Solubility and gelation of chicken breast muscle proteins as affected by salts
Solubility of chicken breast muscle proteins in solutions of very low ionic strength ($<$0.3 mM) reached 95-98% after certain washing/extracting treatments. Treatment by washing with salt and pH adjusted to neutrality was found critical for complete solubilization. This procedure extracted specific proteins after which the remaining myofibrillar proteins became soluble in water, i.e., ionic strength $<$0.3 mM. SDS-PAGE indicated that these additional proteins were of relatively high molecular mass, $>$66 kDa. Four of the six were tentatively identified as X-protein, C-protein, $\alpha$-actinin and amorphin. These first two proteins are associated with the thick filaments and the latter two with the Z-discs. The other two proteins had molecular masses of 95 and 105 kDa. Those high molecular weight proteins ($>$66 kDa) may restrict the solubilization of the other myofibrillar proteins in water and thus were called the "solubility-inhibiting proteins".
At neutral pH, the presence of sodium pyrophosphate and magnesium chloride (10 mM each) and 0.2 M sodium chloride solubilized 57.2 $\pm$ 6.1% of the washed minced chicken breast muscle ("water washed mince") and 60-65% of the myosin. The pyrophosphate may have induced the dissociation of actomyosin into actin and myosin and the presence of magnesium ions may have shifted the isoelectric point of the released myosin toward an acidic pH. These two factors would improve myosin solubility.
Solubilization of the myofibrillar proteins, especially myosin, has been considered a prerequisite step for gelation. Good gels could be formed from washed minced chicken breast muscle ("water washed mince") at low (26 mM) or intermediate (150 mM) salt concentrations. The formation of good gels at "low or intermediate" salt levels could be achieved by adjusting the pH of the surimi sample to neutrality prior to heating. The importance of maintaining a neutral pH for gel formation was further demonstrated when gels with low strain values were produced after the pH of the surimi samples previously adjusted to neutrality were brought back to the original low value at 150 mM sodium chloride. At both 26 and 150 mM sodium chloride there was no measurable solubility of myosin. A neutral pH might cause disorganization which might be necessary for gelation.
Lack of a relationship between solubility properties and gelation characteristics of the chicken breast myofibrillar proteins were noted under conditions of physiological concentrations or less of salt. Generally, low solubility of myofibrillar proteins did not prevent good gels from forming. Likewise, better solubility did not improve gelation.