A survey was performed to estimate the frequency of enterohemorrhagic Escherichia coli O157:H7 or O157:nonmotile (EHEC O157) in feces and on hides within groups of fed cattle from single sources (lots) presented for slaughter at meat processing plants in the Midwestern United States, as well as frequency of carcass contamination during processing from cattle within the same lots. Of 29 lots sampled, 72% had at least one EHEC O157-positive fecal sample and 38% had positive hide samples. Overall, EHEC O157 prevalence in feces and on hides was 28% (91 of 327) and 11% (38 of 355), respectively. Carcass samples were taken at three points during processing: preevisceration, postevisceration before antimicrobial intervention, and postprocessing after carcasses entered the cooler. Of 30 lots sampled, 87% had at least one EHEC O157-positive preevisceration sample, 57% of lots were positive postevisceration, and 17% had positive postprocessing samples. 43% (148 of 341), 18% (59 of 332) and 2% (6 of 330), respectively. Reduction in carcass prevalence from preevisceration to postprocessing suggests that sanitary procedures were effective within the processing plants. Fecal and hide prevalence were significantly correlated with carcass contamination (P ‫؍‬ 0.001), indicating a role for control of EHEC O157 in live cattle. Prevalence of EHEC O157 in the three postprocessing samples wasAbbreviation: EHEC, enterohemorrhagic Escherichia coli.

A number of studies have suggested that Z-disk degradation is a major factor contributing to postmortem tenderization. These conclusions seem to have been based largely on experimental findings showing that the calpain system has a major role in postmortem tenderization, and that when incubated with myofibrils or muscle strips, purified calpain removes Z-disks. Approximately 65 to 80% of all postmortem tenderization occurs during the first 3 or 4 d postmortem, however, and there is little or no ultrastructurally detectable Z-disk degradation during this period. Electron microscope studies described in this paper show that, during the first 3 or 4 d of postmortem storage at 4 degrees C, both costameres and N2 lines are degraded. Costameres link myofibrils to the sarcolemma, and N2 lines have been reported to be areas where titin and nebulin filaments, which form a cytoskeletal network linking thick and thin filaments, respectively, to the Z-disk, coalesce. Filamentous structures linking adjacent myofibrils laterally at the level of each Z-disk are also degraded during the first 3 or 4 d of postmortem storage at 4 degrees C, resulting in gaps between myofibrils in postmortem muscle. Degradation of these structures would have important effects on tenderness. The proteins constituting these structures, nebulin and titin (N2 lines); vinculin, desmin, and dystrophin (three of the six to eight proteins constituting costameres); and desmin (filaments linking adjacent myofibrils) are all excellent substrates for the calpains, and nebulin, titin, vinculin, and desmin are largely degraded within 3 d postmortem in semimembranosus muscle. Electron micrographs of myofibrils used in the myofibril fragmentation index assay show that these myofibrils, which have been assumed to be broken at their Z-disks, in fact have intact Z-disks and are broken in their I-bands.

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Tenderness has been repeatedly reported as the most important quality aspect of meat. However, a number of studies have shown that a significant portion of retail meat can be considered tough. As a consequence, a significant consumer segment is willing to pay a premium for guaranteed tender meat. However, apart from measuring the shear force, there is no reliable method to predict tenderness. Most of the branded meat programs therefore attempt to ensure eating quality by controlling some of the factors that affect tenderness. Meat tenderness is determined by the amount and solubility of connective tissue, sarcomere shortening during rigor development, and postmortem proteolysis of myofibrillar and myofibrillar-associated proteins. Given the effect of postmortem proteolysis on the muscle ultrastructure, titin and desmin are likely key substrates that determine meat tenderness. A large number of studies have shown that the calpain proteolytic system plays a central role in postmortem proteolysis and tenderization. In skeletal muscle, the calpain system consists of at least three proteases, μ-calpain, m-calpain and calpain 3, and an inhibitor of μ- and m-calpain, calpastatin. When activated by calcium, the calpains not only degrade subtrates, but also autolyze, leading to loss of activity. m-Calpain does not autolyze in postmortem muscle and is therefore not involved in postmortem tenderization. Results from a number of studies, including a study on calpain 3 knockout mice, have shown that calpain 3 is also not involved in postmortem proteolysis. However, a large number of studies, including a study on μ-calpain knockout mice, have shown that μ-calpain is largely, if not solely, responsible for postmortem tenderization. Research efforts in this area should, therefore, focus on elucidation of regulation of μ-calpain activity in postmortem muscle. Discovering the mechanisms of μ-calpain activity regulation and methods to promote μ-calpain activity should have a dramatic effect on the ability of researchers to develop reliable methods to predict meat tenderness and on the meat industry to produce a consistently tender product.