B.B. Marsh

Beef tenderness and sarcomere length

A wide range of muscle glycolytic rates was produced in 60 beef carcasses by applying different forms and periods of electrical stimulation immediately after decapitation; a further seven carcasses were not stimulated. The sides were subjected to normal chilling, and at 48 h one short loin per carcass was taken for tenderness evaluation and determination of sarcomere length (SL). In the 19 relatively slow-glycolysing loins-those of 3-h pH (pH(3)) above 6·3-tenderness extended over almost five panel units on a 1-8 scale, and included both the tenderest two and the toughest 10 muscles of the entire study. In the 48 loins of pH(3) below 6·3, on the other hand, tenderness ranged over only 2·5 units, and on average was a full panel unit higher than that of the high-pH(3) muscles. The most striking difference between the two groups, however, was in the relationship between toughness and shortening; the correlation of panel tenderness on SL was remarkably high in the slow glycolysers (r = 0·84), but negligible in those of faster pH decline (r = 0·16). Thus although shortening occurred to about the same extent and over the same range in both groups, it influenced tenderness and tenderness variability only when glycolysis was quite slow. A loin pH(3) of below 6·3, however, is unusual in non-stimulated carcasses, so slow glycolysis and the very wide tenderness diversity accompanying it must be expected in commercial operations that do not use electrical stimulation.

Early-postmortem cooling rate and beef tenderness

Four distinct early-postmortem cooling rates (as measured within the longissimus muscle) were obtained by exposing one side of each of five fat and five lean steers to moving air at -2° and the other to 9° static air. Loin steaks of the fastest cooling group were found to be the least tender and to have the shortest sarcomeres; the three remaining groups differed significantly from each other in tenderness but not in sarcomere length. For all twenty sides, regardless of treatment group, tenderness was highly dependent on, and almost linearly related to, the muscle temperature attained at 2 hours post mortem (27-40°), the relationship deteriorating rapidly as longer time intervals and lower temperature ranges were considered. The results indicate that (1) except in very rapidly chilled lean carcasses, cold shortening is not a significant determinant of tenderness; (2) the enhanced tenderness of slowly chilled beef is not due primarily to the relatively prolonged avoidance of shortening-inducive temperatures but to the accompanying retardation of cooling during the first 2-4 hours post mortem, when muscle temperatures are still far above those associated with cold shortening and (3) the generally recognised superior tenderness of well-finished beef is largely (and perhaps entirely) a consequence of slower cooling during this very early-postmortem period.

Effects of early-postmortem glycolytic rate on beef tenderness

Electrical stimulation (ES) was applied at 500 V to one side from each of 60 beef carcasses at 30 to 40 min postmortem. Wide ranges of early-postmortem glycolytic and cooling rates were produced in the musculature by use of three different forms of ES (in addition to non-ES treatment of the contralateral sides) and application of chilling routines of four different degrees of severity. Panel and Warner-Bratzler shear evaluations of loin steaks from all 120 sides showed that tenderness was highest when glycolysis had proceeded at an intermediate rate (corresponding to the attainment of a 3-h pH of about 6·1) and was appreciably lower on both sides of this mid-value. The toughening effect of rapid glycolysis (relative to that of a moderately increased glycolytic rate) persisted through 14 days of aging at 2°C. These observations appear to be incompatible with the view that lysosomal enzymes contribute significantly to ES-induced tenderization. They also indicate that the effect of ES on tenderness is highly dependent on the subsequent cooling rate, very slow chilling sometimes accelerating the already high rate of pH fall to such an extent that the tissue is significantly toughened. Finally, they suggest that the goal of maximizing the early-postmortem rate of pH decline in bovine muscle is misguided and, if attained, will cause sub-optimal tenderness.

Effects of early post-mortem pH and temperature on beef tenderness.

The tenderness of loin steaks, prepared from beef sides after chilling, is strongly influenced by muscle temperature in the first 3 h after slaughter. Maintenance of about 37°C within the Longissimus muscle during this time, whether by heavy fat cover or by ambient-temperature manipulation, results in appreciable tenderness enhancement. Early post-mortem muscle pH (which varies over a wide range) also affects tenderness significantly; provided early and exceptionally fast chilling does not induce cold shortening, slow glycolysis promotes tenderness. Low-frequency (2 Hz) electrical stimulation, which accelerates glycolysis but causes negligible tissue disruption, significantly toughens the loin; in its normal mode (50-60 Hz), therefore, stimulation produces its desirable tenderising effect mainly-and perhaps solely-by fibre fracture.

Effects of low-frequency electrical stimulation on beef tenderness.

The application of 2 Hz, 500 V electrical stimulation to early post-mortem beef sides causes rapid muscle glycolysis, yet produces no trace of the extensive tissue rupture effected by 60 Hz current. By means of this low frequency technique, coupled with a mild or delayed-chill routine, we have examined the supposed tenderizing effect of rapid pH decline alone, without concurrent fiber fracture or cold-shortening intervention. Loin steaks from sides receiving this treatment were significantly tougher than those from paired control sides, an observation strongly suggesting that tenderization is hindered by rapid glycolysis. In other experiments, 60 Hz 500-V current was found to produce very extensive fracturing with breaks appearing (on average) every 6 mm of fiber length; this treatment resulted in a very significant tenderizing relative to that observed in the unstimulated control sides. It is concluded that, apart from its ability to prevent cold-shortening, normal-frequency high-voltage stimulation exerts its beneficial tenderizing action by fracturing the fibres; the accompanying acceleration of glycolysis actually causes a small, but definite, negative effect on eating quality.

Effects of 2-Hz and 60-Hz electrical stimulation on the microstructure of beef.

A comparative electron-microscope study was made of the structural changes brought about in bovine longissimus dorsi muscle by 2-Hz and 60-Hz electrical stimulation. Apart from the frequency difference, the two treatments were identical: 500V AC, 600 pulses (2Hz × 300s and 60 Hz × 10 s), and a time of application of about 40 min post mortem. The low-frequency current produced no detectable tissue disruption or irregularity in sarcomere length, whereas the 60-Hz treatment caused widespread supercontraction and the formation of contraction nodes; many sarcomeres in the internodal zones were stretched or fractured. The observations confirm and extend those of our earlier light-microscope study, and support the view that fiber disruption contributes significantly to the tenderizing effect of high-voltage electrical stimulation.

Low voltage electrical stimulation of lamb carcasses: effects on meat quality.

The effects of an early post mortem low voltage electrical stimulation (28 V, 60 Hz) on biochemical changes and on final tenderness in muscles Longissimus thoracis et lumborum and Semimembranosus from lamb carcasses were studied. It was shown that electrical stimulation accelerated the glycolytic process resulting in a significant fall in pH during the first 6 h post mortem in both muscles examined and in a significant reduction in adenosine triphosphate (ATP) content in muscle Longissimus thoracis et lumborum. The effect of electrical stimulation on tenderness was recorded by measuring shear force values 2 and 7 days post mortem. Tenderness was significantly improved by electrical stimulation for the Longissimus thoracis et lumborum both at 2 and at 7 days post mortem, while for Semimembranosus electrical stimulation significantly increased tenderness just at 7 days post mortem.

Purification of skeletal-muscle mitochondria by density-gradient centrifugation with Percoll

Abstract Mitochondria prepared from skeletal muscle are typically contaminated with sarcoplasmic reticulum fragments and salt-soluble proteins. These contaminants can be removed by density-gradient centrifugation in Percoll. The resulting mitochondria retain both their original state three respiratory rates and their respiratory control ratios. The method has also been adapted to prepare mitochondria from very small tissue samples.

Structural studies of rigor bovine myofibrils using fluorescence microscopy. II. Influence of sarcomere length on the binding of myosin subfragment-1, alpha-actinin and G-actin to rigor myofibrils

Sarcomere length influences the textural quality of meat, yet the molecular basis for the mechanism of post-mortem shortening and the toughness associated with shortened muscle remain obscure. Bovine and rabbit myofibrillar structure was investigated over a range of sarcomere lengths (SL), using the high affinity of the myosin head for actin and of alpha-actinin for the Z-line. Myofibrils were incubated with fluorescent conjugates of myosin subfragment-1 (S1), alpha-actinin and actin. When S1 and alpha-actinin were added together, S1 bound in the I-band and A-band but not at the Z-line, while alpha-actinin bound at the Z-line. The pattern of S1 binding was highly influenced by SL, and could be explained using a model with the ratio of myofibrillar actin to myosin heads in the overlap region of 2:1 and thin filament penetration into opposing half sarcomeres. Double staining with S1 and actin demonstrated that, once the tip of the thin filament reached the bare zone, few intrinsic myosin heads were available for fluorescent actin. The patterns observed for both S1 and actin staining suggest that myofibrillar rigor bonds form even at very short SL. These observations lead to the hypothesis that the toughness associated with short sarcomeres is due to thick-filament interactions and not to the density of rigor bonds in the myofibril. Regulation of S1 binding was investigated by incubating myofibrils with low levels of fluorescent S1 in the presence and absence of calcium; S1 binding was in the overlap region when calcium was absent, but in the I-band when it was present. These results suggest that the thin filament can be activated for muscle shortening by the binding of myosin heads, a mechanism that may contribute to post-mortem muscle shortening.

Electrical-Stimulation Research: Present Concepts and Future Directions

Many discoveries in science have lain unused and dormant for years until their significance was recognized and they could be used in practical situations. Recent and continuing advances in electronics technology and the spectacular progress anticipated in genetic engineering exemplify this “learn now, apply later” category, for, without prior knowledge on which they could be constructed, these subjects would not exist at all.