with consequent changes in spectral absorption. 

 Replicate nonhonnogenized samples from a 

 given fish loin gave an average precision of 

 + 1.5 percent reflectance units. 



THE COLOR OF COOKED TUNA 



A visual examination of different samples 

 of cooked tuna flesh immediately reveals the 

 complexity of the color system involved. First, 

 the variability of the absolute content of pig- 

 ment in the flesh of various fish should be 

 recognized. We have encountered variations 

 from highly bleached meat to a very high pig- 

 ment concentration with resultant abnornnal 

 redness. Whether these variations are physio- 

 logical or due to postmortem changes has not 

 been determined. Secondly, the condition of 

 "browning" can be recognized. As usually 

 understood, this seems to b e a postmortem 

 and even a postcooking phenomenon which is 

 due to oxidation on exposure to atmospheric 

 oxygen. Thirdly, the greening phenomenon , 

 which is the main subject of this research, is 

 observed. This is a color phenomenon that 

 permeates the flesh of the fish; its character- 

 istics will be described later in this report. 

 Also an orange coloration is frequently noted 

 which seems to occur in much the same manner 

 as the green color. Finally, it became very 

 evident after a modicum of work on the pigment 

 systems of the meat that the distribution of 

 pigment within a single fish loin was quite vari- 

 able, although it might seem uniform to the eye. 

 This was confirmed by nnicroscopic examina- 

 tion which showed the pignnent to be present in 

 bands within the meat. Spectral reflectance 

 measurements gave widely variable estimates 

 of the amount of pigment present. Uniformity 

 could be assured only by grinding and thoroughly 

 mixing all the meat needed in a given sample 

 sequence. This was not usually done, however, 

 since finely ground meat was found to change 

 color even when held in a frozen condition. 



At the beginning of this research the reality 

 of the green of certain offcolor samples of tuna 

 flesh was questioned by one of the investigators. 

 The color is of such a subtle shade and exhibits 

 so many variations that it was supposed that 

 the so-called greenness might be a lack of pig- 

 ment. In order to test the reality of the color, 

 the ICI (International Commission of Illumina- 

 tion) system of color specification was plotted 

 for typical cooked green and normal samples. 

 The results are shown in figure 1. It will be 

 noted that loci of the coordinates for both sam- 

 ples are in a region of lightness or brightness 

 indicative of the general light color of cooked 

 meat. The green flesh is situated nearer the 



0.8 



0.4 -I 



0.2 



0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 



X 



Figure 1 . --Chromaticity coordinates for green 

 and normal tuna meat. ICI systenn. 



yellow-green color region. The normal, on the 

 other hand, has more of the orange component 

 present. The reality of the green cast of cer- 

 tain samples seems to be indicated. 



Another method used in an attempt to eval- 

 uate greenness objectively was to compare 

 absorption in the red and green parts of the 

 spectrum (640 and 540 millimicrons, respec- 

 tively) for cooked green and nornnal meat by 

 reflectance measurements. A green sample 

 would be expected to show greater absorption 

 of light in the red end of the spectrum (640 

 millimicrons) when compared with normal meat. 

 The ratio of the absorbancy at the 540/640 

 nnillimicron absorption peaks therefore should 

 be greatest for normal and least for green meat 

 if pigments that absorb in these regions are 

 involved. In addition, we have been conscious 

 throughout this work that pigment content must 

 be a factor affecting the degree or type of alter- 

 ation that might occur. If we assume that we 

 can use the height of the nnost characteristic 

 absorption peak for heme pigments (the Soret 

 peak, at about 415 millimicrons) as an indica- 

 tion of the absolute pigment content of the cooked 

 meat and plot this against the above indicated 

 greenness index (540/640 absorption peak ratio) 

 it is possible to observe seversLl significant 

 relationships. Such a plot has been made in 

 figure 2, and it will be noted that normal sam- 

 ples, in general, occupy the portion of the 



