Fishy off-flavor

Meijboom and Stroink (76) found that 2-trans, A-cis, 7-c/i-decatrienal was the compound responsible for the fishy off-flavors occurring in autoxidized oil containing 0)-3 fatty acids. This trienal was also found in autoxidized methyl DHA by Noble and Nawar (75) and in autoxidized mackerel oil by Ke et al. (77). 

There are conflicting reports as to whether fishy off-flavors, from fish oils and flaxseed, in eggs can be eliminated by incorporating tocopherols into the feed (Sim, 1998 Surai and Sparks, 2001), although off-flavors are apparently absent if the level of fish oil (preferably stabilized) in the diet is 1.5% or less, and for flaxseed 5% or less (Surai and Sparks, 2001). Off-flavors have been reported to be absent if the feed contains micro-algae (Haumann, 1998). The cooking characteristics of co3 eggs are reported to be normal (Surai and Sparks, 2001). 

The overwhelming consideration in regard to lipid deterioration is the resulting off-flavors. Aldehydes, both saturated and unsaturated, impart characteristic off-flavors in minute concentrations. Terms such as painty, nutty, melon-like, grassy, tallowy, oily, cardboard, fishy, cucumber, and others have been used to characterize the flavors imparted by individual saturated and unsaturated aldehydes, as well as by mixtures of these compounds. Moreover, the concentration necessary to impart off-flavors is so low that oxidative deterioration need not progress substantially before the off-flavors are detectable. For example, Patton et al (1959) reported that 2,4-decadienal, which imparts a deep-fried fat or oily flavor, is detectable in aqueous solution at levels approaching 0.5 ppb. 

Comparative studies by Forss and co-workers (1960A.C) on the fishy, tallowy, and painty flavors of butterfat tended to emphasize the importance of the relative and total carbonyl contents in dairy products with different off-flavors. These researchers showed that three factors distinguished painty and tallowy butterfat from fishy butterfat. First, there was a relative increase in the n-heptanal, re-octanal, n-non-... 

Forss et al. (1960a,c) compared the qualitative and quantitative distribution of carbonyl compounds in dairy products with fishy, tallowy or painty off-flavor. Total content of volatile carbonyl compounds was approximately 10 times greater in the tallowy and 100 times greater in the painty butterfat than in fishy butterfat. Tallowy butterfat contained greater amounts of -heptanal, -octanal, -nonanal, 2-heptanone 2-heptenal and 2-nonenal, while painty butterfat contained greater amounts of K-pentanal and C5 to C10 alk-2-enals. 

Induced changes in food aroma are frequently assessed by consumers as objectionable, for example, as rancid, fishy, metallic or cardboardhke, or as an undefined old or stale flavor. On the other hand, the fact that some volatile compounds, at a level below their off-flavor threshold values, contribute to the pleasant aroma of many fruits and vegetables and to rounding-off the aroma of many fator oil-containing foods should not be neglected. 

Trimethylamine also has a fishy odor. However, its odor threshold at the pH of fish meat is very much higher than that of the potent lipid peroxidation products, e. g., (Z)-l,5-octadien-3-one (cf. 3.7.2.1.8 and 11.2.4.4.4). Therefore, it plays a role as an off-flavor substance only on stronger bacterial infection of fish at temperatures >0 °C. 

C. Masanetz, H. Guth, and W. Grosch, Fishy and hay-like off-flavors of dry spinach, Z. Lebensm. Unters. Forsch. 206 108 (1998). 

---