Fish oils flavor compounds

Additionally, key secondary oxidation products contribute distinctive aromas characteristic to certain fish species. In salmon, co-oxldatlon of polyunsaturated fatty acids of fish oils with salmon-specific carotenoid pigments leads to the formation of a characterizing cooked salmon flavor compound, and changes the ratio of carbonyl compounds formed compared to that for pure fish oil. 

Autoxldatlve reactions In fish or fish oils can also lead to green flavors (32), but these flavors are accompanied by elevated concentrations of compounds contributing general oxidized flavors as well as the 2,4,7-decatrlenals and -4-heptenal which are primarily responsible for burnt, oxldlzed-flshy or cod liver oll-llke flavors. 

Its ability to more effectively direct toward the t, , -lsomer Is seen by an elevated Isomer ratio of 3.08 In the presence of Trolox C (Table II). As noted earlier, low concentrations of the , , -lsomer of 2,4,7-decatrlenal exhibit a more pronounced flshy/burnt flavor quality In oxidizing fish oils while the , , -lsomer causes more green-fishy, burnt character notes (28). Although substantial concentrations of the Cg unsaturated compounds were produced In the fish oils containing Trolox C or alpha-tocopherol (Table I), the production of 2,4,7-decatrlenals provided the underlying unpleasant, characterizing fish flavor notes. 

We originally believed that the short chain saturated fatty acids In oxidizing fish lipids contributed to burnt/flshy flavors. Saturated fatty acid concentrations (C4 - Cg) measured by volatile headspace analysis (32, 59) reached levels as high as 3 ppm In highly oxidized fish oils (32). Flavor threshholds for these short n-chaln fatty acids In oil systems In the literature (>.66 ppm, 63) Indicate that they could contribute notes to oxidizing fish oils. However, studies designed to document the role of short chain acids as flavor compounds detracting from the flavor quality of fish oils did not confirm earlier beliefs. 

The high oxidation rates of EPA and DHA and the instability of their hydroperoxides caused the rapid formation of secondary products such as volatile aldehydes and other compounds, which, in turn, impart flavor reversion in fish oils (56). The hydroperoxides produced from autoxidation of EPA (73) and DHA (74) have been identified but not quantified. They form eight and ten isomers, respectively. Noble and Nawar (75) analyzed the volatile compounds in autoxidized DHA and identified a number of aldehydes. Most of the aldehydes identified could be explained by the p-scission of alkoxy radicals generated by the homolytic cleavage of each isomer of the hydroperoxides as shown in Figure 9. 

The oxidation products of lipids include volatile aldehydes and acids. Therefore, lipids are one of the major sources of flavors in foods. For example, much of the desirable flavors of vegetables such as tomatoes, cucumbers, mushrooms, and peas (Ho and Chen, 1994) fresh fish (Hsieh and Kinsella, 1989), fish oil (Hu and Pan, 2000) and cooked shrimp (Kuo and Pan, 1991 Kuo et al., 1994), as well as many deep-fat fried foods such as French-fried potatoes (Salinas et al., 1994) and fried chicken (Shi and Ho, 1994), are contributed by lipid oxidation. LOX-catalyzed lipid oxidation produces secondary derivatives, e.g., tetradecatrienone, which is a key compound of shrimp (Kuo and Pan, 1991). The major difference between the flavors of chicken broth and beef broth is the abundance of 2,4-decadienal and y-dodeca-lactone in chicken broth (Shi and Ho, 1994). Both compounds are well-known lipid oxidation products. A total of 193 compounds has been reported in the flavor of chicken. Forty-one of them are lipid-derived aldehydes. 

Flavors and aromas commonly associated with seafoods have been intensively investigated in the past forty years ( l-7), but the chemical basis of these flavors has proven elusive and difficult to establish. Oxidized fish oils can be described as painty, rancid or cod-liver-oil like (j ), and certain volatile carbonyls arising from the autoxidation of polyunsaturated fatty acids have emerged as the principal contributors to this type of fish-like aroma ( 3, 5, 9-10). Since oxidized butterfat (9, 11-12) and oxidized soybean and linseed oils (13) also can develop similar painty, fish-like aromas, confusion has arisen over the compounds and processes that lead to fish-like aromas. Some have believed that the aromas of fish simply result from the random autoxidation of the polyunsaturated fatty acids of fish lipids (14-17). This view has often been retained because no single compound appears to exhibit an unmistakable fish aroma. Still, evidence has been developed which indicates that a relatively complex mixture of autoxidatively-derived volatiles, including the 2,4-heptadienals, the 2,4-decadienals, and the 2,4,7-decatrienals together elicit unmistakable, oxidized fish-oil aromas (3, 9, 18). Additionally, reports also suggest that contributions from (Z -4-heptenal may add characteristic notes to the cold-store flavor of certain fish, especially cod (4-5). 

Karahadian, C. and Lindsay, R.C. Evaluation of compounds contributing characterizing fishy flavors in fish oils. J. Am. Oil Chem. Soc. 66, 953-960 (1989). 

A large number of volatile compounds have been identified in beef fat (Table 11.17). The flavor significance of individual volatile compounds can be calculated on the basis of their threshold values. Thus, two of the carbonyl compounds formed in the smallest concentrations, c/5-4-heptenal and rran, tran5 -2,6-nonadienal, were the most flavor significant. More sensitive GC-MS analyses reported odor threshold values of 1260 ppb for cw-4-heptenal and 1231 for tra 5, d5 -2,6-nonadienal found among fish oil volatiles (Section E.3 and Chapter 5). On the other hand, the most abundant carbonyl, 2-decenal, was the least flavor significant. 

Oxidized fish oils, rich in n-3 polyunsaturated fatty acids, produced volatile compounds more readily than oxidized vegetable oils, rich in linoleic acid. Activation energy for the formation of propanal from fish oils was lower than for the formation of hexanal from vegetable oils. A mixture of aldehydes contributed to the characteristic odors and flavors of oxidized fish, described as rancid, painty, fishy and cod liver oil-like (Table 11.21). Oxidation of unsaturated fatty acids in fish was related to the formation of 2-pentenal, 2-hexenal, 4-heptenal, 2,4-heptadienal and 2,4,7-decatrienal. The fishy or trainy characteristic of fish oil was attributed to 2,4,7-decatrienal. Studies of volatiles from boiled trout after storage showed significant increases in potent volatiles by aroma extraction dilution analysis (Table 11.22). Volatiles with the highest odor impact included l,5-octadien-3-one, 2,6-nonadienal, 3-hexenal, and 3,6-nonadienal. 3,6-Nonadienal and 3-hexenal were considered to contribute most to the fatty, fishy flavor in stored boiled fish. 

C. Karahadian and R. C. Lindsay, Evaluation of compounds characterizing fishy flavors in fish oils, J. Am. Oil Chem. Soc. 66 953 (1989). A. Kobayashi, K. Kubota, M. Iwamoto, and H. Tamura, Syntheses and sensory characterization of 5,8,11-tetradeca-trien-2-one isomers, J. Agric. Food Chem. 37 151 (1989)... 

However, when the carotenoids and acylglycerols were recombined, the salmon aroma developed. Combinations of alternate sources of fish acylglycerols along with crayfish carotenoids revealed that the necessary component for salmon flavor development was the presence of carotenoids specifically derived from salmon oil (Table III). Such results strongly suggest that the compound Is derived by co-oxldatlon of fish acylglycerols with salmon carotenoids, and that the precursor Is located In the carotenoid fraction. 

The formation of polymers leads to an increase in viscosity. The various lipids that can leach into the frying oil change the properties and the performance of the frying oil. Colored lipids solubilized in the oil contribute to the darkening. Phospholipids are emulsifiers. Traces of liposoluble metal compounds may act as prooxidants. Liposoluble vitamins and phenolic compounds are antioxidants. Volatile compounds (e.g., from fish or onions) contribute to off-flavors. 

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