presumed to be: ATP^ADP^AMP^Inosine 

 Monophosphate-^Inosine-^Hypoxanthine. The 

 intermediate compound, inosine monophosphate 

 (IMP), and the end product, hypoxanthine, 

 have been credited under some circumstances 

 with affecting the flavor of fish. IMP has a 

 sweet, flavor-enhancing effect, and hypoxan- 

 thine is said to contribute a bitter flavor. If 

 this applies in the case of tuna under commer- 

 cial storage conditions, it could lead to a de- 

 terioration from a pleasant to an unpleasant 

 flavor as the fish ages. Investigations so far 

 do not indicate that flavor changes in canned 

 tuna are closely associated with the changes in 

 purine compounds (Crawford and Finch, 

 1968). This may be because in the canning 

 of tuna, other flavors, notably those relating 

 to the degradation of the oils, and/or other 

 oxidation degradations may dominate. 



A more useful aspect of these changes is to 

 provide measures of the storage history of the 

 raw fish. Fortunately, the rates of degrada- 

 tion of purine compounds in tuna appear to be 

 a good deal slower than in most other fish. 

 Thus, in albacore held at 30° F, a progressive 

 formation of hypoxanthine was recorded, and 

 after 35 days a quarter of the purine compounds 

 had completely degraded to form hypoxanthine 

 (Crawford and Finch, 1968) . This means that 

 it should be possible to use the development 

 of changes in hypoxanthine as an index of stor- 

 age over a prolonged period of time. This is 

 unlike many other fish in which the changes 

 are essentially complete in a few days after 

 which measurements of hypoxanthine cannot 

 be used as a measure of continued storage. In 

 the case of tuna, where storage temperature 

 measurement in the well is very difficult and 

 time consuming, measurement of hypoxanthine, 

 which is quite simple, could provide an easier 

 method to assess the time-temperature storage 

 history and also the parallel changes in con- 

 ditions before canning (Crawford, 1970) . The 

 natural fish-to-fish and other variations are 

 likely to preclude this method from giving a 

 precise indication of storage; however, it may 

 well serve to classify tuna into broad categories 

 of storage history such as excellent, good, fair, 

 poor. This is especially interesting since the 

 method lends itself to automation whereby 

 large numbers of samples may be examined by 

 one person. 



Other factors. — We have looked at several 

 other chemical or physical factors to see if they 

 appear to relate to the quality of the canned 

 product. It seems likely that the oil content 

 of the fish muscle is related to texture, low oil 

 content being associated with a tough dry tex- 

 ture and a high oil content with a soft tender 

 texture. This was found to be true of raw fish 

 muscle and also to some extent of the drained 

 canned tuna in texture studies carried out by 

 the University of Maryland. 



We have attempted to apply the Torry cell 

 fragility test to measure protein changes on 

 storage. After a considerable number of ex- 

 periments, we were able to modify the tech- 

 nique to give consistent results, but we have 

 insufficient results so far to indicate whether 

 this is likely to give results which correlate 

 usefully with the storage history of the tuna. 

 Other measurements we have made, such as 

 the free fatty acid content of the extracted 

 lipids, the nonprotein nitrogen, or the percent- 

 age of denatured protein offer little promise as 

 indices of storage change. 



The Effects of Premortem Stress and 



Postmortem Biochemical Changes on 



the Quality of Canned Skipjack Tuna 



Analyses of the aforementioned experiments 

 and a perusal of the literature indicated that 

 a knowledge of the sugars and nucleotides in 

 tuna may very well elucidate some of the mech- 

 anisms evolved in causing quality changes in 

 tuna (especially with reference to color changes 

 and the presence of scorch). Tarr (1969) re- 

 ported that fish acclimated to warmer climates 

 make use of the Embden-Meyerhof pathway 

 while those acclimated to colder temperatures 

 prefer the hexosemonophosphate shunt. Tarr 

 also stated that almost all of the sugars result 

 from the glycolytic pathway. It has been pos- 

 tulated that the tuna uses the white dorsal mus- 

 cles only as an emergency supply of energy for 

 rapid swimming while using the red muscle for 

 the constant swimming typical of pelagic fish 

 that do not have swim bladders to keep them 

 afloat. Peterson (1970) agrees with this con- 

 cept of a voluntary and involuntary muscula- 

 ture in skipjack tuna. He showed with electron 

 micrographs that the red muscle is indeed more 



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