Effect of pH on the functional properties of myofibrillar proteins at reduced salt concentrations
This work focused on the effect of pH on the solubilization, water-uptake and gelation of myofibrillar proteins at reduced salt concentrations (≤150 mM). Solubilization of myofibrillar proteins in water was affected by certain possible solubility-inhibiting (PSI) polypeptides and postmortem exposure to a low pH. These PSI polypeptides might act like a binder that prevented the rest of the myofibrillar proteins from disorganization, swelling and subsequent solubilization in water. M-protein (166 kDa), a-actinin (95 kDa) and desmin (56 kDa) were tentatively identified as the PSI polypeptides in the mackerel light muscle. Exposure of myofibrillar proteins to the low pH that accompanies postmortem glycolysis could cause protein denaturation and subsequently the loss of extractability in water. However, over 96% of muscle proteins were solubilized after a pre-wash in a solution of physiological ionic strength at neutral pH.
The water-uptake of twice water-washed minced chicken breast muscle at physiological ionic strength was governed by the balance between the driving forces for, and the constraint components against, swelling. pH adjustment from 6.4 to 7.0 increased electrostatic repulsive forces and the osmotic potential of myofibrillar proteins. It also solubilized the constraint components associated with myofibrillar structure. Therefore, it increased the water-uptake.
pH adjustment from 6.4 to neutrality improved the gel strength and water-holding capacity significantly. After pH adjustment, the net negative charges of muscle proteins increased; the proteins unfolded more extensively during the heating process. Gels formed at physiological ionic strength consisted of mainly myofibrils. These myofibrils, which tended to form a network of localized aggregates at pH 6.4, formed a more evenly distributed network of myofibrils at neutral pH. Cell segments were capable of expansion before gelation (40–50°C) at neutral pH and their final volumes after heating were larger than those at pH 6.4. The thick filaments formed a porous network within the myofibrillar structure at neutral pH. During the cooling process, the gel strength was improved more at neutral pH than at pH 6.4. Structural disorganization imposed by pH adjustment from 6.4 to 7.0 was found not sufficient to improve the gelation significantly. It is suggested that pH adjustment from 6.4 to 7.0 introduced several favorable effects for gelation and water-holding capacity.