Background

Protein degradation and denatuation during postmortem storage of beef are known to influence beef quality, especially tenderness of beef. The early postmortem environment in muscle can have a significant effect on the processes that dictate fresh meat quality. It is known that the rate and extent of pH decline influences protein denaturation as well as enzyme function. Small changes in enzyme activity or protein solubility have the capacity to alter the rate and extent of changes in muscle and meat ultrasturcture.

For many years it was understood that protein changes were not significant until after several days of postmortem storage. However, it is likely that this observation is due to the sensitivity of methods used. New approaches to determine protein modifications (solubility, denaturation, degradation) have provided insight in the response - at the molecular and cellular level - to changes in the early postmortem environment. These techniques have been focused on directed experiments focused on the activity of singular enzymes, inhibitors and substrates. The development of technologies to monitor changes in the entire proteome provides the means for a global inquiry of the extent to which the early postmortem environment causes changes in protein profile. Alteration in the postmortem environment by early postmortem electrical stimulation of beef carcasses is known to increase protein degradation and the rate of tenderization during postmortem storage. In previous experiments, researchers have documented the early postmortem electrical stimulation results in a more rapid pH decline, more rapid μ-calpain autolysis and more rapid tenderization in bovine Top Loin (Longissimus dorsi) steaks.

Methodology

Findings

Application of ES resulted in lower LD pH at 3.5, 5.5 and 8.5 hours. No treatment differences were detected in ultimate pH. Application of ES resulted in greater autolysis of μ-calpain and greater sensory tenderness scores in LD steaks evaluated 1 day postmortem.

ES reduced the abundance of several enzymes in the soluble protein fraction of samples evaluated at days 1 and 9 (Table 1) in experiments that utilized both pH 3-10 pH gradient strips as well as pH 4-7 pH gradient strips. Spots with homology to creatine kinase, annexin and pyruvate dehydrogenase were more soluble in response to ES. 

Fewer differences were detected in the experiments designed to investigate the effect of ES on the insoluble fraction (Table 2). The corresponding loss of several enzymes (fructose bisphosphate aldolase, glyceraldehyde 3 –P dehydrogenase) in the soluble fraction resulted in an increase in those proteins in the insoluble fraction. ES resulted in a greater abundance of myosin light chain 2, tropopomyosin beta chain and tropomyosin alpha chain in the insoluble fraction of steaks aged 9 days. 

Implications