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Food Chemistry and Toxicology

Production of Volatiles from Amino Acid Homopolymers by Irradiation ABSTRACT: Amino acid homopolymers were used to determine production of radiolytic volatiles by irradiation. Many new volatiles were generated, and the amounts of volatiles in amino acid homopolymers changed after irradiation. Each amino acid homopolymer group produced different odor characteristics, but the intensities of odor from all amino acid groups were weak, except for sulfur-containing amino acids. Sulfur-containing amino acids produced various sulfur compounds; the overall odor intensity of irradiated sulfur amino acids was very high and the odor characteristics of sulfur amino acids were similar to irradiation odor of meat. Our results indicated that the contribution of methionine to the irradiation odor would be far greater than that of cysteine. Keywords: irradiation, volatiles, amino acid homopolymers, sulfur amino acids, irradiation odor

Introduction

O

UR STUDIES SHOWED THAT ALL IRRADIATED MEAT PRODUCED

characteristic irradiation odor regardless of degree of lipid oxidation. Also, irradiated meat produced more volatiles than nonirradiated meat, and the chromatograms of raw and cooked irradiated meat suggested that lipid oxidation could be responsible for a small part of the off-odor in irradiated meat (Ahn and others 1997; Ahn and others 1998a, 1998b, 1999). 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-1-en-3one and bis(methylthio-)methane. Hashim and others (1995) reported that irradiating uncooked chicken breast and thigh produced a characteristic “bloody and sweet” aroma that remained after the thighs were cooked, but was not detectable after the breasts were cooked. Heath and others (1990) reported that irradiating uncooked chicken breast and leg at 2 or 3 kGy produced a “hot fat,” “burned oil,” or “burned feathers” odor that remained after the thighs were cooked. Chen and others (1999) reported that irradiation before cooking did not influence lipid oxidation of cooked pork during storage. These studies also supported the concept that the changes that occur following irradiation are different from those of warmed-over flavor in oxidized meat, and that other mechanisms such as radiolysis of proteins play an important role in the production of volatiles in irradiated meat. Irradiation of amino acids produced distinct volatile compounds via the radiolytic degradation of amino acids (Jo and Ahn 2000). However, radiolytic degradations of amino acids not only occurred at side chains, but also at amino and carboxyl groups. Thus, the volatile compounds produced from amino acid monomers by irradiation cannot represent the volatiles produced from proteins. To overcome this problem, we used amino acid homopolymers in this study. The objective of this study was to determine the volatile compounds produced from each amino acid polymer by irradiation and the odor characteristics of the irradiated amino acid polymers.

Materials and Methods Sample preparation A total of 15 amino acids that included 12 amino acid homopolymers, a dimer, a trimer, and a tetramer were used in this study and were purchased from Sigma Chemical Co. (Sigma Co., © 2002 Institute of Food Technologists

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St. Louis, Mo., U.S.A.). The molecular weights of these amino acid homopolymers ranged from 2,000 to 40,000 depending upon amino acid. Each individual amino acid (homopolymer or oligomer) (40 mg) was dissolved in 20 mL of citrate-phosphate buffer (100 mM, pH 6.0). Two 10-mL portions were transferred to scintillation vials; one was used as a nonirradiated control and the other was irradiated at 5.0-kGy absorbed dose using an electron beam irradiator (Circe IIIR Thomson CSF Linac, St. Aubin, France). To confirm the target dose, 2 alanine dosimeters per cart were attached to the top and bottom surfaces of a sample vial. Immediately after irradiation, a 2-mL portion of the amino acid (homopolymer or oligomer) solution was transferred to sample vials, flushed with helium gas, and analyzed for volatiles using a purge-and-trap dynamic headspace/gas chromatography-mass spectrometry (GC-MS), and the rest was also portioned and used to evaluate odor characteristics using trained sensory panelists. Four replications were prepared for volatiles.

Dynamic headspace/GC-MS method A purge-and-trap apparatus (Precept II and Purge & Trap Concentrator 3000, Tekmar-Dohrmann, Cincinnati, Ohio, U.S.A.) connected to a GC/MS (Hewlett-Packard Co., Wilmington, Del., U.S.A.) was used to analyze volatiles produced (Ahn and others 2001). Sample solution (2 mL) was placed in a 40-mL sample vial, and the vials were flushed with helium gas (40 psi) for 5 s. The sample was purged with helium gas (40 mL/min) for 12 min at 40 °C. Volatiles were trapped using a Tenax column (TekmarDohrmann) 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 m mm nominal; Hewlett-Packard Co), and an HP-Wax column (7.5 m x 0.25 mm internal dia, 0.25 mm nominal) were connected using zero dead-volume column connectors (J & W Scientific, Folsom, Calif., U.S.A.). Ramped oven temperature was used to improve volatile separation. The initial oven temperature of 0 °C was held for 2.50 min. After that, the oven temperature was increased to 15 °C at 2.5 °C per min, increased to 45 °C at 5 °C per min, increased to 110 °C at 20 °C per min, increased to 210 °C at 10 °C per min, then was held for 2.5 min at that temperature. Constant Vol. 67, Nr. 7, 2002—JOURNAL OF FOOD SCIENCE

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Volatiles from amino acids by irradiation . . . Table 1-Production of volatile compounds from acidic group amino acid homopolymers by irradiation Volatiles

0 kGy

5 kGy

SEM

Table 3-Production of volatile compounds from aliphatic hydroxyl group amino acid homopolymers by irradiation Volatiles

0 kGy

——— Total ion counts x 103 ———


Poly-aspartic acid 2-Propanone Hexane Methyl cyclopentane Benzene Toluene Poly-glutamic acid Acetaldehyde 2-Propanone Hexane Cyclohexane

253b 295b 0b 352a 0b

1735a 636a 204a 104b 359a

424 29 2 9 20

0b 366b 236 184a

5452a 1784a 277 0b

198 316 17 7

a,b Means with no common superscript differ significantly (P < 0.05), n = 4

Table 2-Production of volatile compounds from aliphatic group amino acid homopolymers by irradiation Volatiles

0 kGy

5 kGy

SEM


Poly-alanine Acetaldehyde 979b 2-Propanone 8502 Acetonitrile 0b Acetic acid, methyl ester 1106a Hexane 1600a Butanal 110a Methyl cyclopentane 265a Methyl propionate 0b Cyclohexane 258a Poly-glycine Acetaldehyde 0b 2-Propanone 22555 a 2-Methyl propanal 0b Hexane 782b Methyl cyclopentane 174b Benzene 0b 3-Methyl butanal 0b 2-Methyl butanal 0b Poly-proline 2-Methyl-1-propene 88a 2-Propanone 4353a Hexane 1063 Cyclohexane 991a Hexanal 67a Toluene 978a

2228a 10974 374a 49b 929b 0b 195b 522a 0b

91 1535 7 109 46 9 17 13 6

419a 3752b 409a 1477a 260a 1008a 91a 78a

115 5402 47 87 21 31 26 18

0b 1227b 944 0b 0b 0b

9 402 100 28 4 60


column pressure at 20.5 psi was maintained. The ionization potential of the mass selective detector (Model 5973; Hewlett-Packard Co.) was 70 eV, and the scan range was 18.1 to 350 m/z. Identification of volatiles was achieved by comparing mass spectral data of samples with those of the Wiley Library (Hewlett-Packard 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 103) was reported as an indicator of volatiles generated from the sample.

Odor characteristics Ten trained sensory panelists characterized overall odor characteristics of the samples. Panelists were selected based on interest, availability, and performance in screening tests conducted with samples similar to those to be tested. During training, a lexicon of aroma terms to be used on the ballot was developed. Sam2566

Poly-serine 2-Methyl-1-propene 492a Acetaldehyde 0b 1,1-Oxybis ethane 0b 2-Propanone 4229b Butanal 193a Hexane 1315a Cyclopentanol 122b Methyl cyclopentane 173 2-Butanone 334a Cyclohexane 1913a Toluene 2379a Poly-threonine Acetaldehyde 13693 b 1,1-Oxybis ethane 323445b 2-Propanone 0b Hexane 1391a Butanal 142b 2-Butanone 276b Acetic acid ethyl ester 475b 2-Ethoxy butane 0b Cyclohexane 787a 2,3-Dihydro-1,4-dioxin 0b 1,4-Dioxane 0b Methyl butyrate 0b Toluene 61a

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SEM

0b 1990a 316a 11555 a 0b 713b 349a 206 118b 0b 0b

20 35 4 864 3 74 5 16 38 20 38

60479 a 117262a 47097 a 788b 228a 845a 4305a 90a 0b 233a 125a 272a 0b

1078 6037 2086 55 11 28 126 3 49 7 3 20 2


ples 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.

Statistical analysis Data were analyzed using the generalized linear model procedure of SAS software (SAS Institute Inc. 1989), and the Student’s t-test was used to compare differences between irradiated and nonirradiated means. Mean values and standard error of the means (SEM) were reported. Significance was defined at P < 0.05.

Results and Discussion Acidic amino acid group Methyl cyclopentane and toluene, not found in nonirradiated polyaspartic acid, were produced by irradiation. The amounts of 2-propanone and hexane in irradiated polyaspartic acid increased, but that of benzene decreased after irradiation. Irradiation of polyglutamic acid newly produced acetaldehyde and increased the amount of 2-propanone ( Table 1). Neither of the acidic group amino acid homopolymers produced detectable odor after irradiation, however.

Aliphatic amino acid group Acetonitrile and methyl propionate were newly produced, and the amount of acetaldehyde increased from polyalanine after irradiation (Table 2). The amounts of acetic acid methyl ester, hexane, butanal, and cyclohexane, however, decreased after irradiation. The odor of irradiated polyalanine was characterized as seaweed odor, but the intensity was very weak. Irradiation of polyglycine produced 5 new volatiles not found in the nonirradiated control: acetaldehyde, 2-methyl propanal, benzene, 2-methyl butanal, and 3-methyl butanal. The amount

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Volatiles from amino acids by irradiation . . .

Volatiles

0 kGy

5 kGy

SEM

Table 5-Production of volatile compounds from aromatic group amino acid homopolymers by irradiation Volatiles

0 kGy


Poly-asparagine 2-Methyl-1-propene 2-Propanone Hexane Methyl cyclopentane Cyclohexane Benzene Toluene Poly-glutamine 2-Methyl-1-propene Acetaldehyde 1,1-Oxybis ethane 2-Propanone Butanal Hexane Cyclopentanol Methyl cyclopentane 2-Butanone Cyclohexane Toluene

258a 2963a 309b 0b 1923a 0b 0b

0b 729b 813a 419a 0b 1183a 6385a

10 411 42 40 15 11 83

465a 0b 0b 4708b 191a 1336a 121b 170 374a 1926a 2355a

0b 2029a 316a 11102 a 0b 649b 343a 192 114b 0b 0b

21 53 5 648 4 92 5 18 37 24 38

5 kGy

SEM


Poly-tyrosine Acetaldehyde Oxirane 2-Methyl butane Ethanol 2-Propanone Hexane Butanal Methyl cyclopentane Tetrahydrofuran 2-Methyl-1,3-dioxolane Benzene Cyclohexene 2,3-Dihydro-1,4-dioxin Toluene 2-Ethyl butanal

460b 1258a 349a 2445 1385b 525b 100b 87b 0b 0b 0b 0b 0b 89b 179a

17532 a 0b 0b 741 3774a 751a 230a 243a 1863a 107a 986a 78a 409a 5299a 0b

645 57 78 562 571 34 12 13 128 16 11 2 28 103 31



Aromatic amino acid group

of 2-propanone decreased, but that of hexane increased after irradiation. Irradiated polyglycine produced a weak seashore odor. Irradiation did not produce any new volatiles nor increase the amounts of volatiles in polyproline. Only 2 volatiles, 2-propanone and hexane, remained after irradiation; 2-methyl-1-propene, cyclohexane, hexanal, and toluene detected in nonirradiated polyproline disappeared after irradiation ( Table 2). Irradiated polyproline produced a solvent-like odor, but its intensity was very weak.

Aliphatic hydroxyl amino acid group Polyserine produced acetaldehyde and 1,1-oxybis ethane after irradiation (Table 3). The amounts of 2-propanone and cyclopentanol increased, but those of hexane and 2-butanone decreased. Two-Methyl-1-propene, butanal, cyclohexane, and toluene were detected in nonirradiated control, but were not found in irradiated polyserine. The odor of irradiated polyserine was characterized as a cattle barn odor. New volatiles produced in polythreonine after irradiation included 2-propanone, 2-ethoxybutane, 2,3-dihydro-1,4-dioxin, and methyl butyrate. The amounts of acetaldehyde, butanal, 2butanone, and acetic acid ethyl ester increased, but those of 1,1oxybis ethane, hexane, cyclohexane, and toluene decreased after irradiation (Table 3). The odor of irradiated polythreonine was characterized as a Chinese herbal medicine or as caramel-like.

Amide amino acid group Irradiation of polyasparagine produced 2 new volatile compounds, methyl cyclopentane and benzene, and increased toluene and hexane. The amount of 2-propanone decreased after irradiation (Table 4). Irradiated polyasparagine, however, produced no odor. Acetaldehyde and 1,1-oxybis ethane were newly produced from polyglutamine after irradiation. The amounts of 2-propanone and hexane increased, but those of 2-methyl-1-propene, butanal, hexane, 2-butanone, cyclohexane, and toluene decreased after irradiation ( Table 4). The odor of irradiated polyglutamine was hospital odor.

New volatiles such as tetrahydrofuran, 2-methyl-1,3-dioxalone, benzene, cyclohexane, and 2,3-dihydro-1,4-dioxin were produced after irradiating polytyrosine. The amounts of acetaldehyde, 2-propanone, butanal, methyl cyclopentane, and toluene increased, but those of oxirane, 2-methyl butane, and 2-ethyl butanal decreased (Table 5). The odor of irradiated polytyrosine was characterized as that of seaweed or seashore.

Basic amino acid group Irradiation increased the production of acetaldehyde, 2-propanone, hexane, methyl cyclopentane, and toluene, but decreased amounts of 2-methoxy-2-methyl-propane and cyclohexane in polyhistidine (Table 6). Irradiated polyhistidine produced a sweet odor. Irradiation greatly increased the production of acetaldehyde and also newly produced propanal, butanal, 2-methyl dioxalone, and benzene from polylysine. The amounts of cyclopentane and toluene increased, but that of cyclohexane decreased (Table 6). The odor of irradiated polylysine was characterized as somewhat sour, coleslaw, or hospital odor.

Sulfur-containing amino acid group No S-containing amino acid homopolymers are available, so glutathione (Glu-Cys-Gly) was used to determine the volatiles produced from cysteine upon irradiation, and Met-Ala for methionine. Irradiation produced 2 sulfur volatiles, carbon disulfide and dimethyl sulfide, in addition to methyl cyclopentane from glutathione ( Table 7). The odor of irradiated glutathione was characterized as hard-boiled egg or sulfur, and the odor was strong. Irradiation of Met-Ala produced many more new sulfur volatiles than irradiating glutathione. Mercaptomethane, dimethyl disulfide, methyl thiirane, (methylthio-) ethane, 3-(methylthio)1-propene, 2-methyl-2-(methylthio-) propane, ethanoic acid Smethyl ester, dimethyl disulfide, methyl ethyl disulfide, and 2,4dithiapentane were sulfur volatiles produced from the irradiated Met-Ala. The amounts of other volatiles were also changed by irradiation, but were minor compared to sulfur volatiles. Irradiation of Met-Gly-Met-Met produced a few more nonsulfur volatiles than Met-Ala and the total amount of sulfur compounds from the irradiated Met-Gly-Met-Met was greater than that from the Vol. 67, Nr. 7, 2002—JOURNAL OF FOOD SCIENCE

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Table 4-Production of volatile compounds from amide group amino acid homopolymers by irradiation

Volatiles from amino acids by irradiation . . . Table 6-Production of volatile compounds from basic group amino acid homopolymers by irradiation Volatiles

0 kGy

5 kGy

SEM

ion counts x

103 ———

385a 969b 992b 98866 a 501b 106 65b 2277a 0b

0b 1382a 2137a 19884 b 1037a 136 350a 0b 119a

18 82 196 2181 36 13 7 28 15

116a 0b 0b 24797 634b 0b 242b 90006 2161a 0b 0b 122b

0b 11729 a 460a 35641 1484a 252a 775a 69419 0b 284a 310a 469a

8 421 16 5693 35 8 33 8755 20 40 2 13

——— Total


Poly-histidine 4,4-Dimethyl-2-oxetanone Acetaldehyde 2-Propanone 2-Methoxy-2-methyl-propane Hexane Butanal Methyl cyclopentane Cyclohexane Toluene Poly-lysine 2-Methyl-1-propene Acetaldehyde Propanal 2-Propanone Hexane Butanal Methyl cyclopentane Tetrahydrofuran Cyclohexane 2-Methyl dioxolane Benzene Toluene


irradiated Met-Ala because of higher methionine content per unit weight (Table 7). The odor of irradiated Met-Ala was much stronger than that of glutathione and was characterized as boiled cabbage, sulfury, or rotten vegetable-like. The odor characteristics of Met-Gly-Met-Met were also similar to those of Met-Ala. Table 7 also suggests that methionine produced far greater amounts of sulfur compounds than cysteine, and is the most important amino acid in the production of irradiation off-odor.

Results and Discussion

M

ANY NEW VOLATILES WERE GENERATED AND THE amounts of volatiles produced from amino acid homopolymers were changed after irradiation. The production of many new volatiles from amino acids upon irradiation indicated that more than 1 site in amino acid side chains was susceptible to free radical attack and many volatiles were apparently 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. The perception of odor from samples containing sulfur volatiles changed greatly depending upon their composition and amounts present in the sample. Although volatiles generally found in all amino acid homopolymers could provide dilution effect or produce interactions with sulfur compounds, their roles in the odor characteristics of irradiated sulfur-containing amino acids are expected to be minor. The result of this study also indicated that sulfur compounds produced by S-containing amino acids such as cysteine and methionine played the major role in irradiation odor. The volatile profiles and sensory characteristics of amino acids (Tables 1 to 8) clearly explained why irradiation odor was different from lipid oxidation odor, and why lipid oxidation was responsible for only a small part of the off-odor in irradiated meat (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 2568

Table 7-Production of volatile compounds from sulfurcontaining amino acid dimer or oligomers by irradiation Volatiles

0 kGy

Glutathione (g-Glu-Cys-Gly) Carbon disulfide 0b Hexane 316b Methyl cyclopentane 0b Cyclohexane 119a Dimethyl disulfide 0b Met-Ala 2-Methyl-1-propene 614a Acetaldehyde 0b Mercaptomethane 0b 2-Propanone 1244a Dimethyl sulfide 0b 2-Methyl propanol 0b Hexane 281b Methyl thiirane 0b (Methylthio) ethane 1376a 2-Ethoyxy2-methyl propane1 299a Acetic acid ethyl ester 3290 Cyclohexane 1565a 3-(Methylthio)-1-propene 0b Ethanoic acid, S-methyl ester 0b 2-Methyl-2-(methylthio) propane 86a Dimethyl disulfide 5043b Methyl benzene 591a Methyl ethyl disulfide 0b 2,4-Dithiapentane 0b Met-Gly-Met-Met 2-Methyl-1-propene 270a Acetaldehyde 2264a Mercaptomethane 0b Pentanal 0b Dimethyl sulfide 0b 2-Propanone 4010a Acetonitrile 3485a Hexane 285b 2,2-Oxybis propane 17951 a (Methylthio) ethane 0b 2-Butanone 206a Acetic acid ethyl ester 116873a Cyclohexane 988a Benzene 0b 1-Heptanethiol 0b 3-(Methylthio)-1-propene 0b Ethanethioic acid, S-methyl ester 0b 2-Butanamine 0b 2-Methyl-2-(methylthio) propane 92b Dimethyl disulfide 1430b Methyl ethyl disulfide 0b Ethyl benzene 0b 1,3-Dimethyl benzene 0b 1,4-Dimethyl benzene 0b Isopropyl benzene 0b

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SEM

589a 496a 82a 0b 214a

24 39 5 2 47

0b 2910a 11842 a 0b 166244a 114a 1146a 4177a 0b

11 230 709 456 6183 3 47 174 47

344b 4467 0b 186a

114 415 13 11

106a

7

0b 346229a 0b 2221a 825a

1 9385 23 80 25

0b 0b 17325 a 341a 201541a 0b 356b 780a 3843b 2053a 0b 77893 b 0b 210a 94a 122a

8 224 866 18 939 289 414 26 183 15 35 4084 21 1 1 1

170a 156a

8 6

149a 351320a 1935a 38116 a 60346 a 11550 a 725a

2 1247 15 322 823 164 20


chicken meat, but our data indicated that many other sulfur compounds could be produced from methionine and cysteine (Table 7). The volatility of aroma compounds depends on the vapor-liquid partitioning of volatile compounds, which determines the affinity of volatile molecules for each phase (Buttery and others 1973), and the interactions among food components such as carbohydrates and proteins affect the release of volatile compounds in foods (Godshall 1997). Physicochemical conditions of


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Volatiles from amino acids by irradiation . . . Table 8-Major volatile compounds from irradiated amino acid homopolymers and oligomers and their odor characteristics Amino acid polymer

Major volatiles

Odor characteristics

Poly-aspartic acid Poly-glutamic acid Poly-alanine Poly-glycine Poly-proline Poly-serine Poly-threonine

2-propanone, methyl cyclopentane, toluenea acetaldehyde, 2-propanone acetonitrile, methyl propionate, acetaldehyde, 2-propanone acetaldehyde, 2-methyl propanal, 2-methyl butanal, 3-methyl propanal 2-propanone, hexane acetaldehyde, 1,1-oxybis ethane, 2-propanone acetaldehyde, 2-propanone, acetic acid ethyl ester, 2-ethoxy butane, 2,3-dihydro-1,4-dioxin, 1,4-dioxin, methyl butyrate, 1,1-oxybis ethane methyl cyclopentane, benzene, toluene acetaldehyde, 1,1-oxybis ethane, 2-propanone acetaldehyde, tetrahydrofuran, 2-methyl-1,3-dioxalane, benzene, toluene, cyclohexane, 2,3-dihydro-1,4-dioxin 2-methoxy-2-methyl propane, toluene acetaldehyde, propanal, butanal, 2-methyl dioxalone, benzene, 2-propanone carbon disulfide, dimethyl disulfide, methyl cyclopentane acetaldehyde, mercaptomethane, dimethyl sulfide, methyl thiirane, 3-(methylthio)-1-propene, ethanoic acid- S-methyl ester, dimethyl disulfide, methyl ethyl disulfide, 2,4-dithiapentane, 2-methyl propanal mercaptomethane, pentanal, dimethyl sulfide, (methylthio)-ethane, benzene, 1-heptanethiol, 3-(methylthio)-1-propene, ethanoic acid-S-methyl ester, dimethyl disulfide, methyl ethyl disulfide, 2-butanamine, 1,3-dimethyl benzene, 1,4-dimethyl benzene, isopropyl benzene, ethyl benzene

no odor sweet, honey seaweed seashore odor organic solvent cattle barn odor Chinese herbal medicine

Poly-histidine Poly-lysine Glutathione Met-Ala Met-Gly-Met-Met

no odor hospital odor seaweed or seashore sweet coleslaw, sour hard-boiled eggs, sulfury boiled eggs, sulfury rotten vegetable boiled cabbage, sulfury, rotten vegetable

a Volatiles written in italic did not produce detectable odor at the levels found in the samples

foods, which influence conformation of proteins, also are closely related to flavor release (Lubbers and others 1998). Jo and Ahn (1999) reported that the amount of volatiles released from oil emulsion correlated negatively with fat content. The release of nonpolar hydrocarbons was not influenced, but polar compounds, such as aldehydes, ketones, and alcohols, were greatly influenced by water. This indicated that the relative amounts of volatile compounds released from meat systems could be significantly different from those in the aqueous system tested here. However, the results from this study confirmed the sources of volatile compounds critical to irradiation odor reported earlier by Jo and Ahn (2000). 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. No single group amino acid homopolymers provided such odor characteristics. The odor intensity of sulfur-containing amino acids was much stronger and stringent than that of other amino acid groups. This indicated that sulfur compounds were the most influential in irradiation off-odor, but volatiles from other amino acid groups also played an important role in overall odor perception. Sulfur compounds have very low odor thresholds and most of them are considered to be the major cause of off-odor. However, some sulfur compounds such as 2-pentylthiophene are important for freshly cooked meat flavor (Tang and others 1983). The amounts of aldehydes, especially those of hexanal and pentanal, are highly correlated with oxidation of lipids (Ahn and others 1998a). This study indicated that some aldehydes such as acetaldehyde, 2-methyl propanal, 3-methyl butanal, and 2-methyl butanal could be newly generated from amino acid side chains after irradiation. Benzene and toluene (methyl benzene) are considered as carcinogens. Irradiation of glycine, asparagine, lysine, and tyrosine newly produced benzene, but irradiation reduced or destroyed toluene in nonirradiated proline, asparagine, glutamine, lysine, serine, and tyrosine. Although toluene was detected in both irradiated and nonirradiated broiler meats (Du and others 2001a, 2001b), it is difficult to assess its human

health implications. It is apparent, however, that toluene could be produced from the components naturally present in meat even without irradiation.

Conclusion

S

ULFUR COMPOUNDS PRODUCED FROM THE SIDE CHAINS OF ME-

thionine and cysteine were the most important volatiles for off-odor production in irradiated meat. Sulfur compounds were not only produced by the radiolytic cleavage of side chains (primary reaction), but also by the secondary reactions of primary sulfur compounds with other compounds around them. The amounts and kinds of sulfur compounds produced from irradiated methionine and cysteine indicated that methionine is the major amino acid responsible for irradiation off-odor. The total amount of sulfur compounds produced from cysteine is only about 0.25 to 0.35% of methionine even after the proportion of cysteine or methionine in each of the dimmer, trimer, or tetramer was considered. Therefore, the contribution of methionine to the irradiation odor is far greater than that of cysteine.

References 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. Buttery RG, Guadagni DG, Ling LC. 1973. Flavor compounds: Volatiles in vegetable oil and oil-water mixtures. Estimation of odor thresholds. J Agric Food Chem 21(1):198-201. Chen X, Jo C, Lee JI, Ahn DU. 1999. Lipid oxidation, volatiles and color changes of irradiated pork patties as affected by antioxidants. J Food Sci 64(1):16-19. Du M, Nam KC, Hur SJ, Ismail H, Ahn DU. 2001a. Effect of dietary conjugated linoleic acid, irradiation and packaging conditions on the quality character-

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Poly-asparagine Poly-glutamine Poly-tyrosine

Volatiles from amino acids by irradiation . . . istics 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 oxidation of broiler breast fillets irradiated before and after cooking. Poultry Sci Forthcoming. Godshall MA. 1997. How carbohydrates influence flavor. Food Technol 51(1):6367. Hashim IB, Resurreccion AVA, McWatters 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. 1999. Fat reduces volatiles production in oil emulsion system analyzed by purge-and-trap dynamic headspace/gas chromatography. J Food Sci 64(4):641-643. 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.

Lubbers S, Landy P, Voilley A. 1998. Retention and release of aroma compounds in foods containing proteins. Food Technol 52(5):68-74, 208-214. 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, N.C.: SAS Institute Inc. Tang J, Jin QZ, Ho CT, Chang CT. 1983. Isolation and identification of volatile compounds from fried chicken. J Agric Food Chem 31(3):1287-1292. MS 20010619 Submitted 11/10/01, Accepted 1/10/02, Received 3/25/02 Journal paper number J-19616 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA 50011-3150. Project No. 6523, supported by the National Research Initiative Competitive Grant.

Author is with the Dept. of Animal Science, Iowa State Univ., Ames, IA 500113150. Direct queries to author (E-mail: [email protected]).

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