| During the postmortem storage there are numerous changes in the protein profile of beef. It is well documented that some of these changes directly affect the structure of the myofibril during postmortem aging. Our hypothesis was that there are alterations to proteins during postmortem storage that affect tenderness development. By identifying these alterations, these proteins can be used as indicators of tenderness and can aid in the identification of consistently tender beef.;The first experiment was designed to define the biochemical differences that govern tenderness and palatability of economically important muscles from the beef round using muscles with known tenderness differences. At 24 h postmortem, the longissimus dorsi (LD), gracillus (GR), adductor (AD), semimembranosus (SM), sartorius (SAR), vastus lateralis (VL), and vastus intermedius (VI) muscles were removed from ten market-weight beef cattle. Sensory and biochemical characteristics were determined in each muscle and compared with the LD. This study showed that the AD and VL had poor sensory traits compared to the LD. However, the GR, SAR, and VI had sensory traits similar to the LD.;Results from our first study suggested that proteolysis is the key process in indicating postmortem tenderness and that the AD had little proteolysis from one to 14 days postmortem. The lack of change in protein degradation in the AD provided a model to determine the changes in protein abundance over time in a muscle with little proteolysis from one to 14 days postmortem. Once these changes were determined, they were then compared to the changes in protein abundance over time in a muscle with greater proteolysis (LD) from one to 14 days postmortem to identify the proteins that may play a role in postmortem tenderization. So, this second experiment was designed to determine the identity of proteins that are altered during the postmortem aging process by using muscles that differ in tenderness and postmortem protein degradation. This study illustrated that both glyceraldehyde-3-phosphate dehydrogenase and beta-enolase are not expected to be robust indicators of tenderness across muscles because alterations to the relative abundance of these proteins were not directly related to the amount of tenderization that occurred in both the AD and LD during postmortem storage. Postmortem modifications, through proteolysis or solubility changes, of the structural proteins found to change in relative abundance during aging in the LD (actin, myosin heavy chain, myomesin-2, tropomyosin alpha 1 chain and alpha-actinin-3) can play a key role in postmortem tenderization by disrupting the structure of the myofibril. As a result, actin, myosin heavy chain, myomesin-2, tropomyosin alpha 1 chain and alpha-actinin-3 are anticipated to be good indicators of tenderness across muscles.;While actin, myosin heavy chain, myomesin-2, tropomyosin alpha 1 chain and alpha-actinin-3 appeared to be important to the process of postmortem tenderization, it was also important to verify that differences in these proteins could be detected between high and low star probe samples. Therefore, the next study was designed to identify proteins in bovine longissimus dorsi that differed in steaks from high or low star probe samples. Of the proteins identified in the previous study as changing in relative abundance over time, myomesin-2, tropomyosin alpha 1 chain, and actin were identified as being more abundance in the sarcoplasmic fraction of low star probe samples compared to high star probe samples. Therefore, myomesin-2, tropomyosin alpha 1 chain, and actin are important to the process of postmortem tenderization and can be detected between high and low star probe samples. Because of this, these proteins have the greatest potential to become indicators of postmortem tenderness.;In addition to myomesin-2, tropomyosin alpha 1 chain, and actin, both phosphoglucomutase 1 (PGM1), and myosin light chain 1 (MLC1) differed in relative abundance in the sarcoplasmic fraction between high and low star probe samples. The second portion of this study was designed to determine the extent to which mu-calpain activity could be responsible for the appearance of MLC1 in the highly soluble sarcoplasmic fraction during aging and examine the rate of the appearance of MLC1 in the highly soluble fraction during incubation with mu-calpain. Myofibril incubation with mu-calpain revealed that mu-calpain catalyzes the rapid release of MLC1 from the myofibril.;In conclusion, myomesin-2, tropomyosin alpha 1 chain, actin, and myosin light chain 1 show promise as potential indicators of tenderness. Because variation in posttranslational modifications have effects on protein activity, cellular locations and signaling pathways, the investigation into the posttranslational modifications of any candidate proteins may help in understanding usefulness of these proteins as biomarkers and the mechanism of tenderization during postmortem storage of meat. (Abstract shortened by UMI.)... |
