Contribution of Lipid-Derived Components to the Flavor of Alligator

Chapter 13

Contribution of Lipid-Derived Components to the Flavor of Alligator Meat Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 30, 2018 | https://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch013

Κ. R. Cadwallader1, H. H. Baek1, Η. Y. Chung1, and M . W. Moody2 1Department of Food Science, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803 2Louisiana Cooperative Extension Service, P.O. Box 25100, Baton Rouge, LA 70894-5100 Volatile compounds were isolated from tail meat of both wild (ca. 4-, 7-, and 10-ft in length) and farm-raised (ca. 4-ft in length) alligators by vacuum simultaneous distillation-solvent extraction (SDE) and dynamic headspace sampling (DHS). Analysis of SDE and DHS extracts by aroma extract dilution analysis (AEDA) revealed that six lipid-derived compounds were predominant among the odor-active compounds isolated. A seventh unknown compound having a pungent, metallic aroma was also predominant. Hexanal and (Z)-4heptenal were found to contribute undesirable green, cut-grass and fishy, rancid aromas, respectively, and are thought to be primarily responsible for the off-flavor in alligator meat. Other predominant odor-active compounds included l-octen-3-one, (E)-2-nonenal, (E)-2octenal, and (E,Z)-2,6-nonadienal. Alligator production has become an important industry in Louisiana and Florida. Traditionally, hides were considered the only marketable product, but in recent years, the sale of meat has accounted for over 25% of the total revenue generated. The value of farm-raised alligator meat in Louisiana was over $2.2 million in 1992. An additional $1.0 million was generated from the sale of wild alligator meat during the September harvest. Alligator tail meat is high in protein (29.1%), low in fat (2.9%), and low in cholesterol (64.8 mg/100 g) (i). Wild alligator meat is considered inferior to farm-raised due to a fishy or rancid off-flavor (2), which may be particularly strong in meat from older or larger animals Qualitative and quantitative information on the volatile composition of fat from both wild and farm-raised alligators has been reported previously (5). It was believed that certain volatile lipid-derived compounds, e.g. hexanal, play important roles in the off-flavor associated with alligator fat due to their predominance and low aroma thresholds. However, the relative impact of volatile lipid-derived compounds on alligator meat flavor, especially in comparison with other odor-active compounds present, is unknown. 0097-6156/94/0558-0186$08.00/0 © 1994 American Chemical Society Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 30, 2018 | https://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch013

13.

CADWALLADER ET AL.

Lipid-Derived Components & Alligator Meat

Aroma extract dilution analysis (AEDA) has been successfully employed for the evaluation of odorants in cucumbers (4) and dill seed and herb (5) as well as for the determination of relative intensities of volatile flavor compounds during the autoxidation of linoleic acid (6). AEDA involves the analysis of a serially diluted flavor extract by gas chromatography/olfactometry (GC/O) to obtain a flavor dilution factor (FD-factor) for each odor-active substance present in the original extract. An FD-factor for a particular component can be defined as the highest dilution at which it is detected by GC/O. Results from AEDA help provide a better understanding of the role each compound plays in the overall flavor. The objective of the present study was to determine the importance of volatile lipid-derived components to the flavor of alligator tail meat by identifying predominant odor-active compounds by AEDA. To further validate AEDA results, two different extraction techniques were employed for isolation of volatile compounds. Materials & Methods Sample Preparation. Tail meat from wild alligators (ca. 4-, 7-, and 10-ft in length) was obtained from a processor in St. Martinville, LA. Tail meat from farm-raised alligators (ca. 4-ft in length) was obtained from a processor in Scott, LA. Three animals were sampled for each treatment. All meat was obtained immediately after processing. The external fat layer was removed with just lean meat used for this study. Meat was vacuum packaged in FreshPak 500TM plastic bags (Koch Supplies, Inc., Kansas City, MO) and stored at -20°C until analyzed. Preparation of meat prior to extraction of volatile compounds was done as follows: meat was thawed at 4°C, sliced into small pieces (ca. 1 cm ), and cooked in a convection oven at 350°F (176.7°C) for 30 min in a covered baking dish. 3

Simultaneous Steam Distillation-Solvent Extraction (SDE). SDE was carried out under vacuum (ca. 24-26 in.Hg; b.p. 45-60°C) to minimize artifact formation during extraction. It was necessary to modify the standard SDE apparatus (Cat. No. K5230101-0000, Kontes, Vineland, NJ) as follows: 1) standard valve at U-joint was replaced with vacuum valve; 2) liquid nitrogen cold trap was installed between the SDE apparatus and vacuum pump to minimize solvent entrainment into pump; 3) vacuum valve was installed between SDE apparatus and vacuum pump to prevent back-flush of vacuum oil or other contaminants during venting of sample flask; and 4) sample flask consisted of a 4-L, 3-neck round bottom flask; one neck for vacuum valve and one for thermometer. Cooked alligator meat (300 g) plus 1 L distilled water and 80 /*g of 2,4,6-trimethylpyridine (TMP) was extracted for 2 h with 75 mL of redistilled dichloromethane. Air was evacuated for 30 min prior to heating of sample flask. SDE extracts were kept at -20°C overnight to facilitate water removal. Volume of SDE extract was reduced to ca. 10 mL under a gentle stream of nitrogen, dried over 3 g anhydrous sodium sulfate, and then further reduced to 100 μL prior to analysis.

Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Dynamic Headspace Sampling (DHS). Cooked alligator meat (300 g) plus 1 mg TMP was placed into a purge and trap vessel (1 L , Cat No. 991780, Wheaton, Milleville, NJ). Vessel was oriented in the DHS mode. DHS conditions were as follows: sample temperature was maintained at 60°C; helium was swept over sample at 30 mL/min; purge time was 2 h; and volatiles were trapped onto a Tenax-TA trap (600 mg). Volatiles were eluted from trap with 10 mL redistilled diethyl ether. Ether extract was dried and concentrated to 100 \£L prior to analysis as described above. Gas Chromatography/Mass Spectrometry/Olfactometry (GC/MS/O). In order to achieve more timely and accurate correlations between spectral data and olfactory or sniffing port responses we have equipped our GC systems with sniffing ports, allowing simultaneous spectral scanning and sniffing of GC effluents. GC/MS system consisted of an HP 5790A GC equipped with a flame ionization detector (FID) and connected to an HP 5970B mass selective detector (MSD) (HewlettPackard Co., Palo Alto, CA). Five ptL of each extract was injected (splitless mode; 155 °C injector temperature; 30 sec valve delay) simultaneously into dual (closely matched) fused silica open tubular (FSOT) columns (Supelcowax 10, 60 m length χ 0.25 mm i.d. χ 0.25 /xm film thickness; Supelco, Inc., Bellefonte, PA). Injector effluent, after passing through a FSOT precolumn (1 m length χ 0.25 mm i.d.), was split 1:1 to each column using a glass Y-splitter. Column 1 was connected to the MSD, while the end of the column 2 was split 1:1 to an FID and sniffing port supplied with humidified air. FID and sniffing port transfer line temperatures were 250 and 200°C, respectively. Helium was used as carrier gas at a linear velocity of 25 cm/s. It was necessary to partially restrict the flow between column 1 and MSD (using ca. 1 m length χ 0.1 mm i.d. FSOT column) in order to achieve the same linear velocity for both columns. Oven temperature was programmed from 40 to 175°C at a rate of 2°C/min with initial and final hold time of 5 and 30 min, respectively; temperature was then further increased at 6°C/min to 200°C and held for 30 min. MSD conditions were as follows: capillary direct MS interface temperature, 200°C; ion source temperature, 200°C; ionization voltage, 70 eV; mass range, 33-300 a.m.u.; and electron multiplier voltage, 2200 V . Gas Chromatography/Fourier Transform Infrared Spectrometry/Olfactometry (GC/FTIR/O). GC/FTIR system consisted of an HP 5890 GC/HP 5965B infrared detector (IRD) (Hewlett-Packard Co.). Effluent from the end of GC column was split 1:1 between IRD and sniffing port. GC conditions were the same as above except that a 0.32 mm i.d. Supelcowax 10 column was used. IRD conditions were as follows: light pipe and transfer lines temperature, 200°C; narrow band mercury cadmium telluride (MCT) IR detector; optical resolution, 8 cm ; and coadd factor, 2 scans/spectrum. 1

Compound Identification and Quantification. Compound identifications were based on comparison of GC retention indices (RI)(7), mass and infrared spectra, and odor properties of unknowns with those of authentic standard compounds analyzed under identical experimental conditions. Tentative identifications were based either


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CADWALLADER ET AL.



on matching mass spectra of unknowns with those in the Wiley/NBS mass spectral database (Hewlett-Packard, Co., 1988) or on matching RI values and odor properties of unknowns with those of authentic standards. Concentrations of positively identified compounds were determined using MS response factors for each compound relative to the internal standard. Response factors were determined by analyzing standard compounds at three levels . Aroma Extract Dilution Analysis (AEDA). GC/O system consisted of an HP 5790A GC (Hewlett-Packard Co., Palo Alto, CA) equipped with a flame ionization detector (FID) and a sniffing port. Effluent from the end of the GC column was split 1:1 between FID and sniffing port. Serial dilutions of SDE and DHS extracts were prepared using dichloromethane as diluent. GC conditions were the same as for the GC/IRD system except that oven temperature was programmed from 40°C to 200°C at a rate of 67min with initial and final hold times of 5 and 20 min, respectively. GC/O was performed by three panelists familiar with alligator meat flavor. Panelists were asked to assign aroma properties to each compound detected in the sample dilutions. The highest dilution at which an individual component was detected was defined as its FD-factor. FD-chromatograms presented herein are those of only one panelist, since all responses were essentially the same. Results & Discussion Predominant odor-active compounds in alligator tail meat having FD-values higher than 8 for DHS extracts and 128 for SDE extracts are listed in Table I. A combined total of 17 odor-active compounds were detected in DHS extracts. FDchromatograms of volatiles isolated by DHS were similar for all alligators (Figure 1). Among these, compounds 5, 10, and 18 had the highest FD-factors. Compounds 5 and 10 were identified as hexanal and (Z)-4-heptenal, respectively. Compound 18 was unidentified and possessed a pungent, metallic aroma. Other compounds having relatively high FD-factors included l-octen-3-one (No. 13), (E)-2octenal (No.21), (E)-2-nonenal (No.28), (E,Z)-2,6-nonadienal (No.29), and an unknown compound (No. 3) having a sour, rotten onion aroma. Identification and description of other detected odor-active compounds are presented in Table I. All compounds detected in DHS extracts also were detected in SDE extracts with the exception of the internal standard (2,4,6-trimethylpyridine)-detected only in DHS extracts. FD-chromatograms of volatiles isolated by SDE show that 37 odor-active compounds were detected (Figure 2). FD-chromatograms for farmraised and 10-ft wild alligators had a greater number of odor-active compounds compared with 4- and 7-ft wild alligators. Compounds exhibiting highest FD-factors in SDE extracts included hexanal (No.5), (Z)-4-heptenal (No. 10), l-octen-3-one (No. 13), (E)-2-octenal (No. 21), (E)-2-nonenal (No. 28), (E,Z)-2,6-nonadienal (No. 29), and three unknown compounds having pungent, metallic (No. 18), nutty, baked potato (No.25) and stale, nutty (No.26) aromas. The main difference between FDchromatograms of DHS and SDE extracts was the appearance of odor-active compounds with RI values over 1600 in SDE extracts . Most of these compounds


189

190


Table I. Odor-active compounds in alligator tail meat


No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 a

b

c

d

a

b

Compound

Methods of Identification

RI

Pentanal 2,3-Butanedione unknown 2,3-Pentanedione Hexanal unknown unknown unknown Heptanal (Z)-4-Heptenal unknown Octanal/Cyclohexanone l-Octen-3-one unknown unknown unknown TMP (LSJ* unknown Nonanal unknown (E)-2-Octenal unknown unknown l-Octen-3-ol unknown unknown Benzaldehyde (E)-2-Nonenal (Ε, Z)-2,6-Nonadienal unknown Dimethybenzaldehyde unknown unknown 2,4-Decadienal unknown unknown (dienal?) unknown unknown (dienal?)

IR,MS,RI,odor RI,odor

976 979 1000 1053 1078 1082 1095 1160 1177 1233 1272 1280 1392 1319 1328 1347 1357 1374 1388 1400 1423 1430 1435 1438 1445 1499 1516 1528 1583 1639 1702 1713 1745 1758 1767 1823 1914 2024

IR,MS,RI,odor IR,MS,RI,odor

IR,MS,RI,odor MS,RI,odor IR,MS,RI,odor MS,RI,odor

IR,MS,RI,odor IR,MS,RI,odor

IR,MS,RI,odor

IR,MS,RI,odor IR,MS,RI,odor IR,MS,RI,odor IR,MS,RI,odor

IR,MS IR IR

0

Odor Description

pungent, green creamy, buttery sour, rotten onion creamy, buttery green, cut-grass skunky sour, rotten onion nutty, chocolate green, sweet fishy, rancid musty, mushroom sweet, wine-like metallic, mushroom nutty nutty, popcorn sweet, floral pungent, catty pungent, metallic sweet, floral mushroom peanut skin sweet, floral nutty, potato chip mushroom nutty, baked potato stale, nutty sweet, honeysuckle stale, sour cucumber-like sweet, burnt sugar spicy, floral sweet, melon-like sweet, fatty sweet, fatty nutty, popcorn sweet, fatty sweet, burnt sugar sour, cardboard

Numbers correspond to those in figures 1 and 2 Retention Index on Supelcowax 10 OTGC column Aroma description as perceived at GC-sniffer port I.S. = internal standard


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CADWALLADER ET A L

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Figure 1. Flavor dilution chromatograms of volatiles isolated from alligator tail meat by DHS. Peak numbers correspond to those in Table I. FA: farm-raised alligator; 4A: 4-ft wild alligator; 7A: 7-ft wild alligator; 10A: 10-ft wild alligator.


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ure 2. Flavor Dilution Chromatograms of volatiles isolated from alligator tail meat by SDE. Peak numbers correspond to those in Table I. FA: farmraised alligator; 4A: 4-ft wild alligator; 7A: 7-ft wild alligator; 10A: 10ft wild alligator.



13.

CADWALLADER ET AL.


had sweet, fatty and sweet, tallowy aromas reminiscent of cooked alligator meat. The additive contribution of these compounds to alligator seems important despite their lower FD-factors. Based on the results of AEDA of both DHS and SDE extracts, it is apparent that hexanal, (Z)-4-heptenal, (E)-2-octenal, (E)-2-nonenal, (E,Z)-2,6-nonadienal, 1-octen3- one, and one unidentified compound having a pungent, metallic aroma (No. 18), are the predominant odor-active compounds in alligator tail meat flavor. Hexanal had an undesirable green, cut-grass aroma, whereas the aroma of (Z)-4-heptenal was perceived as fishy and rancid. Both compounds, as well as (E,Z)-2,6-nonadienal, (E)-2-octenal, (E)-2-nonenal, and l-octen-3-one, are lipid-derived compounds (6,813). Hexanal has been shown to be an important element in the aroma of fresh whitefish (11), but its contribution to the aroma of alligator meat flavor is thought to be undesirable since consumers prefer alligator to have chicken or pork-like flavor (2). Accumulation of (Z)-4-heptenal in certain foods, e.g., cold stored cod and oxidized soybean oil, has been reported as undesirable (14,15); however, (Z)-4heptenal imparted a desirable earthy/potato-like aroma in boiled potatoes (16). (Z)4-Heptenal is believed to negatively impact alligator meat flavor. (E,Z)-2,6Nonadienal was regarded as having a desirable cucumber-like aroma; however, this compound is readily converted to (Z)-4-heptenal through a water mediated retroaldol condensation process (8). This conversion may have occurred during cooking of the samples. (E,Z)-2,6-Nonadienal may have been derived from omega-3 polyunsaturated fatty acids (PUFA) (10). Eicosapentaenoic and docosahexaenoic acids comprise about 3.0 and 6.0%, respectively, of the total fatty acids in alligator tail meat (1). Hexanal is readily formed during the oxidation of PUFA. The omega-3 PUFA are most likely the principal source of hexanal due to their higher susceptibility to autoxidation. l-Octen-3-one and (E)-2-nonenal have been reported to contribute heavy, plant-like and strong, cucumber-like notes, respectively, to the aroma of fresh whitefish (11). (E)-2-Octenal, peanut skin-like aroma, was probably derived from 2,4-decadienal by a water-mediated alpha/beta double bond hydration, retro-aldol condensation (13). The above three compounds (No. 13, 21, and 28) are believed to have a positive impact on the aroma of alligator meat. Based on these results it is apparent that the combined aromas of hexanal and (Z)-4-heptenal mask the desirable aromas of l-octen-3-one, (E)-2-nonenal, and (E)-2-octenal. This is believed to be especially true for (Z)-4-heptenal because of its strong and distinctive fishy, rancid aroma. The relationship between concentration and odor threshold values obtained from the literature for predominant lipid-derived compounds in alligator tail meat is presented in Table II. All compounds were present in concentrations exceeding their threshold values. Odor units for individual compounds correlated well with their FD-factors with the exception of (Z)-4-heptenal, which had an odor unit range lower than expected. This also was true for the comparison of hexanal versus (Z)-4heptenal in DHS extracts (Compounds 13,28, and 29 were below GC/MS detection limits in DHS extracts.). However, the odor of (Z)-4-heptenal was very intense in all extracts. It is possible that the odor threshold reported for this compound is too high, since its flavor (or taste) threshold (0.04 jug/L, 13) is much lower.


193


194

Through use of AEDA it was possible to identify important character-impact volatile flavor compounds in alligator tail meat. Lipid-derived compounds were found to play important roles in alligatorflavorand off-flavor. Results of this study may be helpful in designing production schemes and antioxidant systems leading to the improvement of alligator flavor.


Table Π. Concentrations and odor threshold values for predominant lipid derived odor-active volatile compounds in alligator tail meat

No.' 5 10 13 21 28 29

Compound

Cone Range* Odor Threshold G*g/L)

Hexanal 1506-3815 30-67 (Z)-4-Heptenal l-Octen-3-one 20-40 (E)-2-Octenal 87-211 139-422 (E)-2-Nonenal 35-54 (E,Z)-2,6-Nonadienal

d

5 0.8 0.09' 3 1* 0.1* e

4

Odor Unit Range 0

301-763 38-84 220-440 29-70 139-422 350-540

a

Numbers correspond to those in Table I and in figures 1 and 2 Concentration range for all alligator groups based on SDE data Odor unit = concentration of compound divided by odor threshold in water Threshold (/xL/L) in water (17) Threshold in water (14) Threshold in water (18) * Threshold in water (19)

b

0 d e f

Acknowledgments Approved for publication by the Louisiana Agricultural Experiment Station as manuscript No. 93-21-7193 . Support for this study was provided by the Louisiana Agricultural Experiment Station. Literature Cited 1. Leak, F.W.; Lamkey, J.W.; Johnson, D.D.; Balaban, M.O. A Further Analysis of Florida Alligator Meat as a Wholesome Food Product; Aquaculture Report Series. Florida Department of Agriculture and Consumer Services: Tallahassee, FL, 1988. 2. Tippy, J. Alligator farmers hope to tempt the doubting tastebud. The Daily Reveille, Baton Rouge, LA, Feb. 13, 1990. pp 4,13. 3. Baek, H.H.; Pike, M . M . ; Bidner, T.D.; Moody, M.W.; Cadwallader, K.R. Evaluation of the off-odor associated with wild alligator fat. Abstract No. 359. Presented at the Annual Meeting of the Institute of Food Technologists. New Orleans, LA. 1992.


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4. 5. 6. 7. 8. 9.

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Schieberle, P.; Ofner, S.; Grosch, W. J. Food Sci. 1990, 55, 193. Blank, I.; Grosch, W. J. Food Sci. 1991, 56, 63. Ullrich, F.; Grosch, W. Z. Lebensm. Unters. Forsch. 1987, 184, 277 van den Dool, H . ; Kratz, P.D. 1963. J. Chromatogr. 1963,11,463. Josephson, D.B.; Lindsay, R.C. J. Am. Oil Chem. Soc. 1987, 64, 132. Lamikanra, V.T.; Dupuy, H . J. Food Sci. 1990, 55, 861.

10. Josephson, D.B.; Lindsay, R.C.; Stuiber, D.A. J. Agric. Food Chem. 1984, 32, 1347. 11. Josephson, D.B.; Lindsay, R.C.; Stuiber, D.A. J. Agric. Food Chem. 1983, 31, 326. 12. Buttery, R.G.; Seifer, R.M.; Ling, L.C. J. Agric. Food Chem. 1970, 18, 538. 13. Josephson, D.B.; Lindsay, R.C. J. Food Sci. 1987, 52, 1186. 14. McGill, A.S.; Hardy, R.; Burt, J.R.; Gunstone, F.D. J. Sci. Food Agric. 1974, 25, 1477. 15. Seals, R.G.; Hammond, E.G. J. Am. Oil Chem. Soc. 1970, 47, 278. 16. Josephson, D.B.; Lindsay, R.C. J. Food Sci. 1987, 52, 328. 17. Buttery, R.G.; Turnbaugh, J.G.; Ling, L.C. J. Agric. Food Chem. 1988, 36, 1006. 18. Swoboda, P.A.T.; Peers, K.E.J. J. Sci. Food Agric. 1977, 28, 1019. 19. Forss, D.A.; Dunstone, E.A.; Ramshaw, E.H.; Stark, W. J. FoodSci.1962, 27, 90. RECEIVED November 4, 1993


Contribution of Lipid-Derived Components to the Flavor of Alligator

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