1.4 r—l^ 



400 



450 500 550 600 



WAVE LENGTH, M/J 



650 



700 



Figure 6. --Oxidation of flesh pigment on treat- 

 ment with nitrogen, (a) Reflectance curve 

 of fresh tana flesh, (b) Reflectance curve 

 after alternating evacuation and treatment 

 with nitrogen for 3 hours showing increase in 

 metmyoglobin content. 



Pigment Denaturation 



Another change that takes place in raw 

 flesh pigments is the denaturation of the protein 

 moiety of the heme pigment molecule. Such 

 denaturation produces so-called denatured 

 globin hemichronnes (Lemberg and Legge 1949, 

 p. 228). These are more easily identified from 

 their absorption curves in the reduced condition 

 as the denatured globin hennochromes with pro- 

 nounced peaks at 528 and 558 nnillimicrons. 

 Figure 7 shows such spectral reflectance curves 

 obtained with tuna flesh. Strictly fresh nneat 

 gave the characteristic single absorption peak 

 of myoglobin in the green region at 555 milli- 

 microns (curve a). Mild intentional denatura- 

 tion of this meat by heating to 50°C. for short 

 periods (20 minutes in the case illustrated) gave 

 on reduction the appearance of the distinct hemo- 

 chrome peak at 528 millimicrons (curve b) . 

 Similar denaturation can be noted in the absorp- 

 tion curve (curve c) for raw green tuna flesh, 

 and, as would be expected, for cooked green 

 tuna flesh (curve d). It is remarkable that 

 intense commercial cooking in the last instance 

 does not seem to nnarkedly increase the denatu- 

 ration of the pigment when so judged. There is 

 some indication from a study of a nunnber of 

 siich raw reduced samples that the degree of 

 denaturation is related to the amount of 



400 



450 500 550 600 



WAVE LENGTH , M>J 



650 



700 



Figure 7, --Reflectance curves showing denatu- 

 ration by appearance of hemochrome band at 

 528 m[i 2ifter reduction with dithionite. (a) 

 Reduced raw fresh tuna- -myoglobin; (b) Tuna 

 flesh of curve (a) after denaturation by heat- 

 ing 20 minutes at 50°C. ; (c) Raw green tuna, 

 reduced; (d) Commercially precooked green 

 tuna reduced. 



metnayoglobin originally present in the samples 

 and to the greening tendency. 



Solubility of Metmyoglobin 



Another example of unusual heme pigment 

 changes and behavior noted in situ is the unex- 

 pectedly greater solubility of metmyoglobin 

 relative to the oxymyoglobin present in tuna 

 flesh. The phenomenon is illustrated in figure 

 8. It has been noted previously that aqueous 

 extraction of tuna flesh gave absorption curves, 

 nneasured in transmission, that were charac- 

 teristic of nnetmyoglobin. It was always found 

 that such extraction left a pigmented residue 

 whose color showed but slight tendency to 

 decrease with repetition of the extraction. This 

 was initially assumed to be due to the inefficiency 

 of the extraction procedure. Application of 

 spectral reflectance measurements to the prob- 

 lem showed that the metmyoglobin in tuna flesh 

 is characterized by a rather pronounced solubil- 

 ity, while the oxymyoglobin exhibited a high 

 degree of insolubility (compare curves a and b, 

 fig. 8). This suggests the possibility that pig- 

 ments leaching during thawing and precooking 

 might be a factor in producing a pale condition 

 akin to greening in tuna with a high metmyoglobin 



