The surface SH content of carp actomyosin decreased in a similar way to the total SH content dur ing ice storage, regardless of the presence of sorbitol, as well as the formation of myosin heavy chain (MHC) dimer, indicating that the surface SH groups in myosin molecules were responsible for the oxi dation of actomyosin. The Ca2+-ATPase activity of actomyosin stored in the presence or absence of sorbitol was en hanced by the NEM modification of SH groups. EDTA-ATPase activity was stable during the storage despite the presence of sorbitol. These results indicate that reactive SH, SH1, in myosin head was not ox idized. The increase of Mg2+(EGTA)-ATPase activity suggests that SHa on the tail portion of myosin par ticipates in oxidation during storage, no matter whether sorbitol was added or not. These findings suggest that SHa on the myosin tail portion, not SH, on the myosin head portion, is responsible for the oxidation of MHC and the formation of its dimer.
Summary
Endogenous proteases are responsible for gel weakening of fish muscle protein. We investigated inhibitory effect of four selected protease inhibitors: soya bean trypsin inhibitor, leupeptin, E64 and EDTA, to identify the protease groups responsible for gel degradation in both unwashed and washed rohu (Labeo rohita) gel. The protease inhibitory activity was determined by percentage inhibition and SDS‐PAGE pattern. Leupeptin and E64 showed a strong inhibition on the autolysis of unwashed gel, and myosin heavy chain (MHC) band was recovered by the addition of them. The results suggested that cysteine protease was the major endogenous protease involved in the autolysis. In the case of the washed gel, leupeptin and soya bean trypsin inhibitor were shown to have a strong inhibitory effect. Also, the addition of these inhibitors increased the intensity of the MHC band. The results implied that serine protease played an important role in the degradation of the gel network.
The gel-forming ability of rohu (Labeo rohita) mince with and without egg white powder (EW) was investigated. Gel from washed mince (washed gel) was prepared under two setting conditions: kamaboko (40˚C) and modori (60˚C). The gel-forming ability of kamaboko and modori gels was improved by the addition of EW at 2%. The autolytic inhibition of kamaboko gel was obtained in gel added with 2% EW, and 1% EW of modori gel. No marked change was observed in the TCA-soluble peptide content of either gel with the addition of EW above 1%. No effect on the whiteness of both gels was shown after the addition of EW. The addition of EW exhibited smaller cavities and a more compact fibrous network in microstructure.
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