Degrading fish odor

Wastewater. Phenol is a toxic poUutant to the waterways and has an acute toxicity (- 5 m g/L) to fish. Chlorination of water gives chlorophenols, which impart objectionable odor and taste at 0.01 mg/L. Biochemical degradation is most frequently used to treat wastewater containing phenol. Primary activated sludge, along with secondary biological treatment, reduces phenol content to below 0.1 mg/L (69). 

The alkadienals could be formed from the autoxidation of PUFA and may contribute desirable arenas to freshly prepared foods (19). Further degradation of alkadienals often increased undesirable flavors. Josephson and Lindsay demonstrated that 2,4-decadienal could produce 2-octenal and ethanal (20) and 2,6-nonadienal could produce 4-heptenal and ethanal (21) via retro-aldol condensation mechanisms. Hsieh et al. (22) reported that iscmers of various alkadienals and alkatrienals gave green, greasy and oxidized fish oil odors in crude menhaden fish oil. 

Eleven, thirteen and twenty alkylbenzenes were identified among the volatiles in the boiled crayfish tail meat, hepato-pancreas and the pasteurized crahmeat, respectively. The alkylbenzenes and the naphthalenes might have come from environmental pollutants. Lee et al. (30) reported rapid uptake of naphthalene in marine fish. Several chlorobenzenes identified in the crayfish and crafcmeat samples possibly were degradation products of various pesticides. Neff et al. (29) reported that aromatic hydrocarbons accumulated in fish to a greater extent and were retained longer than the alkanes. Phenol was also identified in all three samples. The medicinal odor of phenol contributed negatively to these products. KUbota et al. (28) identified xylenes and phenol as volatile components that contributed undesirable odors to cooked krill. 

Phenolic compounds are ubiquitous in the environment coming from different sources such as manufacturing processes used in the plastic, dye, drug, antioxidant, and pesticide industries. Chloro- and nitrophenols are the main degradation products of many chlorinated phenoxy acid and organophosphorus pesticides, respectively [1,2]. These compounds are of particular interest and concern to the environment because they are toxic to most aquatic organisms [3,4]. Moreover, they affect the taste and odor of both water and fish even at very low concentrations of phenolic compoimds in water [5]. The US Environmental Protection Agency (EPA) has listed 11 phenols as priority pollutants [6]. 

The typical aroma substances of egg white and egg yolk are still unknown. The fishy aroma defect that can occur in eggs is caused by trimethy-lamine TMA, which has an odor threshold that depends on the pH (25 pg/kg, pH 7.9) because only the undissociated form is odor active. TMA is formed by the microbial degradation of choline, e. g., on feeding fish meal or soy meal. Normally, TMA does not interfere because it is enzymatically oxidized to odorless TMA oxide. However, in feed, e. g., soy meal, substances exist which could inhibit this reaction. 

Secondary and tertiary amines are formed from precursors other than amino acids. Dimethylamine results from degradation of choline (which is present in some phospholipids), some alkaloids (e.g. in beer it is produced from gramine (see 10-198) present in germinating barley grains and also in non-enzymatic browning reactions from methylamine and formaldehyde or by decarboxylation of sarcosine. Trimethylamine, together with dimethylamine, methylamine and ammonia, is an odorous compound of fish and other aquatic animals. It is formed by reduction of the sensory indifferent trimethylamine oxide (trimethylaminoxide, 8-143) in tissues post mortem. 

Most marine fishes contain trimethylamine oxide (TMAO) this colorless, odorless, and flavorless compound is degraded to trimethylamine, which gives a fishy odor and causes consumer rejection. This compound is not present in land animals and freshwater species (except for Nile perch and tila-pia from Lake Victoria). TMAO reductase catalyzes the reaction and is found in several fish species (in the red muscle of scombroid fishes and in the white and red muscle of gadoids) and in certain microorganisms Enterobacteriaceae, She-tvanella putrefaciens). 

---