Transverse anisotropy in beef muscle

In beef sternomandibularis muscles, a variety of treatments—tetanic contraction, cold shortening, cooking shortening and swelling in alkali—caused dimensional changes which differed markedly with the direction chosen within a plane perpendicular to the fibre direction. In all cases a marked increase (up to twice) occurred in the narrow dimension of the muscle, with little change in the wide dimension. In the cooked meat, shear force across the fibre was independent of the transverse direction chosen whether the muscle was relaxed or cold shortened. The effects were also seen in rectus abdominis and psoas, but not in longissimus dorsi muscles. It is suggested that anisotropy is a result of differing organisation of connective tissue in the two directions, although there was little histological evidence. The force to shear parallel to the fibres varied with direction in a manner consistent with this view.     

Osmotic properties of post-rigor beef muscle

The osmotic pressure of beef sternomandibularis and psoas muscles ranged from 480 to 540 mOs, which is almost twice that of pre-rigor muscle (about 300 mOs). The post-rigor osmotic pressure appears to be satisfactorily explained by the low molecular weight components. Muscle samples were soaked at 2°C for 24 h in various solutions and sample weight changes were recorded. Samples soaked in buffered mannitol solutions exhibited weight changes indicative of predictable osmotic behaviour. Those samples soaked in buffered salt solutions (KCl, NaCl), however, exhibited dramatic increases in weight, irrespective of the solute concentration. The reasons for this are not clear. These results indicative that the use of salt solutions, such as 0·15 m KCl, for studies on post-rigor muscle is suspect and conclusions from such studies may be unfounded.     

Measurement of autolysis in beef muscle homogenates

Homogenates of beef longissimus dorsi muscle have been used to study the effect of pH, temperature and Ca(2+) ions on post-mortem autolysis. Measurements of myofibrillar proteins were made after SDS-polyacrylamide gel electrophoresis and it was found that the intensity of the troponin T band could be used as an indicator of autolysis. At pH values below 6·0, the addition of EDTA increased the rate of loss of troponin T: above pH 6·0, the rate of loss of troponin T was accelerated by Ca(2+) ions. It was concluded that there were at least two proteolytic enzyme systems involved-cathepsin B at low pH and a calcium-activated factor at high pH.     

Cold shortening capacity and beef muscle growth

Cold shortening capacity is related to animal growth. For unloaded strips of beef sternomandibularis muscles the degree of cold shortening developed at 2 °C during the pre-rigor period increased from approximately 0.2 for small young calves to 0.6 for large old bulls. This growth-dependent capacity to cold shorten is even more pronounced in muscle strips shortening isometrically. Thus in terms of power development, it increases forty-fold with the growth of beef sternomandibularis muscles to their mature sizes. In contrast, temperature sensitivity of the phenomenon is virtually the same for both small and large animals with the tendency to cold shorten declining in both to zero at 35 °C. These findings are important to meat chilling since they imply that cold shortening will be more pronounced the larger the carcass. This is offset to a degree by the fact that large carcasses will chill more slowly than smaller carcasses.     

Structural changes in beef muscle during ageing

The ageing of beef muscle set in rigor mortis while restrained at twice its resting, excised length has been examined. Ageing leads to opening of gaps between the A and I bands of the sarcomere. With the shrinkage of both bands during cooking, the gaps become the major feature of the sarcomere and develop within the tenderising time-scale. As such they represent the most distinct structural change yet encountered to explain the phenomenon of ageing. A possible reconciling of the contradictory views of ageing—splitting at the A–I junctions and decay of Z lines—is discussed.     

Tensile strength and the tenderness of beef sternomandibularis muscle

In their relationship to shortening, the tensile strength of cooked meat along the fibres and the shearing force measured across them are strikingly similar. Both increase to maximum values at 40% shortening before falling by a half with an approach to 60% shortening. A high correlation (r = 0·81) exists between the two sets of values, whether toughness is increased by cold shortening or reduced by ageing. The mechanical strength of meat along its fibres is therefore a simple measure of tenderness and a useful basis for relating muscle structure to stength.     

Effects of temperature on post-mortem metabolism in beef muscle

Changes in pH and in the levels of some phosphate fractions with time were examined in beef muscle stored at 1°, 15° and 37° soon after slaughter. At 15° the fall in pH of the muscle and decreases in concentration of creatine phosphate and of other acid-labile phosphate occurred more slowly than at 37° and there was also a slower increase in the concentration of inorganic phosphate. Not all of these changes occurred more slowly at 1° than at 15°, however. The alkali-labile phosphate concentration remained virtually constant at 37° but increased at each of the other two temperatures, the increase at 1° being greater than that at 15°. These changes in alkali-labile phosphate concentration were attributable to the accumulation of hexose-6-phosphate. An effect of temperature on the metabolism of an organic phosphate fraction stable to both acid and alkali was noted.     

The kinetics of rigor onset in beef muscle fibres

Force and stiffness were investigated as beef muscle fibre preparations entered rigor mortis. The results show that single fibres enter rigor rapidly (exhibit a small t_10–90) whilst fibre bundles exhibit a much slower rate of entry into rigor (larger t_10–90). The mean ‘rigor time’ in both cases is, however, similar. The difference in kinetics is probably due to the fact that individual fibres within a bundle enter rigor at different times. Effectively anaerobic fibre bundles treated with iodoacetate exhibit a t_10–90 almost as short as do single fibres when similarly treated, suggesting that much of the inferred heterogeneity of response amongst fibres in a bundle arises from differences between fibres in the ability of glycolysis to sustain ‘relaxing’ concentrations of ATP.

Fibres have been chemically ‘skinned’ after entering the rigor state. Measurements of the relationship between stiffness and tension in the skinned fibres indicate that there are two readily distinguishable rigor ‘states’ depending upon whether rigor is entered from the relaxed of from the contracting state. By contrast, the rigor state achieved by intact fibres is similar to that attained in skinned fibres via the contracted state, suggesting that the ‘natural’ rigor state is preceded by a calcium-activated contraction. Interconversion of the two rigor states generated in vitro requires an intermediary relaxed state.     

Changes in extensibility of raw beef muscle during storage

The mechanical properties of raw meat under tension were studied to characterise the factors responsible for tenderisation during storage. Extensibility and irreversible lengthening in the direction of muscle fibres was determined continuously from 2 h to 10 days after stunning by cycling the muscle between two applied forces. In post-rigor meat, extensibility increased slightly and the muscle lengthened by 40%. Lengthening occurred earlier at higher post-rigor temperatures and was more sensitive to changes in temperature than the changes in texture of cooked meat. Increasing the maximum applied stress also caused lengthening to occur earlier in post-rigor meat. No lengthening was detected pre-rigor. The relationship between the maximum applied stress and the rate of lengthening showed that lengthening would not have occurred with a stress of 0·07 Ncm(-2) or less. These findings suggest that weakening which occurs during conditioning commenced at rigor and that the component responsible for tenderising was about 30 times weaker than indicated using previous techniques. Histological sections showed regular cracks spaced at about 17 μm apart. This was believed to be indicative of the involvement of extracellular components in the texture of conditioned meat.     

The tenderising effect of a muscle proteinase on beef

Freeze-dried beef has been reconstituted in a solution of a Ca²⁺ activated muscle proteinase. After storing the reconstituted meat for 5 days, the Z-lines of the myofibrils had disintegrated and the meat, when cooked, became appreciably tender as determined by a tenderometer. Since control samples, without added proteinase, did not become as tender as the enzyme-treated samples it was concluded that this proteinase acts as a meat tenderiser.     

The relationship of muscle fibre size to tenderness of beef

Steaks were removed from loins of beef carcasses at 1, 3, 6 or 14 days post mortem for fragmentation index (MFI), Warner-Bratzler Shear Force (SF) and sensory panel tenderness evaluation. Also, after 1 day of storage, samples were removed for histological observations. Greatest improvement in tenderness, SF and MFI occurred within the first 6 days of storage. Sensory panel tenderness was correlated (P < 0·01) with SF and MFI. Average muscle fibre size was correlated (P < 0·01) with tenderness and SF at days 1 and 3, but not at days 6 and 14. Evidently, muscle fibre size is important to tenderness prior to post-mortem storage of meat and proteolysis, but becomes less of a factor in tenderness after 6 days of storage.     

Mechanisms of ultrastructural changes in electrically stimulated beef Longissimus muscle

Three steers, ranging in grade from USDA low to average Good, were used in the evaluation of the effect of electrical stimulation (ES) on structural properties of the beef longissimus muscle. The left sides were electrically stimulated with 1-A current (delivering 145-250 V through the sides) for 2 min by applying eight bursts of 15-s duration. The right sides were not stimulated (controls). Samples of the longissimus muscle were removed at the sixth lumbar area 48 h post mortem and processed for light and transmission electron microscopy. Light and electron micrographs of stimulated muscle revealed irregular (contraction) bands, superstretching of myofibrils, absent or ill-defined I-, A- or Z-lines, tearing of myofibrils, the presence of empty vesicles and fragmentations of myofibrils at the Z-lines. Therefore, some of the mechanisms of tenderisation of electrically stimulated muscle would appear to result from physical disruption of myofibrils and possibly autolytic proteolysis.     

The effect of hot-boning on glycolysis in beef muscle.

The M. semimembranosus (SM) was excised from one side of six beef carcasses within 1 h of slaughter 3(hot-boned) and held at 10°C until 24 h post mortem and then at 3°C. The contralateral muscle underwent normal chilling in the intact side. Changes in energy phosphates, temperature and pH were followed in the intact SM at depths of 1.5, 5 and 8 cm from the carcass surface and at corresponding locations in the hot-boned SM. Hot-boning had a major effect on the rate of metabolism in the muscle post mortem. A relatively uniform rate of glycolysis was observed throughout the hot-boned muscle compared to the carcass muscle in which the rate of glycolysis increased with depth. The ATP content in the hot-boned muscle fell to 50% at 8 h post mortem and rigo^r was completed within 24 h.     

Effects of ageing and cooking on the tenderness of beef muscle

When beef sternomandibularis muscle was subjected to prolonged cooking, toughness, measured by shearing force across the grain, was reduced by about 50%. The shortened state of the muscle determined the final shear-force value attained as in normal cooking, being considerably higher in meat at 40% shortening, than in either unshortened meat or in meat shortened by 60%. The tenderising effect of ageing was additional to that from long cooking. Ageing reduced the tensile strength of the myofibrils as measured by resistance to the shearing stresses of homogenisation. Cooking tenderising resulted from a breakdown in the collagen of the interstitial connective tissue. In cooked meat distinct linkages were shown to exist between Z-lines of adjacent myofibrils. The mechanical strength of cooked meat is ascribed to the tensile strength of the fibrous components of muscle and these lateral linkages.     

Alternative pre-rigor foreshank positioning can improve beef shoulder muscle tenderness

Thirty beef carcasses were harvested and the foreshank of each side was independently positioned (cranial, natural, parallel, or caudal) 1 h post-mortem to determine the effect of foreshank angle at rigor mortis on the sarcomere length and tenderness of six beef shoulder muscles. The infraspinatus (IS), pectoralis profundus (PP), serratus ventralis (SV), supraspinatus (SS), teres major (TM) and triceps brachii (TB) were excised 48 h post-mortem for Warner–Bratzler shear force (WBSF) and sarcomere length evaluations. All muscles except the SS had altered (P < 0.05) sarcomere lengths between positions; the cranial position resulted in the longest sarcomeres for the SV and TB muscles whilst the natural position had longer sarcomeres for the PP and TM muscles. The SV from the cranial position had lower (P < 0.05) shear than the caudal position and TB from the natural position had lower (P < 0.05) shear than the parallel or caudal positions. Sarcomere length was moderately correlated (r = − 0.63; P < 0.01) to shear force.     

Comparison of methods for measuring sarcomere length in beef semitendinosus muscle

The objective of the study was to compare the precision of the laser method for measuring sarcomere length with the precision of two oil-immersion microscope methods. Eighteen semitendinosus muscles were assigned to one of three post-mortem treatments to provide a wide range in sarcomere length. Two 5·0cm sections were removed from the centre of each muscle. Each section was subdivided into six equal-size pieces and randomly allotted to each of three participating institutions. Analysis of variance revealed that sarcomere length measurement was not significantly affected by the method of measurement or by the technician. For 99% precision, the laser method required 34 measurements, whereas the two microscope methods required 45 and 66 measurements, respectively.