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Casein zymography was used to determine the effect of postmortem storage on the proteolytic activity of [micro]-calpain and m-calpain in lamb longissimus. Casein zymography assays were conducted on crude muscle extracts (only one centrifugation). Six market weight crossbred lambs were slaughtered and a portion of the longissimus lumborum was removed at death (within 15 min of exsanguination) and after 3, 6, 9, 12, 24, 72, and 360 h postmortem. Muscle samples were snap-frozen in liquid nitrogen and stored at -70 [degrees] C. Soluble muscle proteins were extracted from muscle samples and analyzed by in-gel casein assay to measure calpain proteolytic activity. There was a gradual decline in [micro]-calpain activity (P [is less than] 0.05) such that after 24 and 72 h postmortem, [micro]-calpain had lost 42 and 95% of its activity, respectively. After 360 h postmortem, no [micro]-calpain activity could be detected (under the conditions used in this study). Autolysis of [micro]-calpain could be detected as early as 3 h postmortem. It was demonstrated that the detectable level of [micro]-calpain activity is a function of the amount of muscle protein electrophoresed. Hence, the activity data reported are in relative terms, rather than absolute values. Furthermore, it was demonstrated that the activity data also are a function of the assay methods used. Different methods have different lower detection limits. Of the three assays examined, [sup.14]C-labeled casein was the most sensitive, then the ingel casein assay, and the least-sensitive method was the standard casein assay. Unlike [micro]-calpain, postmortem storage had no effect on m-calpain (P [is greater than] 0.05). When the calcium concentration of a muscle extract was increased to the level that induces m-calpain autolysis, m-calpain was autolyzed and its autolysis was readily detected by the in-gel casein assay. Collectively, these results demonstrate that calcium concentration in postmortem muscle is only high enough to activate [micro]-calpain. These results support the widely believed conclusion that [micro]-calpain-mediated proteolysis of key myofibrillar and cytoskeletal proteins is responsible for postmortem tenderization. Hence, understanding the regulation of [micro]-calpain in postmortem muscle should be the focus of future studies. Key Words: Calpains, Meat, Postmortem Changes, Tenderizing

Carcass (n = 854) and longissimus thoracis palatability (n = 802) traits from [F.sub.1] steers obtained from mating Hereford, Angus, and MARC III cows to Hereford or Angus (HA), Tuli (Tu), Boran (Bo), Brahman (Br), Piedmontese (Pm), or Belgian Blue (BB) sires were compared. Data were adjusted to constant age (444 d), carcass weight (333 kg), fat thickness (1.0 cm), fat trim percentage (21%), and marbling ([Small.sup.00]) end points. Results presented in this abstract are for age-constant data. Carcasses from BB- and HA-sired steers were heaviest (P [is less than] 0.05) and carcasses from Bo- and Tu-sired steers were lightest (P [is less than] 0.05). Adjusted fat thickness was greatest (P [is less than] 0.05) on carcasses from HA-sired steers and least (P [is less than] 0.05) on carcasses from BB-and Pm-sired steers. Numerical USDA yield grades were lowest (P [is less than] 0.05) for carcasses from Pm- and BB-sired steers and highest (P [is less than] 0.05) for carcasses from HA- and Br-sired steers. Marbling scores were highest (P [is less than] 0.05) for carcasses from HA- and Tu-sired steers and lowest (P [is less than] 0.05) for carcasses from Br-, BB-, and Pm-sired steers. Longissimus thoracis from carcasses of HA-, Pm-, and Tu-sired steers had the lowest (P [is less than] 0.05) 14-d postmortem Warner-Bratzler shear force values. Carcasses from HA-sired steers had longissimus thoracis with the highest (P [is less than] 0.05) tenderness ratings at 7 d postmortem. Longissimus thoracis from carcasses of Br- and Bo-sired steers had the highest (P [is less than] 0.05) Warner-Bratzler shear forces and the lowest (P [is less than] 0.05) tenderness ratings at 7 d postmortem. Adjustment of traits to various slaughter end points resulted in some changes in sire breed differences for carcass traits but had little effect on palatability traits. Carcasses from BB- and Pm-sired steers provided the most desirable combination of yield grade and longissimus palatability, but carcasses from HA-cross steers provided the most desirable combination of quality grade and longissimus palatability. Tuli; a breed shown to be heat-tolerant, had longissimus tenderness similar to that of the non-heat-tolerant breeds and more tender longissimus than the heat-tolerant breeds in this study. Key Words: Beef, Breeds, Carcass Composition, Meat Quality, Palatability, Tenderness

The myofibril fragmentation index (MFI) is strongly associated with indices of meat tenderness, such as Warner-Bratzler shear force and sensory tenderness. The MFI is normally determined on fresh muscle. It is not known whether this index can be determined on frozen muscle. The objective of this experiment was, therefore, to determine whether there is a difference between MFI values of fresh and frozen lamb and pork longissimus. To compare the effect of freezing on MFI, longissimus samples were obtained from eight lamb carcasses at 1, 3, and 15 d postmortem and longissimus samples were obtained from 12 pork carcasses at 3 d postmortem. For each sample, MFI was conducted on both fresh muscle and snap-frozen muscle (frozen in liquid nitrogen and stored 23 to 26 d at -70 [degrees] C). The [R.sup.2] between MFI of fresh and frozen muscle was 0.94 and 0.92 for lamb and pork longissimus, respectively. The differences between fresh and frozen MFI were not significant for either species (P [is greater than] 0.05). These results indicate that it is not necessary to determine MFI on fresh muscle. Key Words: Fragmentation, Lambs, Meat, Myofibrils, Pork

The objective of this study was to identify quantitative trait loci for economically important traits in two families segregating an inactive copy of the myostatin gene. Two half-sib families were developed from a Belgian Blue x MARC III (n = 246) and a Piedmontese x Angus (n = 209) sire. Traits analyzed were birth, weaning, and yearling weight (kg); preweaning average daily gain (kg/d); postweaning average daily gain (kg/d); hot carcass weight (kg); fat depth (cm); marbling score; longissimus muscle area ([cm.sup.2]); estimated kidney, pelvic, and heart fat (%); USDA yield grade; retail product yield (%); fat yield (%); and wholesale rib-fat yield (%). Meat tenderness was measured as Warner-Bratzler shear force at 3 and 14 d postmortem. The effect of the myostatin gene was removed using phase information from six microsatellite markers flanking the locus. Interactions of the myostatin gene with other loci throughout the genome were also evaluated. The objective was to use markers in each family, scanning the genome approximately every 25 to 30 centimorgans (cM) on 18 autosomal chromosomes, excluding 11 autosomal chromosomes previously analyzed. A total of 89 markers, informative in both families, were used to identify genomic regions potentially associated with each trait. In the family of Belgian Blue inheritance, a significant QTL (expected number of false-positives = 0.025) was identified for marbling score on chromosome 3. Suggestive QTL for the same family (expected number of false-positives = 0.5) were identified for retail product yield on chromosome 3, for hot carcass weight and postweaning average daily gain on chromosome 4, for fat depth and marbling score on chromosome 8, for 14-d Warner-Bratzler shear force on chromosome 9, and for marbling score on chromosome 10. Evidence suggesting the presence of an interaction for 3-d Warner-Bratzler shear force between the myostatin gene and a QTL on chromosome 4 was detected. In the family of Piedmontese and Angus inheritance, evidence indicates the presence of an interaction for fat depth between the myostatin gene and chromosome 8, in a similar position where the evidence suggests the presence of a QTL for fat depth in the family with Belgian Blue inheritance. Regions identified underlying QTL need to be assessed in other populations. Although the myostatin gene has a considerable effect, other loci with more subtle effects are involved in the expression of the phenotype. Key Words: Beef, Double Muscling, Genetic Markers, Loci, Myostatin Gene, Quantitative Traits

The objective of this study was to determine the relationship of longissimus tenderness classes to tenderness of three other major muscles. Ninety-eight crossbred steers and heifers (14 to 17 mo of age) were humanely slaughtered over 9 wk and the carcasses were chilled 48 h at 0 [degrees]C. At 48 h postmortem, carcasses were assigned to one of three tenderness classes (tender = [is less than] 26 kg, intermediate = 26 to 42 kg, tough = [is greater than] 42 kg) using slice shear force from the MARC Beef Classification System (n = 20, 67, and 11, respectively). The longissimus thoracis, gluteus medius, semimembranosus, and biceps femoris were removed, aged at 2 [degrees]C, and frozen at -30 [degrees]C at 14 d postmortem. Two 2.54-cm-thick steaks were obtained from each muscle, thawed to 5 [degrees]C, cooked with a belt grill at 163 [degrees]C for 5.5 min, and served warm to an eight-member trained descriptive attribute panel. Panelists evaluated each sample for tenderness, connective tissue amount, juiciness, and beef flavor intensity on 8-point scales. The mean 2-d longissimus slice shear force values were 20.7, 34.4, and 46.3 kg, respectively, for the 'tender,' 'intermediate,' and 'tough' classes. Tenderness ratings were lowest (P [is less than] 0.05) for the 'tough' class and highest (P [is less than] 0.05) for the 'tender' class for all muscles except the gluteus medius, for which the 'tender' and 'intermediate' classes were not different (P [is greater than] 0.05; longissimus, 7.7, 7.1, 6.3, and 7.1; semimembranosus, 6.4, 5.8, 5.1, and 5.8; biceps femoris, 5.9, 5.4, 4.8, and 5.4; gluteus medius, 6.8, 6.5, 5.8, and 6.5 for the 'tender,' intermediate,' 'tough,' and 'unsorted' classes, respectively). The magnitude of the differences in tenderness ratings between the 'tender' and 'intermediate' classes and between the 'intermediate' and 'tough' classes was similar for all muscles. The percentages of tenderness ratings greater than 5.0 (slightly tender) for the 'tender' and 'unsorted' classes, respectively, were as follows: longissimus, 100 and 95%; semimembranosus, 95 and 85%; gluteus medius, 100 and 94%; and biceps femoris, 95 and 81%. The simple correlations between longissimus and the other muscles for tenderness ratings were as follows: semimembranosus, 0.58; biceps femoris, 0.43; and gluteus medius, 0.68. These data indicate that early-postmortem longissimus slice shear force could be used to classify top sirloin, top round, and bottom round cuts for tenderness. Key Words: Beef, Carcass Grading, Classification, Meat Quality, Muscles, Tenderness