Thermomechanical properties of beef muscle

Differential scanning calorimetry (DSC) was used to follow the three major endothermic transitions (T(1), T(2) and T(3)) of beef muscle during heating. Borchardt and Daniels reaction kinetics were used to predict the three time and temperature treatments required to sequentially eliminate each transition. Longissimus dorsi and semimembranosus muscles were removed from beef carcasses suspended by Achilles tendon or pelvis. Samples prepared by heating for 5 min at 57°C (I), 70°C (II) and 81°C (III) were assessed by sensory panel for tenderness, juiciness and residual connective tissue. Weight loss, Warner-Bratzler (W-B) shear and microstructure using transmission electron microscopy (TEM) were also determined. The I treatment showed a significant difference in tenderness and residual connective tissue between muscles, but not between contraction states. The II treatment produced collagen shrinkage and a significant drop in W-B shear and residual connective tissue, coupled with increased tenderness in semimembranosus muscle. An increased W-B value, decreased juiciness, increased weight loss and a reduction in sarcomere and A-band length accompanied the III transition. Muscles from carcasses that had been suspended by the pelvis were found to be significantly more tender than the same muscles from Achilles hung carcasses. It is concluded that DSC is capable of determining amount of protein denaturation and, hence, degree of cooking.     

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.     

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.     

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.     

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 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.     

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 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.     

牛背最长肌高铁肌红蛋白还原酶的分离及其酶学性质

以冷鲜牛背最长肌为原料,采用硫酸铵盐析、超滤法对原料肉中的高铁肌红蛋白还原酶进行粗分离,并研究温度、pH、金属离子对高铁肌红蛋白还原酶活性的影响.结果表明:在硫酸铵饱和度65％,超滤(＞50 kDa)条件下,牛背最长肌高铁肌红蛋白还原酶可得到分离与纯化.试验范围内,牛背最长肌高铁肌红蛋白还原酶的分子量为29.0～66....     

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.     

Refractometry of pork muscle and beef connective and adipose tissue

Bulk refractive index (RI) was measured with an Abbe refractometer using a red laser for transmittance (T) and a green laser for reflectance. The critical angle, although obscured by scattering, was detected subjectively at the red-green boundary. The refractometer also was operated under computer control, detecting RI photometrically. Pork longissimus thoracis (n=20) had higher RI than biceps femoris (1.357±0.004 versus 1.352±0.005, respectively, P<0.001). Longissimus thoracis also had lower Japanese pork colour scores (JPCS) than biceps femoris (2.92±0.37 versus 3.87±0.48, respectively, P<0.001). In pooled samples (n=40), RI was correlated with JPCS, r=-0.55, P<0.001. RI of bovine tendon (n=10) was higher than for adipose tissue (1.415±0.009 versus 1.343±0.001, P<0.001). Refraction may contribute to the inverse relationship between meat pH and paleness, and may affect signals from fibre-optic meat probes.     

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.