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.     

Modelling the effect of sarcomere length on collagen thermal shortening in cooked meat: consequence on meat toughness

Normal and contracted pieces of Semimembranosus and Longissimus Dorsi muscles from cull cows were cooked for 90 min at temperatures up to 80°C. For both muscles, at 50°C the normal samples have higher breaking stress than contracted samples. The breaking stress of normal samples decreases at 55°C. This decrease is not observed for contracted samples. The contracted samples become the tougher above 60°C. Drip and cooking losses are the highest in contracted samples. Sarcomere length decreases above 60°C whatever the raw sarcomere length. The amplitude of thermal shortening of perimysium collagen fibres in cooked meat has been calculated. This theoretical model takes into account the changes in the waviness of collagen fibres associated with changes in raw sarcomere length and the geometrical changes of fibre bundles due to drip, cooking losses and cooking shortening. The calculations lead to the conclusion that thermal shortening of collagen fibres at 60°C is lower in contracted samples than in normal samples. As the final modulus of collagen fibres decreases when their thermal shortening increases, this can explain part of the differences observed between the toughness of normal and contracted cooked meats. In particular, it can explain why contracted cooked meat becomes tougher than normal meat just above 60°C and why there is a decrease in normal meat toughness between 55 and 60°C. This work therefore emphasises the role of collagen in toughening associated with cold shortening.     

FIBRE FRACTURE OF BOVINE M. PECTORALIS PROFUNDUS MUSCLE COOKED PRE- AND POSTRIGOR

The ultimate tensile strength and extensibility in the direction of the muscle fibres of cooked meat decreased exponentially during storage of the raw meat for up to 3 days at 15C. The mode of fracture was examined by light microscopy of longitudinal sections taken after testing to break. In meat cooked soon after stunning, 'brittle' fracture had been induced across the fibres and the high extensibility was proposed to be due to sequential fibre fracture. At 1 day postmortem, the fractures crossed several fibres occurring in a repetitive fashion every 30 μm along the fibres. This was thought to be due to a stiff fibre embedded in an extensible surface matrix connecting the fibres. In aged meat, discontinuous fractures were prominent within each fibre and which stopped at the surface of each fibre showing further weakening of the fibre and the composite matrix. All the myofibrils remained in register. These changes are consistent with a gradual reduction in cohesion within the meat and indicates that the weakening of the focal adhesions is primarily responsible for postmortem tenderisation.     

Collagen contribution to meat toughness: Theoretical aspects

One of the major changes in connective tissues during heating is the transformation of the quasi-crystalline structure of collagen into a random-like structure. This molecular change induces a shortening of these tissues and gives them a rubber-like behaviour. In this state, their mechanical properties are dependent on the total number of cross-linked chains present per volume, which can be estimated from the number and the functionality of each cross-link present in the sample. The number of cross-linked chains per volume of meat explains a large amount of the tenderness variation, produced by muscle type, animal age, type, and sex in different species. During heating collagen fibres and fibrils shortening produces a pressure which is also dependent on the total amount of cross-linked chains present per volume, but also on the morphology of endomysial and perimysial envelopes. In meat, during heating, collagen fibres and fibrils thermal shortening is restricted by muscle fibres and muscle fibre bundles. This restriction, which depends on several muscle fibre characteristics, has a strong effect on the final elastic modulus of connective tissues, by changing the respective amount of crystalline and rubber-like fractions in collagen fibres and fibrils after heating. The implications of this phenomenon in tenderness variations are discussed.     

Collagen contribution to meat toughness: Theoretical aspects

One of the major changes in connective tissues during heating is the transformation of the quasi-crystalline structure of collagen into a random-like structure. This molecular change induces a shortening of these tissues and gives them a rubber-like behaviour. In this state, their mechanical properties are dependent on the total number of cross-linked chains present per volume, which can be estimated from the number and the functionality of each cross-link present in the sample. The number of cross-linked chains per volume of meat explains a large amount of the tenderness variation, produced by muscle type, animal age, type, and sex in different species. During heating collagen fibres and fibrils shortening produces a pressure which is also dependent on the total amount of cross-linked chains present per volume, but also on the morphology of endomysial and perimysial envelopes. In meat, during heating, collagen fibres and fibrils thermal shortening is restricted by muscle fibres and muscle fibre bundles. This restriction, which depends on several muscle fibre characteristics, has a strong effect on the final elastic modulus of connective tissues, by changing the respective amount of crystalline and rubber-like fractions in collagen fibres and fibrils after heating. The implications of this phenomenon in tenderness variations are discussed.     

Mechanical and structural characteristics of single muscle fibres and fibre groups from raw and cooked pork longissimus muscle

The aim of this study was to analyse the mechanisms that determine why small groups of muscle fibres may have different mechanical properties than single muscle fibres. The method used combined light microscopy and tensile testing on single fibres and small groups of fibres from raw and cooked (80 °C) meat, from both conditioned and unconditioned porcine longissimus muscle. The results showed that small groups of fibres had different breaking properties than constituent single fibres in raw muscle, but that these differences diminished on cooking. Raw groups of fibres showed a more uniform lengthening along their entire length and a higher extension to rupture than single fibres. Conditioning increased maximum strains in both single fibres and small fibre groups. In unconditioned cooked meat, single fibres and fibre groups showed comparable breaking stresses and extensions. Conditioning resulted in a lower strength in fibre groups than in single fibres. These results show that (endomysial) connective tissue linkages between adjacent muscle fibres in a small group significantly alter the breaking behaviour of single fibres. The effects of these connective tissue linkages are not reduced by conditioning alone, but are largely diminished by cooking to 80 °C.     

Tensile strength of heat-coagulated myosin fibres

Myosin fibres, formed by heat-coagulation under conditions of pH and ionic strength similar to those prevailing in post-rigor meat, exhibited tensile strengths which decreased with fibre diameter. Tensile strengths of the smaller diameter fibres (10-100 μm) ranged from 30 to 3 kg/cm(2). These values resemble breaking strengths reported for cooked meat (e.g. Davey & Gilbert, 1977) but are an order of magnitude higher than strengths reported for heat-set myosin gels (Nakayama & Sato, 1971) and myosin-bonded meat pieces (Macfarlane et al., 1977).     

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 histology of blood splash in lamb

Ecchymosis (blood splash), can occur in the carcasses of animals that have been stunned electrically or shot with a captive bolt. While blood splash presents no health hazard, it is likely to detract from the acceptable appearance of the meat. Based on light and electron microscopic studies, ecchymosis has been shown to be due to the localised discharge of blood from ruptured blood vessels into the surrounding muscle tissue. A small proportion of muscle fibres in a splashed region were distorted ( 5 %) with irregular zones of high shortening interspersed with zones of lesser shortening. Such ‘supercontracture’ and its associated blood splash was markedly reduced by neuromuscular blocking agents. It is proposed that through supercontracture of some fibres, a severe strain is placed on adjacent blood vessels, occasionally leading to their rupture and hence to haemorrhaging.     

The effect of cooking temperature on mechanical properties of whole meat, single muscle fibres and perimysial connective tissue

The structural changes in beef semitendinosus caused by cooking were studied by performing tensile tests of the isolated meat components (i.e. single muscle fibres and perimysial connective tissue) and related to the toughness of the whole meat. Whole meat toughness was found to increase in two separate phases upon cooking from 40-50°C, and again from 60 to 80°C with a decrease in meat toughness between 50 and 60°C, in agreement with previous studies. The changes in whole meat toughness at temperatures below 60°C were found to correspond to changes in the mechanical properties of the perimysial connective tissue, whereas changes of whole meat toughness at temperatures above 60°C were found to correspond to increased breaking strength of single muscle fibres. The myofibrillar component explained approximately 47% of the variation in whole meat toughness upon cooking whereas inclusion of the connective tissue component increased the goodness of fit.     

THE EFFECTS OF LONG TERM COOKING ON SHEAR FORCE-DEFORMATION CURVES

The influence of muscle contraction, pressure-heat, and cooking conditions on some parameters of the Warner Bratzler shear force curves were studied. Stretched meat samples, restrained from shortening along the muscle fibers during cooking for a short time at 90°C, had larger peak shear force values than samples cooked unrestrained. As cooking times were increased, peak shear force values decreased until after 8-16 hr the cooked restrained samples had lower values than the unrestrained ones. The initial yield force values obtained for pressure-heat treated samples were not significantly affected by cooking at 90° for up to 32 hr, although the peak shear force values decreased with cooking time. The experimentally determined shear curves were shown to be approximated by summing the estimated individual contributions of the myofibrillar and the connective tissue components. The results are consistent with the suggestion that the initial yield force values represent mainly the myofibrillar strength and that the difference between the peak force and the initial yield force represents the contribution of the connective tissue.     

A comparison between tensile and shear adhesive strength of meat-myosin junctions

A model meat-myosin gel junction was used to compare the adhesive strength of binding junctions when subject to tensile or shear forces. This comparison was made on junctions cooked to 80°C with three different alignments of muscle fibres with respect to the junction plane. Tensile adhesive strength (TAS) and shear adhesive strength (SAS) did not differ significantly when fibres in both of the bound pieces of meat were perpendicular to the plane of the myosin gel function (90°/90° junction). However, SAS was higher than TAS if the muscle fibres in one or both of the meat pieces were parallel to the plane of the junction. This suggests that tensile failure of binding junctions is the more likely mode of failure. Differences between SAS and TAS in any one junction orientation were small compared to the effect of muscle fibre orientation with respect to the junction; both TAS and SAS were highest for 90°/90° junctions and lowest when muscle fibres in both meat pieces were parallel to the junction (0°/0° junctions).     

THERMAL CONTRACTION OF MEAT DURING COOKING AND ITS POSSIBLE INFLUENCE ON TENDERNESS

Samples of beef muscles with different states of myofibrillar contraction were cooked at 80 °C, both restrained and unrestrained from thermal shortening. Samples restrained from thermal shortening lost similar amounts of moisture to the unrestrained samples and had significantly (P < 0.001) larger decreases in the cross-sectional area. The effects of restraint on structural strength were assessed using adhesion, tensile and shear testing. Restraint decreased adhesion between the fibers and increased tensile, initial yield and peak force values in the control and stretched samples where, without the restraint, thermal contraction would normally have been large. Shear force values of the cooked meat showed the increase normally associated with cold shortening whether or not thermal shortening occurred.     

The physical basis of meat texture: Observations on the fracture behaviour of cooked bovine M. Semitendinosus

The fracture behaviour of cooked strips of beef M. semitendinosus was studied by qualitative observation of the manner in which fracture occurred and by quantitative measurements of ultimate tensile strength, work of fracture and notch sensitivity. Qualitative observations showed that fracture started in the perimysial connective tissue in all test configurations used, resulting initially in the separation of intact muscle fibre bundles. The ultimate tensile strength along and across the fibres was ～-300kNm(-2) and ～-25kNm(-2), respectively. The qualitative aspects of fracture were explained on the basis of a uniaxial fibrous composite of strong muscle fibre bundles in a weak connective tissue 'matrix', with poor interfacial strength. Work of fracture through the perimysium was in the range 0·4 to 1·8kJm(-2). The difficulty in propagating fracture across the muscle fibre bundles was explained in terms of the material's complete insensitivity to notches running across the fibres. The results imply that the muscle fibre bundle is an important level of structural organisation as far as fracture is concerned and that the strength of the perimysium, or perimysium/muscle fibre bundle interface, is likely to have a major influence on the toughness of'the cooked meat.     

Development of Muscle Thermal Rigorometer and Characterization of Heat-induced Muscle Shortening of Tilapia

A muscle thermal rigorometer was constructed, allowing muscle shortenings induced by dynamic heating or isothermal aging to be monitored. Operating isothermally like a traditional rigorometer at 10 °C, postmortem dorsal muscle shortening (S_10°C) developing from 0% to 10% of its initial length in corresponding to RI_fiber along fiber‐direction developing from 0% to 100% within 16 h was monitored for freshwater tilapia. Monitoring meat cooking in the dynamic heating mode, heat‐induced shortenings could be observed for all muscle samples possessing different degrees of rigor induced by 10°C aging. The heat‐induced shortening (S_dynamic) plus its 10°C aging shortening (S_10°C) for each sample was the same, S_overall= S_10°C+ S_dynamic= 10%. Their heat‐induced shortening peak temperatures (Ts) from 30°C to 48°C were inversely correlated with the sample RI_fiber from 0% to near 100%. These findings together with an additional calcium/adenosine triphosphate (ATP) model studies showed that the ATPase related myofibrillar contractile system was responsible for these low‐temperature cooking shortenings, which along with Ts values could thus be adopted as new rigor indices.     

The tenderness of cooked and raw meat from young and old beef animals

The shortening-toughness relationships for sternomandibularis muscles of young ox and large old bull have been compared. In both, toughness, measured by tenderometer, increases to a maximum at a shortening of 40%, and declines steeply from there at higher shortenings. This characteristic peaked relationship is obtained for ox and bull cooked both at 60 and 80 °C. However, toughness values for meat cooked to 60 °C are only about half those for meat cooked to 80 °C. In contrast to these similarities in the relationships for young and old animals, the rate of toughness increase with shortening in ox is considerably less than in bull. Thus toughness of ox at a given shortening and cooking temperature is considerably lower than that for bull, especially at high shortenings. A further distinction is made between the animals in that raw ox muscle does not give a peaked shortening-toughness relationship, whereas raw old bull does. The results have shown that for longissimus and sternomandibularis muscles a trained taste panel is only sensitive to variations in toughness in the lower half of the range of shear force values determined by tenderometer. In the light of this, live animal characteristics and muscle shortening are both important in determining toughness measured by taste panel.     

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.     

Shortening as a factor in meat ageing

Summary. Shortening during the slow and rapid phases of rigor mortis onset determines largely the extent to which beef ages. For meat stored at 15°C for 3 days, shear-force values are uniformly low at shortenings of 0% to 20% of the freshly excised muscle length. However, there is a four- to five-fold increase in toughness as shortenings proceed from 20% to 40%. This is followed by a decline in toughness as shortenings increase further from 40% to 55%. With increasing shortenings beyond 20%, progressive decreases occur in the extent to which meat ages until at 40% shortening, ageing has declined to zero.     

The effect of conditioning on the strength of perimysial connective tissue dissected from cooked meat

Tensile tests were carried out on ribbons of perimysial connective tissue dissected from slices of bovine semitendinosus muscles that had been conditioned or not conditioned and then cooked to a range of temperatures. A consistent reduction in the strength of the perimysia was seen in the conditioned samples, both in the raw meat and meat cooked to 50°C. At higher cooking temperatures (60-80°C), no effect of conditioning was seen. The content of collagen or total protein of mechanically extracted perimysia and the collagen content of the test pieces from conditioned and unconditioned muscles was not significantly different. It was concluded that conditioning decreases the breaking strength of the perimysial connective tissue in raw meat or in meat which is subsequently cooked to 50°C, but not in meat cooked to the temperatures normally employed by consumers. The tenderization observed in conditioned meat cooked to 60°C and above is, therefore, due to the weakening of muscle fibres within the fibre bundles.     

Effect of added μ-calpain and post-mortem storage on the mechanical properties of bovine single muscle fibres extended to fracture

The effects of μ-calpain and post-mortem storage on the strength of single muscle fibres were investigated. During the 10 min of incubation at pH 7.5, μ-calpain became evenly distributed throughout the fibre. μ-Calpain-incubation resulted in thinner (P <0.001) Z-lines and reduced (P <0.001) the strength of the fibres compared to controls. These results demonstrate that μ-calpain is capable of mechanically weakening the muscle fibres. Post-mortem storage of meat for 10 days at 2?°C weakened (P <0.001) the muscle fibres compared to 24-h fibres. The presence or absence of Ca(2+) affected fibre stiffness. Fibres incubated at pH 7.5 in 100 μM Ca(2+) were less stiff than fibres incubated in 200 μM EGTA. Breaking stress and strain were not affected by Ca(2+). We hypothesise that Ca(2+) causes conformational changes in some of the load-bearing proteins, which alters their initial resistance to extension, but does not affect the breaking strength of the fibres.