冷凍メカジキの緑変現象に関する研究-II

緑変の発生要因に就て

抄録

The problem of green discoloration in frozen meat of sword fish has drawn much attentions among frozen food packers in this country for recent several years. In the previous paper¹⁾ (1950) the presence of isovaleric acid in ?? green part of the meat was noted as a component of disagreeable smell resembling that of foul cheese.
The purposes of the present work are to ascertain what is the relation between occurrence of green meat and freshness of raw muscle and whether or not the green pigment is identical to that of Pseudomonas fluorescens. It is hoped that the latter of the questions may be answered by throwing light on biochemical agencies, if any, responsible for giving rise the discoloration.
The discoloration takes place just beneath the ventral and lateral sides of the skin and often in the abdominal cavity (Figs. 1-4). It is frequently experienced that formation of the green meat is likely promoted when the presence of the blood or dark flesh is remarkable in the adjacent muscles. It can be also pointed out that the discoloration tends to spread to a certain extent during the first two weeks of storage with temperature kept below -15°C., but no more after that period. The results of comparative examination showed that the green part was almost in an early stage of decomposition as compared with a normal part of the meat. The amount of iron contained in the green meat was rather higher than that of normal one (Table 1).
The green pigment can not be extracted with organic solvents such as ethanol, methanol, buthanol, berzol, ether, and is only soluble in water. The absorption spectrum of this pigment indicates their maximum point at the wave length of 418 milimicrons and differ distinctly from that of the pigment produced in the cultivation of Pseudomonas fluorescens. Furthermore, the green pigment does not show fluorescence under ultra-violet ray. Intensity of the color is neither reduced by addition of acid until pH value of the medium lowers to 4.0 or below, nor recovered when alkali is added to this acidified solution in an attempt at reversing the reaction toward alkaline side.
On the contrary, the pigment of Pseudomonas fluorescens fades out readily when pH of the medium is adjusted under 62. It is as well possible to let the color turn back completely as if to behave as an acid base indicator whenever pH is exceeded above 6.2 by addition of alkali (Table 8).
Since about 1 to 2mg. of hydrogen sulfide was proved to exist in 100 grms. of the green meat, injection of dilute H₂S solution into the normal tissue was applied in an attempt at artificial discoloration of sword fish meat. After a day or so the development of green color was observed at the locality where hydrogen sulfide had been injected (Table 9). Expansion of the green area was noticed during storage at the temperature of -15°C. for artificial discoloration. It is intere-sting to note that just about 1.25 mgms. of H₂S is necessary to form the discoloration, and that this amount roughly corresponds to that of H₂S naturally contained in the green meat Some reducing substances such as cysteine which are believed to probably exist in the muslce tissues, and glucose, , do not exhibit green discoloration as occurred in H₂S injection, regardless of their concentrations.
Artificial discoloration by H₂S was also successful with the muscles of dogfish and flatfishes to which the injection was performed. But it failed in case of Alaska pollack (Table 10).
Contamination of Pseudomoaas fluorescens would have no significance for the discoloration of frozen sword fish.