Heat-induced gelation of myofibrillar proteins and myosin (0.6M; pH 6.0) from rabbit fast-and slow-twitch muscles was analyzed by thermal scanning rheometry. Proteins from slow-twitch muscle exhibited higher thermostability and lower gel strength than those from fast-twitch muscle. Purifying myosin from myofibrillar proteins changed heat-gelation profiles and generally increased gel rigidity at 80ЊC. However, the effect of some proteins on the gelation of myosin was muscle dependent. Complete elimination of actin decreased the heat-gelling ability of slow myosin and increased that of fast myosin. Also, elimination of C-protein led to a greater increase in rigidity of gels from slow myosin than from fast myosin. The heat-behavior of the different protein fractions was related to the degree and type of aggregation in the gel.

The effects of ionic strength on myofibrils and myosin from rabbit fasttwitch Psoas major (PM) and slow-twitch Semimembranosus proprius (SMp) muscles before and after heating were studied by electron microscopy and thermal scanning rheometry. The direct suspension of proteins in low ionic strength (0.2M KCl; pH 6.0) led to very weak gels, whereas a gradual lowering of the ionic strength (by dialysis against 0.2M KCl; pH 6.0) of 0.6M KCl protein solutions induced strand-type networks at low temperature and strong heat-induced gels. As shown by transmission and scanning electron micrographs, in low ionic strength, SMp myosins formed shorter filaments before heating and thinner and shorter structures in heat-induced gels, as well as a lower gel porosity than PM myosins.

Both aliphatic and aromatic surface hydrophobicities of myosins from fast‐twitch psoas majorand slow‐twitchsemimembranosus propriusmuscles were investigated using fluorescence probes (cis‐parinaric acid and 8‐analino‐1‐naphthalene sulphonic acid). Surface hydrophobicity of unheated slow myosin was 1·5‐fold higher than that of fast myosin. However, heating led to an increased enhancement of fast myosin hydrophobicity which became, after heating, higher than that exhibited by heated slow myosin. Whatever the myosin isoforms, most surface hydrophobicity was on the myosin heads. Heating induced a rise in hydrophobicity of the S1‐subfragment from slow and fast myosins. However, the hydrophobicity of rods from fast myosin was increased three‐fold more by heating than did that from slow myosin. Finally, the blocking of hydrophobic binding sites affected differently the heat‐induced gelation in high ionic strength (0·6MKCl) of both myosin isoforms and confirmed that the molecular mechanisms involved in the gelation were muscle‐type dependent.

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