The influence of high temperature, type of muscle and electrical stimulation on the course of rigor, ageing and tenderness of beef muscles

The course of rigor mortis (rigor), ageing and tenderness has been evaluated for three beef muscles; M. biceps femoris (BF), M. semimembranosus (SM) and M. semitendinosus (ST), when entering rigor at constant temperatures of 15 and 37°C respectively, with and without electrical stimulation (ES/NS) (85 V, 14 Hz and 32 s). The course of post-mortem changes has been registered by isometric tension, by shortening of unrestrained muscle strips and by following the pH decline and the changes in metabolites, such as ATP and CP. Ageing at +4°C was recorded by measuring Warner-Bratzler (W-B) shear values 2, 8 and 15 days post mortem. On the last occasion, the sensory properties of the cooked meat were also evaluated. Maximum shortening and isometric tension were higher at 37°C as compared to 15°C, whereas ES did not reduce rigor shortening. A high correlation between maximum shortening and the ATP-level at the onset of the shortening rapid phase was found (r = 0·77(???)), which could explain the greater shortening obtained at 37°C compared to 15°C. Rigor shortening is an important phenomenon governing meat tenderness as tenderness is highly affected by rigor temperature but not by ES. This was the case for muscles SM and ST but not for BF muscle. Even though tenderness was measured after ageing (15 days post mortem), shortening during rigor seems to be more important for toughness when rigor mortis occurs at 37°C than any suggested tenderizing effect due to increased proteolysis in this temperature region.     

The effects of electrical stimulation and ageing on beef tenderness

Seventeen beef carcasses from cattle with a range of breeds, ages and body conditions were used in this trial. The four treatments applied to each carcass were control (C), electrical stimulation (ES), ageing for 28 days (A) and electrical stimulation plus ageing for 28 days (ES + A). Post-mortem muscle pH was measured at 0, 0·5, 4 and 24h post-stimulation. Significantly lower muscle pH values (P < 0·01) were achieved by the stimulated carcass side sompared to the unstimulated side at 0·5 (pH 6·47 vs. 6·91) and 4 h (pH 5·96 vs. 6·44) post-stimulation.

Warner-Bratzler shear and taste panel methods were used to assess the tenderness of Longissimus dorsi muscle samples from each of the four treatments. The ES, A and ES + A treatments were significantly more tender (P < 0·01) than the control treatment. The ES and the A treatments resulted in a similar improvement in tenderness compared to the control. The ES + A treatment was significantly more tender (P < 0·01) than the ES treatment alone, but there was no significant difference in tenderness between the A and the ES + A treatments.     

High and low rigor temperature effects on sheep meat tenderness and ageing

Immediately after electrical stimulation, the paired m. longissimus thoracis et lumborum (LT) of 40 sheep were boned out and wrapped tightly with a polyethylene cling film. One of the paired LT's was chilled in 15°C air to reach a rigor mortis (rigor) temperature of 18°C and the other side was placed in a water bath at 35°C and achieved rigor at this temperature. Wrapping reduced rigor shortening and mimicked meat left on the carcass. After rigor, the meat was aged at 15°C for 0, 8, 26 and 72 h and then frozen. The frozen meat was cooked to 75°C in an 85°C water bath and shear force values obtained from a 1×1 cm cross-section. The shear force values of meat for 18 and 35°C rigor were similar at zero ageing, but as ageing progressed, the 18 rigor meat aged faster and became more tender than meat that went into rigor at 35°C (P<0.001). The mean sarcomere length values of meat samples for 18 and 35°C rigor at each ageing time were significantly different (P<0.001), the samples at 35°C being shorter. When the short sarcomere length values and corresponding shear force values were removed for further data analysis, the shear force values for the 35°C rigor were still significantly greater. Thus the toughness of 35°C meat was not a consequence of muscle shortening and appears to be due to both a faster rate of tenderisation and the meat tenderising to a greater extent at the lower temperature. The cook loss at 35°C rigor (30.5%) was greater than that at 18°C rigor (28.4%) (P<0.01) and the colour Hunter L values were higher at 35°C (P<0.01) compared with 18°C, but there were no significant differences in a or b values.     

The protective effect of electrical stimulation and wrapping on beef tenderness at high pre rigor temperatures

A three factorial experimental design involving electrical stimulation (ES/NES), wrapping (wrapped/unwrapped) and pre rigor temperature (15 °C or 35 °C) was applied to 70 beef M. longissimus lumborum muscles to obtain a wide variation in shear force and drip loss. The shear force of all treatment groups decreased during ageing. As anticipated, wrapping and electrical stimulation had positive effects on shear force. However, high pre rigor temperature (35 °C) did not result in higher shear force values if the muscles were electrically stimulated, wrapped or both. The results suggested that electrical stimulation protects against the negative effects of high pre rigor temperatures. The drip loss of all treatment groups increased during ageing in a manner that was unrelated to treatment but was correlated to tenderness (r² = 0.70; p < 0.0001). It was concluded that the application of electrical stimulation, whatever the pre rigor temperature, protects beef from toughening through the prevention of rigor shortening and the avoidance of inhibition of ageing enzymes.     

High pre rigor temperature limits the ageing potential of beef that is not completely overcome by electrical stimulation and muscle restraining

Two simultaneous trials were conducted to determine the effects of electrical input [electrical stunning and stimulation (ES)], wrapping, pre rigor temperature (15 °C and 38 °C) and different post rigor chilling rates on beef quality using M. longissimus lumborum (n=100). The high pre rigor temperature induced a faster pH decline than ES. The loins at 38 °C had significantly greater protein denaturation, more purge and drip loss, higher shear force values and less desmin degradation compared with the loins at 15 °C. No difference in sarcomere length was determined between the pre rigor temperatures regardless of ES and wrapping. Different post rigor chilling rates did not play a substantial role in water-holding capacity, proteolysis, or shear force values during ageing. These results suggest that high pre rigor temperature induces temperature-related toughness of muscle due to protein denaturation with subsequent limitation of proteolysis by μ-calpain, regardless of ES and wrapping treatments.     

Effects of electrical stimulation and temperature on beef quality and tenderness

Beef carcasses were stimulated with either 50 or 500 V and exposed to three chilling temperatures during the first 3 to 9 h post mortem. Both high (HV) and low voltage (LV) electrical stimulation (ES) treatments improved lean maturity scores, lowered the pH values at 2 h through 12 h and reduced the time required for the ATP level to reach minimum values. Both HV-ES and LV-ES treatments influenced firmness, fragmentation and connective tissue (CT) residue as measured by sensory panel and reduced Instron shear value (ISV) of loin steaks. The 30°C temperature treatment had an adverse effect while the 20°C temperature treatment had a beneficial effect on ISV and panel ratings for firmness and CT residue. There was an interaction between conditioning treatments and ES for ISV and sensory panel traits. The LV-ES-20C treatment resulted in the lowest ISV and superior panel ratings; however, the LV-ES-30C treatment had ISV and sensory panel ratings that were similar to the NES-2C (control) group.     

The influence of temperature on shortening and rigor onset in beef muscle

At sufficient ATP concentration and temperatures below about 15°C, pre-rigor beef muscles (neck muscles) contract; this phenomenon is known as cold shortening. There is also a contracture at higher temperatures occurring just before rigor onset which is called rigor shortening. While rigor shortening starts in neck muscles at pH around 6·3–6·0 and at about 2 μMol ATP/g muscle, cold shortening can begin at pH around 7·0 and the full ATP concentration (4 μMol ATP/g) in the muscle. Shortening can take place as long as there is no irreversible formation of the actomyosin complex in the muscle, i.e. before rigor onset occurs, which can be measured by intermittent loading of the muscle. The degree of extensibility which follows starts to decrease at the moment of rigor onset. This irreversible loss of extensibility at temperatures between the freezing point (?1°C) and physiological temperatures (38°C) starts at various pH values and ATP concentrations in the muscle. At 38°C the rigor onset occurs at pH 6·25 and about 2 μMol ATP/g muscle, dropping at 15°C to pH 5·75 and 1 μMol ATP/g muscle. At 0°C, as at all temperatures below 10°C, the loss of extensibility at medium loads (about 250 g/cm²) begins shortly after cold shortening. This loss of extensibility is reversible by increasing the load or raising the temperature. The irreversible loss, or rigor onset, however, occurs at 0°C with pH of 6·1–6·2 and 1·8–2·0 μMol ATP/g muscle. Thus, the onset of rigor is influenced by more than one factor. Temperature, pH and ATP concentration each play a rôle.Maximum loss of extensibility or completion of rigor is reached between 10°C and 38°C at pH 5·5–5·6 and less than 0·5 μMol ATP/g muscle. At 0°C the completion of rigor takes place at pH 6·0, but still at 0·5 μMol ATP/g muscle. The latter fact shows that the completion of rigor is solely dependent on the ATP concentration in the muscle; nevertheless, the pH of rigor completion is higher in the extreme cold shortening range. This is apparently due to a different pH/ATP relationship in muscles at low temperatures.The results are discussed in terms of changes in the concentration of Ca²⁺ ions and ATP.The results are of particular interest for the handling of hot-boned meat; that is, for both the cooling of pre-rigor muscle and the use of hot-boned meat for processing.     

Effects of electrical stimulation, chilling temperature and hot-boning on the tenderness of bovine muscles

The objective of this study was to determine the effects of hot-boning, low voltage electrical stimulation (ES) and chilling temperature on the tenderness of bovine M. longissimus dorsi (LD) and M. semimembranosus (SM) muscles. LD (n=32) and SM (n=32) muscles were subjected to different post-mortem treatments; hot-boning (before 90min post-mortem), cold-boning (at 48h post-mortem), low voltage ES and rapid or slow chilling. Hot-boned muscles which were not electrically stimulated (NES) had higher Warner Bratzler shear force (WBSF) values (P<0.001) and shorter sarcomeres (P<0.001) than cold-boned muscles. Hot-boned muscles subjected to ES had lower pH values (P<0.001) post-stimulation and up to 8h post-mortem than NES muscles. At both chilling temperatures WBSF values were lower in ES hot-boned LD and SM muscles at days 2, 7 and 14 post-mortem than NES muscles. Hot-boned muscles subjected to slow chilling had longer sarcomeres (P<0.001) than those subjected to fast chilling. In hot-boned SM muscles, ES resulted in longer sarcomere lengths (P<0.001). However, ES did not have a significant effect on the sarcomere length of LD muscles. As indicated by WBSF values all muscles tenderised during ageing, including muscles which were 'cold shortened'. Proteolysis was not the main reason for differences in WBSF values between ES and NES muscles as judged by qualitative sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). A combination of slow chilling and ES had a positive effect on hot-boned muscles with respect to WBSF values.     

Yield point in raw beef muscle. The effects of ageing, rigor temperature and stretch

The yield point of raw sternomandibularis muscle of the ox decreased markedly with ageing. This parameter is the most sensitive and selective indicator of ageing since, unlike shear measurements on cooked meat, it is not complicated by heat denaturation or the contribution of the collagen net. Rigor at 2°C with consequent cold shortening has little effect on yield point, but rigor at 37°C diminishes yield values relative to the 15°C controls. Muscles stretched by 40-60% during rigor show higher yield points. Yield was also studied in other muscles. Unaged strips of bull sternomandibularis, or steer psoas and rectus abdominis tended to break rather than yield, but after ageing usually yielded at the same low loads as aged ox sternomandibularis. The histological changes due to yielding varied widely, but stretched, rather than broken, I-bands were the dominant feature. Our interpretation of the electron micrographs is that in rigor muscle, actin filaments fracture while gap filaments stretch, but in aged muscle both sets of filaments fail simultaneously at low loads.     

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.     

Post-mortem electrical stimulation and high temperature ageing of hot-deboned beef

The effects of electrical stimulation (80-100 V, 15 pps) of hot-deboned, bovine Longissimus dorsi muscles, followed by ageing at 30° or 40°Cfor 5, 7 or 10 h, on the solubility of sarcoplasmic and myofibrillar proteins, soluble non-protein nitrogen, and on the water-holding capacity, colour, tenderness and microbiological status of the meat, was assessed. Comparisons were made with the same traits measured on non-stimulated controls (average muscle temperature, 5°C), which were cold-deboned post rigor, and with hot-deboned, electrically stimulated muscles, subsequently held at 2°C. For each treatment and its corresponding control, six muscles each were studied. Such electrical stimulation produced a typical acceleration of pH fall post mortem and enhanced ageing changes at both 30°C and 40°C. It significantly increased tenderness over non-stimulated, cold-deboned controls. The higher temperature of ageing, however, was significantly associated with adverse colour development, loss of water-holding capacity and increased microbial growth. Electrical stimulation alone would not obviate the need for immediate refrigeration in hot-deboning operations with ambient temperatures of ～ 40°C.     

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.     

The effect of time and type of electrical stimulation on the calpain system and meat tenderness in beef longissimus dorsi muscle

Effects of type and time of electrical stimulation on the calpain system, sensory and objective meat quality in the M. longissimus thoracis et lumborum from 38 pasture-fed carcasses, were investigated under conventional chilling conditions. High voltage stimulation was applied to whole carcasses at 3 min post-mortem (pm) and to sides at either 40 or 60 min pm, whilst low voltage stimulation was applied to whole carcasses at 3 min pm and to sides at 40 min pm. Unstimulated sides served as controls. The levels of extractable μ-calpain and calpastatin decreased during stimulation by 28-44% and 8-17%, respectively. Shear force and adjusted tenderness score showed that stimulation at 3 min, irrespective of type of stimulation, resulted in significantly tougher meat (P<0.05) which was associated with an rapid rate of pH decline, compared to stimulation at 40 min. Higher calpastatin levels soon after stimulation at 3 min (P < 0.05) and lower μ-calpain level at 24 h pm for high voltage stimulation at 3 min (P<0.05) coincided with the tougher meat. On the other hand, high voltage stimulation at 40 and 60 min resulted in similar tenderness and levels of μ-calpain and calpastatin post-stimulation and 24 h pm. Significantly tougher meat from the control sides, with a higher μ-calpain levels at 24 h pm and similar sarcomere length, compared to those from low voltage stimulation at 40 min (P<0.001), appeared to be linked to the later activation of the calpain system. Results from the current study suggest that early application of stimulation may be associated with a very rapid decline in pH and consequently a reduction in meat quality.     

The influence of high post-mortem temperature and differing ultimate pH on the course of rigor and ageing in pig Longissimus dorsi muscle

This study was performed in order to assess the effect of temperature and differing ultimate pH (pH_u, 24 h post mortem) on the development of rigor mortis in pig Longissimus dorsi muscle. The rigor development (isometric tension and shortening) was measured continuously during the first 24 h post mortem, using an apparatus wherein muscle strips were held at constant temperatures of 12 or 35°C. pH_u was manipulated by adrenaline injections preslaughter.

The rates of pH fall, adenosine triphosphate (ATP) and creatine phosphate (CP) breakdown were markedly increased at 35°C compared to 12°C. For both temperatures, no delay phase was observed with regard to the development of shortening. Rigor resulted in higher maximum isometric tension and shortening and in shorter time needed to reach maximum values at 35°C than at 12°C. The results are discussed in connection with pH, ATP and CP data. The extent of ageing from 2 to 4 days post mortem, estimated through myofibrillar length determinations, was higher for 12°C than for 35°C.

pH_u affected significantly most of the traits under study, but its effect depended in some cases upon the rigor temperature. At 12°C, the traits related to the kinetics of rigor development were significantly affected by pH_u, but this was not the case at 35°C. Maximum isometric tension was significantly related to pH_u at 35°C (r = 0·86, P < 0·001), but such a relationship was not found at 12°C. Myofibrillar lengths were significantly affected by pH_u, but in an opposite manner from one temperature to another. A positive relationship was found at 12°C and a negative one at 35°C.

These results illustrate the importance of the interaction between the extent of pH fall and temperature with regard to post-mortem changes in pig muscle.     

Tensile properties of cooked beef in relation to rigor temperature and tenderness

A study of the tensile properties of cooked strips from beef sternomandibularis which had gone into rigor at various temperatures showed no clear relationships with earlier observed variations in shear force with rigor temperature. A consistent feature of the load extension curves was a sudden increase in extensibility at a ‘change point’ of about 1 kg/cm². Some curves, particularly those for strips cooked under restraint, showed a third intermediate extensibility. Evidence points to modification of a myofibrillar component as the reason for the sudden changes.     

Effect of electrical stimulation, high temperature conditions and ageing on muscle myofibrillar proteins

Electrical stimulation, when accompanied by high temperature incubation, was found to lower the extractability of myofibrillar proteins and increase their degradation, particularly that of the myosin component. Troponin T and myosin light chain 2 were similarly affected. Troponin T and troponins I and C were degraded earlier during ageing in electrically stimulated muscles incubated at high temperature than in conventionally chilled muscles.     

Effect of electrical stimulation and temperature on biochemical changes in beef muscle

The effect of low voltage electrical stimulation and storage temperatures of 2, 15 and 25°C on biochemical changes in M. sternocephalicus from beef carcasses was studied. It was shown that electrical stimulation accelerated the glycolytic processes resulting in an immediate increase in muscle lactate and about 30% reduction in ATP. Combination of stimulation and 15 or 25°C temperature further increased the changes with ATP, reaching 2 μM/g muscle in about 2 h post mortem. At 2°C, the biochemical changes were slow. By ensuring that the muscle temperatures in low voltage stimulated carcasses do not fall below 15°C for 2 to 3 h, this should provide favorable conditions for depletion of ATP and glycogen and the prevention of cold shortening at chilling.     

The biochemical and physical effects of electrical stimulation on beef and sheep meat tenderness

Application of electrical stimulation in the sheep and beef processing industry has been erratic around the world and this may reflect an incomplete knowledge of how to optimise the technology. Although it is well established that stimulation increases the rate of post-mortem glycolysis, other biochemical and biophysical effects have been implicated with the use of this technology. This review seeks to examine the current theories about the effect of stimulation on post-mortem muscle. The classical view that stimulation prevents muscle from shortening excessively during rigor development has been expanded to include the possibility that it also results in physical disruption of muscle structure. The interaction of these effects with the acceleration of the rate of proteolysis through activation of the calpain protease system has not been comprehensively reviewed in the past. Thus there are two mechanisms which could explain the effect of stimulation on tenderisation, reduced 'cold-induced' shortening and alteration of protein structure. A secondary effect is the enhancement of the rate of proteolysis stimulated by release of Ca (2+) at a higher temperature. As a result of this review we highlight several areas that may prove fruitful for further research. The challenge for further development of electrical stimulation systems is optimisation of the activation of the enzyme systems in parallel with manipulation of chilling regimes so as to ensure rigor mortis is achieved at temperatures which minimise shortening. These optimal temperatures largely established at a fixed incubation temperature for detached muscle may be different when measured in intact carcasses. The potential of regional stimulation of sections of the carcass to achieve this outcome is worthy of study given the different fibre composition of muscles and temperature gradients. In addition, to ensure that appropriate amounts of energy are applied to individual carcasses, development of self-response stimulation units, which are able to determine carcass resistance and apply appropriate durations or strengths of stimulation is worthy of future research. This would lead to more effective electrical stimulation practices.