FURTHER STUDIES ON GREEN OR OFFCOLOR CONDITION 

 IN PRECOOKED YELLOWFIN TUNA 



By 



John J. Naughton, Michael M. Frodyma, and Harry Zeitlin 

 Chemistry Department, University of Hawaii 



An undesirable "green" color which 

 develops in the flesh of certain tuna onprecook- 

 ing prior to canning, results in considerable 

 economic loss to the fisherman and to the 

 industry through rejection of such fish. The 

 cause of this offcolor condition has been the 

 subject of studies at the Chemistry Department, 

 University of Hawaii, under contract No. 14- 

 19-008-2475 with the Pacific Oceanic Fishery 

 Investigations (POFI), U. S. Fish and Wildlife 

 Service. The background and the results of 

 preliminary studies are embodied in a previous 

 report (Naughton, Frodyma, and Zeitlin, 1956). 



The extension of the work herein described 

 represents an attempt to establish more defi- 

 nitely the nature and genesis of the pigment or 

 pigments responsible for the offcolor. 



We are most grateful to Mr. Fred Jermann 

 of Hawaiian Tuna Packers, Ltd., and to Dr. 

 Albert L. Tester, Director, and other members 

 of the staff of POFI for suggestions and help in 

 the investigation. 



SAMPLES 



The fish used in this study were yellowfin 

 tuna (Neothunnus nnacropterus) which, for the 

 most part, were caught on longline by POFI 

 research vessels in the central equatorial 

 Pacific in 1955 and 1956. The fish were frozen 

 on board ship as soon as possible after capture 

 and were held in a solid frozen condition until 

 they were needed. Sizes ranging from about 

 70 to 200 pounds were selected as it had been 

 noted that the larger fish showed a greater 

 tendency to turn green on precooking. In addi- 

 tion small yellowfin (up to 15 pounds) v/ e t e 

 caught by trolling in local waters by the staff 

 of the Hawaii Marine Laboratory, University 

 of Hawaii, and were sampled while fresh. 

 Samples of fresh, prime fillets ("ahi" samples) 

 of yellowfin caught in the local longline fishery 

 were purchased from local markets for study. 



In order to obtain precooked sannples for 

 the evaluation of color, frozen fish were thawed 

 in tanks at Hawaiian Tuna Packers, Ltd., and 

 cut into loins; one loin from each fish was re- 

 tained as a raw sample. The remaining loins 

 were put through the regular commercial 



precooking process of that company. Judgment 

 of color grade of the precooked samples was 

 made by Mr. Fred Jermann, the company's 

 food technologist. Specific samples noted to be 

 characteristic of their types (normal, green, 

 pink, etc. ) as subjectively judged, were used 

 in much of the work. 



REFLECTANCE 



Early in the research on the color of tuna 

 flesh, an opaque material, the need was recog- 

 nized for a device for nneasuring spectral 

 reflectance, thus making possible an objective 

 evaluation of the actual color of the sannples. 

 A reflectance attachment to the Beckman DU 

 spectrophotometer was secured. With the de- 

 velopment of techniques for reflectance meas - 

 urement, it became apparent that the method 

 had unique value as an analytical tool in addi- 

 tion to its recognized use in the specification 

 of color. Curves exhibiting optical absorption 

 peaks could be obtained by spectral reflection 

 measurements. These were characteristic of 

 the pigments present in tuna flesh and were 

 identical with the absorption curves obtained 

 from transmission measurements made on the 

 same pigments in solution. Beer's law was 

 obeyed, thus justifying the use of absorbancy 

 or optical density in our curve plots. The ob- 

 vious advantages of being able to make meas - 

 urements in situ, and on insoluble systems, 

 such as those encountered in this research, 

 were points that were vital. A separate com- 

 munication has been published (Naughton, 

 Frodyma, and Zeitlin, 1957) which describes 

 in greater detail the advantages of spectral 

 reflectance as an analytical tool, especially in 

 biological systems. 



PRECISION OF MEASUREMENTS 



The variability of the pigment content 

 throughout a sample of meat (see below) intro- 

 duces a difficulty when making reflectance and 

 pigment-content measurements on a series of 

 samples, even from the sanne fish. Ground and 

 thoroughly mixed (homogenized) samples gave 

 readings that were reproducible to + 0.5 per- 

 cent reflectance units or within the error of the 

 measuring instrument. Cn standing, however, 

 oxidative changes took place in such sannples 



