Volatile compounds freshly harvested fish

The definition of oxidized fish oil-like aromas still leave fresh fish aromas undefined. Various freshly harvested fish have distinguishing aromas, but they also are characterized by a common plant-like, seaweed-like aroma. Thus, compounds and reaction pathways different from random autoxidation appear likely and reasonable. Even conflicting descriptions of fishy odors, i.e., including roles for volatile amines (2 19) and sulfur compounds (20-22), can be accommodated by the hypothesis that previously unrecognized biochemical reactions yield characterizing fresh fish aromas. These premises led to investigations (23-26) which have resulted in the identification of a group of enzymically-derived volatile aroma compounds that contribute fresh, plant-like aromas to freshly harvested fish (Table I). 

In addition to the six-carbon volatile compounds, l-penten-3-ol is also found in all freshwater fish. However, concentrations of l-penten-3-ol in fish remain below its recognition threshold (400 ppb 38), and therefore it is unlikely that this volatile contributes strongly to the characteristic aroma of freshly harvested fish. 

In addition to the enzymically-derived volatile aroma compounds (Table I), low levels of autoxidatively-derived carbonyls can also be detected in harvested fish held a day on ice, and these volatiles are listed in Table II. The oxidatively-derived carbonyls modify the fresh plant-like aromas of fresh fish by providing oxidized-oil-like, staling fish-type odor notes (3-5, 9). The formation of hexanal in freshly harvested fish appears to be enzymic because the concentration of this compound can be diminished by lipoxygenase inhibitors (25). However, when fresh fish are stored on ice or are held under frozen storage, hexanal concentrations also increase because of autoxidative processes (40). 

Recently, German and Kinsella (68-69) have demonstrated activity of a 12-lipoxygenase in the gill and skin tissues of trout (Salmo sp.), and these findings identify an appropriate precursor to some of the fresh fish aroma volatiles. This allows association of an appropriate precursor with the occurrence of certain volatiles, and provides very strong support for the view that the volatile aroma compounds characterizing the fresh, plant-like aromas of freshly harvested fish result from lipoxygenase-mediated bioconversions of polyunsaturated fatty acids. 

This work clearly shows that after harvest, polyunsaturated fatty acids are enzymatically converted to physiologically active compounds which are subsequently converted to volatile alcohol and carbonyl compounds that are characteristic of the odor of freshly harvested fish. 

Six-carbon volatile alcohols and aldehydes have been found in all freshwater fish surveyed (23-24). However, these compounds have not been found in either salmon residing in saltwater (unpublished data) or in oysters (26). Hexanal has been found in modestly fresh (5-6 days old) saltwater fish (24), but its formation may be the result of autoxldation rather than via enzyme-mediated reactions. Thus, data for the occurrence of hexanal in freshly harvested saltwater fish remains to be developed. Hexanal and (E)-2-hexenal contribute coarse, green-plant-like, aldehydic aroma notes to freshly harvested finfish where their aroma dominates the overall odors within seconds after the death of the fish. (Z)-3-Hexen-l-ol contributes a clean, green-grass-like aroma note. Hexanal always occurs in substantially greater abundance than 1-hexanol in fish. 

The aromas associated widi very fi esh fish are usually mild, delicate and fi esh (53,54), and generally described as green (hexanal), melon-like ((E,Z)-3,6-nonadienal), iodine-like (bromophenols). Fresh fish and seafood aromas are due to volatile carbonyls and derive fi om lipoxygenase catalyzed oxidation of polyunsaturated fatty acids. The oxidation of Eicosapentaenoic acid (C20 5) leads to C5 to C9 alcohols, aldehydes, ketones and hydrocarbons. The formation of methyl mercaptan, dimethyl sulfide and dimethyl disulfide in fi esh fish at the time of harvest has been reported by Shiomi et al. (55). Although these compounds are usually associated with fish deterioration, they contribute to the fi esh aroma ch cter at low concentrations. For instance, dimethyl sulfide is... 

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