抄録
The discoloration of frozen tuna meat during storage has long been a problem for tuna freezing industry. This is particularly a ploblem in such cases that the fishing grounds are so distant that the fish can no longer be kept by simple icing.
So, it is an urgent need to elucidate the mechanism of the discoloration to provide some means for prevention change in the color during freezing storage.
The author measured the absorption spectra on aqueous extract of frozen tuna meat stored under different conditions.
1. As indicated in Table 1 and Plates 1 and 2, the meat stored at -78°C with dry ice showed a very good retention of color and it was hardly distinguished from the color of fresh unfrozen meat. On the contrary, a faster discoloration took place in the samples of meat kept at -18°C or at -5 to -14°C.
During the process of discoloration of frozen tuna meat, it was clearly observed on the aqueous extract that the absorption of oxymyoglobin (maxima 540 and 576mμ) decreased, while that of metmyoglobin (maxima 503 and 633mμ) increased.
2. In the case of frozen block of tuna meat under the storage temperature of -18°C, the discoloration was more considerable in deep portion than surface portion, as shown in Plate 2, Figs. 7 and 8, Table 3.
3. Absorption spectra of aqueous extract from brownish meat of frozen tuna was almost identical to the absorption spectra of metmyoglobin, and the extract showed the pattern of oxymyoglobin, when shaken in air with sodium hydrosulfite (Na2S2O4).
These observations will indicate that the discoloration of tuna meat during freezing storage is caused by the oxidative change of myoglobin (Fe2+) into metmyoglobin (Fe3+).
