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.     

Effect of rigor temperature on muscle shortening and tenderisation of restrained and unrestrained beef m. longissimus thoracicus et lumborum

Pairs of muscularis longissimus thoracicus et lumborum (LTL) from young bulls were removed within 1h of slaughter. Small portions of the muscles were placed in a rigormeter to continously follow the isometric tension and isotonic shortening developed, at constant temperatures of 15, 20, 25, 30 and 35°C, as the muscle went into rigor. The bulk LTL was placed in water baths at the same temperature. One of the bulk pairs was tightly restrained by wrapping, to reduce muscle shortening, the other was unrestrained free to shorten. For the bulk samples, shear values were measured using a Warner-Bratzler instrument (1, 7 and 14 days post mortem), and sensory attributes were measured using a sensory panel (7 and 14 days post mortem). Minimum tension and shortening occurred at 15°C. The activation energy for the muscle shortening process was larger than for the isometric tension process. This indicates that the isometric tension data, collected during rigor, does not solely reflect muscle shortening. Thus, a counteracting process that decreases the tension response, most likely ageing is simultaneously detected. Meat that went into rigor at 15°C had least shortening and was always more tender than meat going into rigor at higher temperatures. For meat entering rigor at temperatures higher than 15°C, restraining of the muscle by wrapping, significantly (p<0.05) decreased the amount of muscle shortening and resulted in an improved meat tenderness (p<0.001). It was also observed that at rigor temperatures higher than 15°C the meat tenderness is affected negatively by a reduced ageing capacity. It therefore appears that muscle shortening and enzyme activity both affect tenderness and that both are highly affected by rigor temperature and have the greatest beneficial effect at a rigor temperature of 15°C.     

Tenderness in relation to the temperature of rigor onset in cold shortened beef

Beef sternomandibularis muscle was cold shortened at 2°C for 24 h and then transferred to 37°C until rigor mortis was complete. In spite of a final shortening of 33%, the mean shear value after cooking was identical with that of unshortened meat which had gone into rigor at 15°C. Meat sent into rigor at 2°C with the same degree of shortening had twice the shear value. Thus raising the temperature to 37°C in the final stages of rigor completely nullifies the toughness seen in cold shortened meat, without affecting the shortening. Small changes in cooking loss run parallel to tenderness. The effects are not due to ageing, and may arise from modification of actin–myosin bonding. It is concluded that conditions during the last stages of rigor onset are more important to tenderness than the rest of the post-mortem history of the muscle.     

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.     

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

The course of rigor mortis, ageing and tenderness have been evaluated for two beef muscles, M. semimembranosus (SM) and M. longissimus dorsi (LD), when entering rigor at constant temperatures in the cold-shortening region (1, 4, 7 and 10°C). The influence of electrical stimulation (ES) was also examined. Post-mortem changes were registered by shortening and isometric tension and by following the decline of pH, ATP and creatine phosphate. The effect of ageing on tenderness was recorded by measuring shear-force (2, 8 and 15 days post mortem) and the sensory properties were assessed 15 days post mortem. It was found that shortening increased with decreasing temperature, resulting in decreased tenderness. Tenderness for LD, but not for SM, was improved by ES at 1 and 4°C, whereas ES did not give rise to any decrease in the degree of shortening during rigor mortis development. This suggests that ES influences tenderization more than it prevents cold-shortening. The samples with a pre-rigor mortis temperature of 1°C could not be tenderized, when stored up to 15 days, whereas this was the case for the muscles entering rigor mortis at the other higher temperatures. The results show that under the conditions used in this study, the course of rigor mortis is more important for the ultimate tenderness than the course of ageing.     

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.     

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 rigor temperature and frozen storage on functional properties of hot-boned manufacturing beef

Within 45 min post-mortem, 10mm thick strips of semitendinosus muscle from both unstimulated and high voltage stimulated heifer sides were held at 0, 5, 10, 25 and 35 °C for 24 hr, during which they entered rigor. Half the samples were frozen and stored at -20 °C for one month. The pH, sarcomere length, drip, total (TPS), myofibrillar (MPS) and sarcoplasmic (SPS) protein solubilities, and Hunter L (?), a (?) and b (?) values were determined at 24 hr and on thawed samples. Electrical stimulation did not significantly affect any of the parameters measured. The ultimate pH of samples entering rigor at 10 and 25 °C was lower (p < 0.001) than that of samples held at the other temperatures. Rather surprisingly, there was no significant difference in sarcomere length due to rigor temperature. Samples entering rigor at 35 °C had lower TPS, MPS and SPS values than samples held at 0 to 25 °C (p < 0.001). The MPS increased with rigor temperature up to 25 °C (p < 0.001). Drip and total moisture losses, and Hunter L (?), a (?) and b (?) values also increased with rigor temperature (p < 0.001) whereas SPS decreased and NMR meat water spin-lattice relaxation time (T1) shortened with increasing rigor temperature (p < 0.001). Hue angle and cook loss decreased with rigor temperature in 24 hr samples but increased with rigor temperature in frozen samples. After frozen storage, SPS, T1, cook loss and Hunter L (?), a (?), b (?) values decreased, but TPS, MPS, drip losses and hue angle increased. There were significant (p < 0.05) correlations between SPS, hue angle, drip losses and T1.     

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.     

rigor mortis in beef sternomandibularis muscle at 37°c

Beef Sternomandibularis muscle, restrained from shortening, was more tender if it entered rigor at 37°C than at 15°C. Raising the pre-rigor holding temperature of unrestrained muscle above 15°C resulted in progressively greater shortening above 28°C, and greater tenderness, particularly between 34 and 37°C. Muscles allowed to shorten during rigor at 37°C were actually more tender than if restrained. In M. rectus abdominis, the muscles shortened at 37°C were a little tougher, but much less so than the degree of shortening would indicate. A study of the time course of tenderness changes, and other evidence, indicated that these effects were not due to ageing. Muscles which went into rigor at 37°C showed a slightly higher cooking loss than the 15°C controls and were softer, and usually paler in colour. There was an increased tendency of fibres to slip over each other. Loading experiments with raw muscle strips showed that where rigor occurred at 37°C, the strips yielded and “flowed” at lower loads than the 15°C controls. The various physical changes described indicate that rigor at 37°C differs considerably from rigor at lower temperatures. Some of the changes appear to be a mild form of the “pale soft exudative” condition seen in pork.     

Effects of muscle shortening and proteolysis on Warner-Bratzler shear force in beef longissimus and semitendinosus

The effect of the addition of the fungal protease EPg222 on the sensory characteristics of dry fermented sausage "salchichón" ripened with commercial starter cultures was investigated. Sausages were prepared with purified EPg222 and Staphylococcus carnosus, Staphylococcus xylosus, and Lactobacillus sakei as starter cultures, ripened for 145 days and compared with a control batch only inoculated with the starter cultures. Dry fermented sausages ripened with EPg222 and starter cultures showed higher amount of NPN and volatile compounds derived from amino acid catabolism, than control ripened only with starter cultures. Several branched aldehydes, acids and alcohols such as 2- and 3-methylbutanoic acid and 2-methylpropanol were detected only in enzyme treated samples. Sensory analysis reflected higher values for aroma intensity of sausages treated with EPg222 and lower values of hardness than control. The effect of EPg222 may be of great interest to improve sensory characteristics of dry fermented sausages ripened with starter cultures.     

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.     

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.     

Effect of pre rigor stretching on beef tenderness development

The objective of this study was to determine effects of pre rigor stretching on beef tenderness development. Beef loins (M. longissimus dorsi; n=24) were assigned to either stretching or non-stretching treatments and aged for 14days at -1.5°C. Sarcomere length, shear force, water-holding capacity and proteolysis were determined for the loins at 1, 7 and 14days of ageing. Stretching increased the length of the loins by 33% resulting in a trend of increasing sarcomere length (P=0.19). However, no significant differences for shear force values, purge, drip and cooking losses between treatments were found. Shear force values decreased with ageing times (P<0.05). Western blot assay found that stretching did not affect the extent of desmin degradation. The findings from the present study suggest that pre rigor stretching with the current increase in length will not contribute to tenderness improvement of the beef loins.     

Effect of pre rigor stretching on beef tenderness development

The objective of this study was to determine effects of pre rigor stretching on beef tenderness development. Beef loins (M. longissimus dorsi; n = 24) were assigned to either stretching or non-stretching treatments and aged for 14 days at − 1.5 °C. Sarcomere length, shear force, water-holding capacity and proteolysis were determined for the loins at 1, 7 and 14 days of ageing. Stretching increased the length of the loins by 33% resulting in a trend of increasing sarcomere length (P = 0.19). However, no significant differences for shear force values, purge, drip and cooking losses between treatments were found. Shear force values decreased with ageing times (P < 0.05). Western blot assay found that stretching did not affect the extent of desmin degradation. The findings from the present study suggest that pre rigor stretching with the current increase in length will not contribute to tenderness improvement of the beef loins.     

Cold shortening and cooking changes in beef

Quantitative relationships have been established between pre-rigor shortening in beef sternomandibularis muscle and cooking shrinkage and weight loss. Shortening induced by cold increases cooking weight loss and shrinkage across the meat grain, but reduces shrinkage along the fibre direction. The mechanism of fluid discharge from meat on cooking is determined by the degree of pre-rigor shortening. In unshortened meat, cooking shrinkage along the fibre precedes discharge and transverse swelling occurs to accommodate entrapped fluids. During the cooking of highly shortened meat, fluid discharge occurs mainly across the fibres, the sarcolemmas of which presumably have been split during shortening. Consequently early swelling on cooking does nccur.