal, pale soft exudative (PSE) and dark firm dry (DFD) pork with different packaging and storage. Meat Sci 57:419-426. Diehl JF. 1995. Chemical effects of ionizing radiation. In: JF Diehl, editor. Ch. 3. Safety of irradiated foods. 2nd ed New York: Marcel Dekker Inc. P. 43-87. Du M, Nam KC, Hur SJ, Ismail H, Ahn DU. 2001a. Effect of dietary conjugated linoleic acid, irradiation and packaging conditions on the quality characteristics of raw broiler breast fillets. Meat Sci 60(1):9-15. Du M, Nam KC, Hur SJ, Ismail H, Ahn DU. 2001b. Volatiles, color, and lipid oxida- JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 7, 2002 jfsv67n7p2659-2665ms20020084-MO.p65 2664 9/20/2002, 10:36 AM Volatiles by protein-lipid interactions . . . formation of radiolysis products in meat and meat substances. J Agric Food Chem 26:29-31. Patterson RLS, Stevenson MH. 1995. Irradiation-induced off-odor in chicken and its possible control. Br Poultry Sci 36:425-441. SAS 1989. SAS user’s guide. Cary, NC: SAS Institute Inc. MS 20020084 Submitted 2/8/02, Accepted 5/12/02, Received 5/12/02 Journal paper number J – 19734 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA 50011-3150. Project No. 6523, and supported by the National Research Initiative Competitive Grant/USDA. The authors are with the Department of Animal Science, Iowa State University, Ames, Iowa 50011-3150. Direct inquiries to author Ahn (E-mail: duahn@iastate.edu). Food Chemistry and Toxicology tion of broiler breast fillets irradiated before and after cooking. Poultry Sci Forthcoming. Hashim IB, Resurreccion AVA, MacWatters KH. 1995. Disruptive sensory analysis of irradiated frozen or refrigerated chicken. J Food Sci 60(4):664-666. Heath JL, Owens SL, Tesch S, Hannah KW. 1990. Effect of high-energy electron irradiation of chicken on thiobarbituric acid values, shear values, odor, and cook yield. Poultry Sci 69:313-319. Jo C, Ahn DU. 2000. Production of volatiles from irradiated oil emulsion systems prepared with amino acids and lipids. J Food Sci 65(4):612-616. Lee EJ Ahn DU. 2002. Production of off-odor volatiles from fatty acids and oils by irradiation. J Food Sci Submitted. Merritt C Jr, Angelini P, Wierbicki E, Shuts GW. 1975. Chemical changes associated with flavor in irradiated meat. J Agric Food Chem 23:1037-1043. Merritt C Jr, Angelini P, Graham RA. 1978. Effect of radiation parameters on the Vol. 67, Nr. 7, 2002—JOURNAL OF FOOD SCIENCE jfsv67n7p2659-2665ms20020084-MO.p65 2665 9/20/2002, 10:36 AM 2665
