Production of Volatiles from Fatty Acids and Oils by

JFS:

Food Chemistry and Toxicology

Production of Volatiles from Fatty Acids and Oils by Irradiation E.J. LEE AND D.U. AHN


ABSTRACT: To understand the mechanisms of off-odor production in irradiated meat, the volatile compounds produced from individual fatty acids by irradiation were identified. Nonirradiated oil emulsions prepared with polyunsaturated fatty acids (PUFAs) produced many volatile compounds, but the amounts of volatiles generally decreased after irradiation. Although volatile profiles of fatty acid emulsions were changed by irradiation, the odor characteristics and intensity between irradiated and nonirradiated fatty acid emulsions were not different. Thiobarbituric acid-reactive substances (TBARS) values indicated that irradiation accelerated lipid oxidation during subsequent storage, but the volatiles produced by lipid oxidation were not the major contributors of off odor in irradiated samples. Keywords: volatiles, fatty acid, irradiation, TBARS, odor characteristics

Introduction

I

RRADIATION IS AN EFFECTIVE METHOD FOR PATHOGEN CONTROL IN RAW

meat. It is permitted for use in both poultry and red meats. A major concern, however, is its effect on meat quality. When molecules absorb ionizing energy, they become reactive and form ions or free radicals that react to form stable radiolytic products (Woods and Pikaev 1994). These reactive substances oxidize myoglobin and fat and thus cause discoloration, rancidity, and off odor in meat (Murano 1995). Ionizing radiation is known to generate hydroxyl radicals in aqueous ( Thakur and Singh 1994) or oil emulsion systems (O’Connell and Garner 1983). The hydroxyl radical is the most reactive oxygen species. It can initiate lipid oxidation by abstracting a hydrogen atom from a fatty acyl chain of a polyunsaturated fatty acid (PUFA) and form a lipid radical. In the presence of oxygen, the lipid radical rapidly reacts with oxygen to form a peroxyl radical which, in turn, can extract a hydrogen atom from another fatty acyl chain, yielding a new free radical that can perpetuate the chain reaction and a lipid hydroperoxide that can be degraded into various volatile compounds after a series of secondary reactions (Gray 1978; Enser 1987). Kanatt and others (1998) reported that irradiation increased 2thiobarbituric acid-reactive substances (TBARS) values and carbonyl content in ground chicken meat. Ahn and others (1997) reported that irradiation increased lipid oxidation in raw turkey breast and thigh meats that were aerobically packaged, but had limited effects on the formation of total volatiles during storage at 4 °C for 7 d or longer. Lipid oxidation is known to generate aldehydes, ketones, hydrocarbons, esters, furans, and lactones, which can be responsible for rancid flavors and sensory defects in meat products (Ladikos and Lougovois 1990). Aldehydes contributed the most to oxidation flavor and rancidity in cooked meat (Shahidi and Pegg 1994). Hexanal was the predominant aldehyde produced by lipid oxidation, and hexanal content correlated the best with TBARS of meat (Ang and Lyon 1990; Spanier and others 1992; Ahn and others 1998a, 1998b, 1999b). The objective of this study is to determine the volatile com-

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JOURNAL OF FOOD SCIENCE—Vol. 68, Nr. 1, 2003

pounds produced from individual fatty acids by irradiation as a step toward understanding the mechanisms of off-odor production in irradiated meat.

Materials and Methods Sample preparation Selected fatty acids generally found in meat (palmitoleic, oleic, linoleic, linolenic, and arachidonic), corn oil (refined), and fish oil (refined) were purchased from Sigma Chemical Co. (St. Louis, Mo., U.S.A.) and used to determine their contribution to lipid oxidationdependent production of volatiles by irradiation. An oil-in-water emulsion system was used in this study because it can increase the surface area of fatty acid or oil. Oil emulsion was prepared by blending 0.5 g of fatty acid or oil in 50 mL deionized distilled water (Waring blender; 22,000 rpm for 2 min; Dynamics Corp. America Co., Conn., U.S.A.). An aliquot of oil emulsion sample (15 mL) was transferred to a scintillation vial and irradiated at 0 or 5.0 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 99.3 kGy/min. The max/ min ratio was approximately 1.39 (avg.). To confirm the target dose, 2 alanine dosimeters per cart were attached to the top and bottom surface of a sample vial. The alanine dosimeter was read using a 104 Electron Paramagnetic Resonance Instrument (Bruker Instruments Inc., Billerica, Mass., U.S.A.). Immediately after irradiation, 2-mL portions of the oil emulsion were transferred to sample vials, flushed with helium gas (99.999%) for 5 s at 40 psi, and capped. Two of them were used to obtain volatile profiles, two for TBARS, and the rest were used to determine odor characteristics. Volatile profiles, TBARS, and odor characteristics of irradiated and nonirradiated oil emulsions were compared. Fatty acid compositions of corn oil and fish oil were analyzed using the method described by Nam and others (2001).

TBARS analysis Oil emulsion (1 mL) was transferred to a 13 ⫻100 mm disposable glass tube and butylated hydroxyanisole (50 ␮L, 7.2% in ethanol) © 2003 Institute of Food Technologists

Volatiles from fatty acid by irradiation. . .

0 kGy Volatiles 1-Pentene Pentane 1-Methoxy-2-methyl-1-propene 2-Methyl pentane 3-Methyl pentane 2,2-Dimethyl pentane 2,3-Dimethyl pentane 3,3-Dimethyl pentane 1-Hexene Hexane 2-Methyl hexane 3-Methyl hexane 3-Ethyl hexane 2,4-Dimethyl hexane 1-Octene Octane 2-Octene 3-Octene 3-Methyl octane 2,6-Dimethyl octane 1-Heptene Heptane 2,6-Dimethyl heptane 1,2,4-Trimethyl heptane Ethyl benzene 1,3-Dimethyl benzene 2,2,3-Trimethyl butane 3-Nonen-1-ol Undecanenitrile Octahydro-1H-indene 1,3-Cyclopentadiene 4-Methyl cyclopentene 3-Methyl cyclopentane Methyl cyclopentane 1,1,3-Trimethyl cyclopentane Cyclohexane Cyclohexene Methyl cyclohexane 1,3-Dimethyl cyclohexane Ethyl cyclohexane 1,1,3-Trimethyl cyclohesane 1,2,4-Trimethyl cyclohexane 1,2,3,5-Tetramethyl cyclohexane 1-Ethyl-3-methyl cyclohexane Propyl cyclohexane 1-Ethyl-2,3-dimethyl cyclohexane Butyl cyclohexane 1,1,2,3-Tetramethyl cyclohexane 1-Methyl-4-(1-methylethyl) cyclohexane 1,1,4-Trimethyl cyclohexane 1,2-Dimethyl cyclooctane Total volatiles Odor characteristics

5 kGy

SEM

total ion counts ⫻ 104 0b 0b 81a 870a 5163a 3850a 256a 579 0b 10691 a 288a 986a 211 50b 0b 56b 305 280b 518 1467 0b 127 120b 445 767a 4246a 632a 1010 2476 1231 114a 0b 0b 32889 a 813 9492a 0b 526a 774a 490a 910 1238 577 5092a 5539 707 975 3447 952 809 2116 104749 chickeny, fishy, wet dog

66a 571a 0b 514b 2875b 2808b 165b 595 88a 6466b 178b 272b 176 74a 162a 80a 296 413a 456 1464 300a 133 218a 362 188b 903b 436b 952 2254 1104 0b 214a 117a 11605 b 799 2922b 91a 348b 233b 385b 768 1074 474 3443b 4691 635 947 2123 829

1 18 2 37 234 176 11 25 2 479 12 22 12 3 6 5 17 25 44 277 10 6 12 26 16 119 26 166 457 221 2 6 3 1235 153 269 2 17 18 21 54 99 38 401 599 110 214 593 116

725 129 2064 457 59615 chickeny, fishy, metallic, rancid

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

and thiobarbituric acid (TBA, 20 mM)/trichloroacetic acid (TCA, 15% wt/vol) solution (2 mL) were added. The mixture was vortexed and then incubated in a boiling water bath for 15 min to develop color. The sample was then cooled in cold water for 10 min, mixed, and centrifuged for 30 min at 3000 ⫻ 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. If the absorbance of supernatant solution after color development was above 1.0, the solution was diluted properly with water and TBA/TCA mixture (1:2) until the absorbance became ⬍ 1.0. The amounts of TBARS were expressed as milligrams of malondialdehyde per L of oil emulsion.

Determination of volatile compounds 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 volatiles produced (Ahn and others 2001). Oil emulsion samples (2 mL) were placed in 40-mL sample vials, and the vials were flushed with helium gas (40 psi) for 5 s. The maximum waiting time of a sample in a refrigerated (4 °C) holding tray was less than 6 h to minimize oxidative changes before analysis (Ahn and others 1999a). The sample was purged with helium gas (40 mL/min) for 12 min at 40 °C. Volatiles were trapped using a Tenax-charcoalsilica 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 ⫻ 0.25 mm i.d., 1.4 ␮m nominal), an HP-1 column (52.5 m ⫻ 0.25 mm i.d., 0.25 ␮m nominal; HewlettPackard), and an HP-Wax column (7.5 m ⫻ 0.25 mm i.d., 0.25 ␮m 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 2.5 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 2.5 min at the temperature. Constant column pressure at 20.5 psi was maintained. The ionization potential of mass selective detector (Model 5973; Hewlett-Packard) 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). 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), and the total peak area (pA*s ⫻ 104) 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. Samples were placed in glass scintillation vials, 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. Sensory panelists were asked to write all odor characteristics that they could recognize and the odor characteristics that all panelists agreed on were used to determine the odor intensity of emulsions.

Statistical analysis Data were analyzed using the generalized linear model procedure of SAS software (SAS Institute Inc. 1989); 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. Vol. 68, Nr. 1, 2003—JOURNAL OF FOOD SCIENCE

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Table 1—Volatile compounds in irradiated oil emulsion prepared with arachidonic acid

Volatiles from fatty acid by irradiation. . . Table 2—Volatile compounds in irradiated oil emulsion prepared with linolenic acid 0 kGy Volatiles

5 kGy

total ion counts ⫻ 0b


Propanal 2,3-Dimethyl butane 531 2,2,3-Trimethyl butane 17893 1-Pentene 82b 2-Pentene 3517a 2-Methyl-1-pentene 0b 3-Methyl-2-pentene 0b Pentane 611a 2-Methyl pentane 8092 3-Methyl pentane 44759 2,3-Dimethyl pentane 1224 3,3-Dimethyl pentane 2410b 2,2,4,4-Tetramethyl pentane 354 1-Hexene 0b 2-Hexene 0b 3,3-Dimethyl-1-hexene 95b Hexane 79779 2-Methyl hexane 1578 2,4-Dimethyl hexane 232 2,5-Dimethyl hexane 138a 3-Methyl hexane 2385a 4-Propyl-3-heptene 1258a Heptane 968 2-Methyl heptane 136 2,6-Dimethyl heptane 209 3-Methyl heptane 150 3-Methyl heptane 2996 Octane 311a 3-Methyl octane 1119 3-Ethyl-1-octene 350a 2,5-Octadiene 100b Piperidine 511 Ethyl benzene 6791a 1,4-Dimethyl benzene 34507 a p-Xylene 15141 a 3-Nonen-1-ol 1512a Undecanenitrile 4169a cis-Octahydro-1H-indene 2055a o-Menth-8-ene 892a 1-Ethyl-1-methyl cyclopropane 0b 1,2-Dimethyl cyclopropane 561a 1,1,2-Trimethyl-2-cyclopropane 1254a Methyl cyclopentane 168782a Ethyl cyclopentane 205a 3-Methyl cyclopentene 0b 3-Methyl-1-cyclopentene 0b 1,3-Dimethyl cyclopentane 202b 1,2-Dimethyl cyclopentane 247a 1,2,3-Trimethyl cyclopentane 100a Cyclohexene 78560 a Methyl cyclohexane 3377a Ethyl cyclohexane 2156a Propyl cyclohexane 11800 a Butyl cyclohexane 1539a 1,2-Dimethyl cyclohexane 624a 1,3-Dimethyl cyclohexane 3025a 1,4-Dimethyl cyclohexane 873a 1,1,3-Trimethyl cyclohexane 2098a 1,2,3-Trimethyl cyclohexane 1080 1,2,4-Trimethyl cyclohexane 2863 1,1,2,3-Tetramethyl cyclohexane 3688a 1,1,4,4-Tetramethyl cyclohexane 3141a 1-Ethyl-2,3-dimethyl cyclohexane 966a 1-Methyl-4-(1-methyle)-cyclohexane 2068a 4-Methyl cyclohexene 812a Total volatiles 526876 Odor characteristics fishy, glue-like, fruity

SEM 104

1519a

32 321 65 12065 2336 136a 13 196b 215 a 473 56 467a 58 185b 44 5285 975 31244 4795 756 141 4908a 552 60 189 a 646 84 811a 94 212a 20 65482 6067 958 187 175 30 0b 6 1376b 258 b 826 105 955 138 109 11 170 25 140 16 1869 352 142b 36 771 126 b 200 38 172a 16 354 57 1276b 156 b 6814 693 5503b 477 991b 111 2637b 400 b 1245 156 598b 65 461a 47 319b 39 b 768 92 123599b 11090 101b 18 1538a 158 a 2829 294 905a 90 138b 27 0b 7 b 33688 7718 1832b 328 1273b 168 6856b 797 b 963 142 381b 55 746b 142 509b 63 1344b 201 756 100 1889 294 2301b 275 b 1814 269 623b 81 1031b 116 522b 66 339233 beany, fishy, painty, fruity, metallic


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Table 3—Volatile compounds in irradiated oil emulsion prepared with linoleic acid 0 kGy

5 kGy

SEM

total ion counts ⫻ 104

Volatiles 1,1-Oxybis ethane Butane 2,3-Dimethyl butane Butanal 1-Pentene 2-Pentene Pentane 3-Methylene pentane 3-Methyl pentane Pentanal 1-Hexene 2-Hexene 3-Hexene Hexane Hexanal 1-Heptene Heptane 2-Heptenal 1-Heptyne 1-Octene 2-Octene Octane 2-Propyl furan 2-Buthyl furan Benzene Ethyl benzene 1,3-Dimethyl benzene 1,4-Dimethyl benzene 1,2-Dimethyl cyclopropane Methyl cyclopentane 1-Methyl cyclopentene Cyclohexane Total volatiles Odor characteristics

1600a 352b 150a 1204a 79b 0b 4994b 0b 4339a 29390 a 27772 a 0b 62678 a 131b 47947 a 0b 141b 92a 0b 0b 103a 0b 65a 560a 302a 1318a 4717a 1184a 0b 9111a 0b 182a 198411 glue-like, metallic, cardboard

96b 49 1336a 34 112b 5 205b 50 958a 9 232a 2 10743 a 174 70a 5 1328b 113 b 3833 1208 9213b 664 132a 4 27231 b 1047 a 494 10 6399b 4980 644a 22 203a 9 b 0 12 101a 2 108a 8 0b 6 a 70 8 0b 3 150b 39 0b 13 266b 83 1106b 763 229b 92 525a 3 b 2599 332 110a 3 119b 9 68612 glue-like, metallic, cardboard, rancid


Results and Discussion

T

ABLES 1 TO 3 SHOW THE VOLATILES PRODUCED FROM OIL EMULSIONS

of PUFAs (arachidonic, linolenic, and linoleic acid) before and after irradiation. Many new volatiles were generated from oil emulsions of PUFAs by irradiation: pentane, 1-heptene, 4-methyl cyclopentene, and 1-octene were newly generated from arachidonic acid; 3-methyl-1-cyclopentene, propanal, and 3-methyl cyclopentene from linolenic acid; 1-heptene, 1,2-dimethyl cyclopropane, and 2-pentene from linoleic acid. The amounts of most of the volatiles in the oil emulsions of PUFAs, however, decreased after irradiation: methyl cyclopentane, cyclohexane, hexane, and 1,3-dimethyl benzene decreased the most in arachidonic acid emulsion; methyl cyclopentane, cyclohexene, 1,4-dimethyl benzene, hexane, and 3-methyl pentane in linolenic acid emulsion; and hexanal, 3hexene, pentanal, and 1-hexene in linoleic acid emulsion. Nawar (1986) reported that a series of dienes, trienes, and tetraenes were formed from unsaturated triacylglycerols by irradiation at 60 kGy under vacuum conditions. Although the irradiation dose used in this study is much lower, the number of volatiles produced from fatty acid emulsions was greater than that of Nawar (1986). Instead of irradiating oil or fatty acids directly, this study used an oilin-water emulsion system because the emulsion system would be

Volatiles from fatty acid by irradiation. . .

0 kGy Volatiles Propanal 1-Pentene Pentane Pentanal 1-Hexene Hexane Hexanal 1-Heptene Heptane 1-Octene Octane Toluene Methyl cyclopentane Total volatiles Odor characteristics

5 kGy

total ion counts ⫻ 0b 0b 0b 141a 0b 842b 65a 0b 370b 0b 335b 65a 374a 2192 bloody, fishy, antiseptic

SEM 104

709a

8 127a 1 974a 19 0b 8 120a 2 a 2095 212 0b 4 403a 8 5832a 125 a 308 10 3649a 145 0b 1 94b 11 14311 bloody, fishy, rancid, boiled beef, buttery

a,b Means with no common superscript differ significantly (P