Effect of Ultrastructural Disruption and Protein Dispersion on Gel-forming in Myofibrillar Gels.

Abstract

Gelled muscle foods span a diverse range of products. The texture and water⁄fat binding properties of these food products is largely determined by their gel structure. A means to increase the functionality of meat proteins with respect to gel forming and water-holding ability would thus be beneficial.

This research conducted experiments that utilized three different approaches to enhancing the dispersion of muscle proteins during comminution: salt addition, alkaline solubilization⁄reprecipitation (pH shifting) and calpain-induced pre-degradation of meat ultrastructure. Data from these experiments were considered as to whether they supported our hypothesis: increased disruption of muscle ultrastructure and dispersion of myofibrillar proteins will lead to formation of gels which exhibit properties of increased fracture stress and strain (i.e., improved gelling properties).

The pH shifting process involves chopping the muscle with water, adjustment low (2-3) or high (10.5-11) pH values, and then precipitation by adjusting to the isoelectric point (5.3-5.5). The resulting meat protein isolate is then dewatered and ready for further use. Interestingly the pH shifting treatment eliminates the necessity for salt addition in order to maximize the strength and deformability of cooked gels made from the meat so treated. Experiments were carried out using this process on King Mackerel. The slurries produced by the process and gels made from the slurries were evaluated for disruption and dispersion by transmission election microscopy, TEM. The evaluation of these micrographs was assisted by a trained sensory panel to generate quantative data describing the micrographs. The gel-forming ability was measured using torsion.

The process results in disruption of ultrastructure and better dispersion of proteins. These two factors positively affect the gel forming ability (fracture stress, strain values) of the meat. It appears that maximal effects on disruption and dispersion occurs when the pH is raised to (at least) 11, likely because of the removal of ultrastructural proteins, including the so-called SIPs identified by previous authors.

Unfortunately, the role of protein conformational changes could not be separated from that of muscle ultrastructure disruption and dispersion effects. To assist in deconvoluting these two effects, experiments which replicated the disruption and dispersion process, but which minimized the conformational changes induced in the individual proteins were done.

To affect disruption⁄dispersion without conformational change; calpain, a naturally occurring endopeptidases was utilized. Experiments were carried out on isolated chicken myofibrils. Calpain isolated from King Mackerel was introduced into a isolated myofibrils solution to affect a change. Disruption/dispersion and gel-forming ability were measured as in the pH shifting experiments.

The data from the calpain experiments support our hypothesis. Increases in both properties (disruption and dispersion) generally correlated well to increases in fracture stress. These data reinforce the observations in the literature that stronger gels display greater homogeneity of protein dispersion, when viewed by TEM.

Calpain pretreatment produced the strongest gelling properties in subsequently comminuted myofibrils, but the degree of disruption/dispersion as scored by the panel did not increase significantly over other treatments that yielded somewhat less strong gels than this treatment. However, almost certainly the greater effect of calpain pretreatment on gelation can be attributed to its ability to better disperse the myosin and actin, due to its known ability to disrupt the Z-line and titin; it is also does not degrade the gelling proteins and its action should have little effect on the native conformation of these proteins.

It is quite significant that this is the first report that a proteolytic treatment, in particular that using calpain, results in an increased gelling ability of comminuted meat.

The discussion of published evidence to support, plus data from experiments conducted to test, our hypothesis hints at a new paradigm in myofibrillar protein gelation.