360 FOOD SCIENCE APPLICATIONS 



nism differs markedly from that of other lipids involving hydroperoxide 

 autoxidation^-. In fact, it was an observation by Farmer "^ on autoxidizing 

 fish oil fatty acids which led to the development of the modern theories 

 on nonconjugated polyunsaturated autoxidation. Such theories will not 

 be discussed here, but some minor deviations in the behavior of fish oils 

 from that of other lipids during oxidation will be noted. The course of 

 oxidation is often quite different in extracted fish oils than when the oils 

 remain in the fish tissue so that these cases will be discussed separately. 



Oxidation of Extracted Fish Oils. Extracted fish oils oxidize at a con- 

 siderably greater rate than do most vegetable oils or animal fats, and 

 this is scarcely surprising in view of the much greater content of highly 

 polyunsaturated (five and six double bonded) fatty acids. In most cases 

 the induction curve for the oxidation of fish oils is considerably flattened 

 with not such a pronounced break in the curve at the end of the induction 

 period. This is particularly the case with crude, unrefined fish oils. Anti- 

 oxidants which give high protective factors with many other lipids some- 

 times give much lower protective factors for fish oils. There seems to be 

 in many cases a lack of consistency between the effect of antioxidants on 

 fish oils as reported by different workers or even as found by the same 

 worker at different times. Some of these discrepancies can be explained 

 on the basis of the effect of such naturally occurring impurities in crude 

 fish oils as free fatty acids and amines which, as reported by Olcott, 

 et aZ.^'^'^°, exert respectively a negative antagonistic and a positive syner- 

 gistic action on the effectiveness of certain antioxidants on the oxidation 

 of fish oils. The situation is quite complex such that a given substance 

 may act as a synergist with one antioxidant and as an antagonist with 

 another. 



Oxidation of Oil in Fish Tissue. Oil in fresh fish tissue held at refriger- 

 ated temperatures above freezing seldom exhibits any marked tendency 

 toward oxidation or development of rancidity. In almost every instance 

 the fish has become inedible from bacterial spoilage before any indication 

 of rancidity appears. This surprisingly great stability of fish oils in the 

 tissue of fresh fish would appear to be caused by something more than 

 the mere overshadowing of the onset of rancidity by the much more rapid 

 development of bacterial decomposition. It has been suggested^- ^^ that 

 this behavior, which has also been noted in the case of bacon stored just 

 above and just below the freezing point, may be due to bacterial action 

 in the chilled product limiting the oxygen tension and hence greatly re- 

 tarding the rate of potential oxidation of the oil. 



When fish is stored at above-freezing temperatures and the bacterial 

 decomposition is reduced or eliminated by such techniques as dehydra- 



