Connective tissue differences in the strength of cooked meat across the muscle fibre direction due to test specimen size

Systematic variations in the tensile strenght of cooked beef M. semitendinosus across the muscle fibre direction due to the cross-sectional size of specimens are demonstrated in specimens from (a) longitudinal and (b) transverse slices. The strength perpendicular to the fibre direction of longitudinal slices of thickness 0·25-5·75 mm varied by a factor of 2, thicker slices being stronger. This factor of 2 is in approximate agreement with the difference in strength of transverse versus longitudinal slices across the fibre direction. These variations of strength due to specimen geometry are explained on the basis of the increasing likelihood of including a ribbon of the perimysial connective tissue network which is continuous along the whole length of the test piece in larger samples. The breaking strength of small cross-sectional area specimens is likely to be dominated by the strength of the endomysial-perimysial junction. Larger cross-sectioned specimens, by including continuous strands of the perimysial network, have higher strengths resulting from the necessity to break these strands. These findings highlight the need to specify specimen dimensions in tensile test results. They also show that by manipulating specimen geometry, the relative magnitude of the two mechanisms of connective tissue fracture (endomysial-perimysial separation and perimysial strand fracture) may be assessed.     

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.     

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.     

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.     

The influence of the collagen fibre network of muscle on the compliance and tensile strength of muscle samples subjected to loads at right angles to the muscle fibre axis

The pattern of subdivision of a muscle into fibre bundles by the perimysium was examined as a possible source of within sample variability of meat toughness attributable to the connective tissue component. It was shown that the pattern of subdivision into fibre bundles relates to the tendom surfaces and influences the compliance and toughness of a meat sample. In addition it was shown that greater plasticity of raw meat samples results in greater compliance under load. The procedure of defining sample orientation relative to the geometry of the component parts enables some of the within sample variability to be explained and controlled.     

Structural and mechanical changes in raw and cooked single porcine muscle fibres extended to fracture

Tensile tests on single muscle fibres from raw and cooked porcine longissimus thoracis muscle were performed to explore the structural mechanisms responsible for their deformation and fracture properties. Measurements of load and deformation were made simultaneously with light microscopy observations of the structural changes which occur on extension. On extending the fibres to fracture, an r-shaped stress-strain curve was observed and the structural changes which occurred during this process could be divided into three phases. Phase one, was characterised by a rapid increase in stress with little change in strain and ended at the yield point. Sarcomere length was uniform along the fibre in this initial phase. Raw fibres yielded at strains of between 2 and 5% of their resting lengths and cooked fibres at strains of between 10 and 20%. In phase two, there was rapid increase in strain with minimal changes in stress. In most fibres this phase was characterised by multiple cracks on the fibre surface and unequal sarcomere stretching. Sarcomeres in the regions where the surface had ruptured extended faster than those in areas still covered by the surface membrane, where sarcomere length remained relatively unchanged. In some cooked fibres, there was little or no surface cracking and all the sarcomeres in these fibres extended almost uniformly. Phase three was characterised by a rise in stress as strain increased and then a final fall in stress at the breaking point. This was accompanied by myofibrillar failure and finally breakage of the whole fibre. The myofibrils did not always fail as one unit; a progressive snapping of small bundles of myofibrils was seen in some raw fibres. Muscle fibres could be stretched to 10·9 ± 1·45% of their resting length before breaking when raw, but to 130 ± 42% of their rest lengths after they were cooked for 1 h at 80°C. Where multiple surface cracking was observed in phase two, sarcomeres in some cracked areas lengthened faster than others and the cracked areas which extended fastest were usually the focus of the eventual failure of the fibre. In raw fibres, sarcomeres in the areas where the fibre surface had ruptured could be stretched up to 107·7% before failure, while those in areas of the fibre with an intact surface remained relatively unchanged. In cracked areas of cooked fibres the sarcomeres were more extensible and could be stretched to 169·7% before breaking. The order-of-magnitude increase in overall extension to failure of fibres resulting from cooking is only partially due to this increase in sarcomere extensibility in cracked areas. Mechanically demembranating raw fibres depressed the stress at which yielding occurred and doubled their breaking strain. However, this process had no effect on the stress at which the fibres fractured. The results show that deformation is not uniform along individual fibres, especially in the raw case and that the endomysium has an important contribution to this non-uniform deformation.     

Composition of muscle fibre types and connective tissue in bovine M. semitendinosus and its relation to tenderness

The distribution of muscle fibre types and connective tissue in bovine M. semitendinosus is described. A parallel increase in the volume fraction of type I muscle fibres (from 10% to 30%) and a decrease in the IIB volume fraction (from 58% to 34%) was recorded from superficial to deep layers. A positive correlation was observed between the frequency and the cross-sectional area of both type I and IIB fibres. The elastic fibres formed irregularly shaped bundles that made up about 50% of the volume of the perimysium. Thin elastic fibres extended into the endomysium. The relative proportion of elastic fibres in the perimysial connective tissue increased towards the deeper layers of the muscle. A taste panel evaluation of the sensory properties was performed and the data were correlated to the histological observations. A gradual decrease in scores of four tenderness-related traits was recorded from the superficial to the deep layer of the muscle. The superficial layer was rated as most tender, whereas the consecutive layers were rated less tender. The possible relationship between the composition of muscle and the meat quality is discussed.     

ANALYSING WARNER-BRATZLER CURVES

Forces required to deform and rupture samples of meat were measured in a modified Warner-Bratzler device. A model is proposed to describe the shape of the plot of force against distance. This model was fitted by a least squares method to experimental curves obtained from (1) bovine longissimus dorsi muscles aged at 15°C for 1–7 days and then cooked at 80°C for 4 h, and (2) bovine biceps femoris muscles conditioned at 15°C for 24 h and then cooked at 80°C for 1–22 h. A plot of the variance between replicate forces, as a function of distance, exhibits two prominent peaks, one of which is thought to be associated with rupturing of fibre bundles, and the other with rupturing of perimysial connective tissue. Parameters calculated by fitting the model to replicate curves indicate that muscle fibres rupture in groups of fibre bundles. Hence, contrary to previous interpretations, initial yield involves rupture of connective tissue, as well as myofibrils. However, the number of fibre bundles forming a group capable of independent rupture varies according to the time-temperature history of the sample, as do other parameters. The model permits relative contributions of fibre bundles and perimysial connective tissue to be estimated. In the samples studied here, perimysial connective tissue offered greater resistance to passage of the blade than did the fibre bundles.     

Effects of carcass weight and muscle on texture, structure and myofibre characteristics of wild boar meat

Texture, histology and muscle fibre characteristic of selected muscles: m. quadriceps femoris (QF), m. biceps femoris (BF), and m. semimembranosus (SM) of wild boars of different carcass weight (20±2 and 60±3kgSD) were compared. Muscle texture (hardness, cohesiveness, springiness, chewiness) was determined with the double penetration test performed with the Instron 1140 apparatus. Structural elements (muscle fibre cross-section area, perimysium and endomysium thickness) and percentage of myofibres of each type: I (slow oxidative), IIA (fast oxidative-glycolytic) and IIB (fast glycolytic) per muscle fibre bundle, were measured in muscle samples using a computer image analysis program. The young wild boar muscles showed significantly lower values for the textural parameters (p<0.05). The muscle fibre cross-sectional areas of the juvenile wild boar muscles were significantly lower and the perimysium and endomysium thinner (p<0.05) than those in the old wild boar meat, while the percentage of type IIB fibres was higher. Of all the wild boar muscles tested, the highest hardness and chewiness values were found in BF which, at the same time, showed the highest fibre cross-sectional area and the thickest perimysium and endomysium. The highest percentage of I and IIA fibre types was typical of BF and SM either in young or in old wild boars with the lowest percentage of type I and the highest percentage of type IIB fibres being found in the QF. The results suggest that a higher hardness of wild boar muscles can be connected with a thicker perimysium and endomysium, fibres of higher cross-sectional area and probably a higher content of red fibres (type I).     

The effect of temperature and ultimate pH on the increase in meat toughness resulting from restraint during cooking

Samples of stretched muscle cooked at 50, 60, 70 or 80°C, while restrained at either their original pre-cooking length or further tensioned at about 130% of their original pre-cooking length, had significantly (P < 0·001) greater Warner-Bratzler (WB) peak shear force values for all temperatures than similar samples cooked without restraint except for those restrained at their original length and cooked at 50°C. Restraint during cooking at 80°C increased the peak shear force values of stretched sheep muscles with ultimate pH values in the range 5·5-7·0. This increase, which has been related to connective tissue strength, was not significantly related to ultimate pH. Both initial yield and peak force values, for samples cooked either restrained or unrestrained, decreased significantly (P < 0·001) and at similar (not significantly different) rates with increase in ultimate pH.     

The strength and stiffness of perimysial connective tissue isolated from cooked beef muscle

Thick transverse slices of bovine M. semitendinosus were cooked for 1 h at 50°–90° and then cooled. Perimysial connective tissue was dissected from the cooked meat and subjected to mechanical testing in a small-scale device. The initial ‘toe’ region of the J-shaped load-extension curve was progressively lost with increasing temperature, the curve becoming more nearly linear after cooking at 90°C. These effects are explained on the basis of the progressive straightening out of the crimps from the collagen fibres, the crimps becoming finally lost at approximately 70°C. The final stiffness of the perimysium at greater extensions was unchanged at higher temperatures. Breaking strength increased from raw to cooked at 50°C, thereafter decreasing at cooking temperatures up to 90°C. it is proposed that this technique of testing isolated perimysium gives a valuable means of directly measuring the effects of cooking, or other treatments, on the intrinsic properties of perimysial collagenous material. Quantitative knowledge of these will help to determine its contribution to the overall mechanical properties, and hence eating quality, of cooked meat.     

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.     

Variations in water-holding capacity due to changes in the fibre diameter, sarcomere length and connective tissue morphology of some beef muscles under acidic conditions below the ultimate pH

The water-holding capacities (WHC) of six different beef muscles were measured over the pH range 5·7 to 4·0. Corresponding changes in the morphology of muscle fibres and connective tissue were observed by light microscopy. WHC increased over the pH range 5·1 to 4·0 in all muscles, with the M. longissimus dorsi (LD) having significantly higher (p < 0·05) swelling ratios than the other muscles at pH 4·3 and pH 4·0. In all muscles, swelling increased across and along the muscle fibre axis between pH5·1 and pH4·4. However, towards pH4·0, increased muscle fibre swelling occurred in predominantly 'white' fibre-type muscles, in particular the LD, whereas muscle fibre shrinkage occurred in predominantly 'red' fibre-type muscles. Increased swelling of perimysial collagen and endomysial reticulin was observed in all muscles between pH4·5 and pH4·0, while the appearance of elastin was unaffected by pH. Consequently, interactions between muscle fibre swelling and connective tissue swelling determined the extent of total muscle swelling in different muscles between pH 4·5 and pH 4·0.     

束纤维的拉伸特性与强度—伸长间的关系

束纤维实验与单纤维实验存在差别 ,不仅测试方法和仪器有所不同 ,而且所测得的结果也有差异。采用束纤维强伸性测量方法能够得到羊毛纤维束比强度 (BS)与纤维束伸长率 (BE) ,但在分析纤维束强力曲线时存在局限性。文中提出了新的参量 ,如纤维束初始模量 (BM)、断裂比功 (BW)、峰值前断裂比功 (WBP)、峰值后断裂比功 (WAP)以及二者的比率 (WAP WBP)等。同时比较了羊毛、涤纶及粘胶人造丝三者纤维束的强伸性能。从 32批精梳毛条中抽取试样进行测试 ,结果表明 ,三者的束强力特征间 (如BS BE ,BS BM ,BS BW ,BE BW和BM BW)存在相关性。但是 ,各变量的变异系数彼此几乎不存在相关性 ,并且与束纤维力学参数平均值 (除BS CVBM和CVBM BW外 )不相关。     

ACETIC ACID MARINADING—THE RHEOLOGICAL CHARACTERISTICS OF SOME RAW AND COOKED BEEF MUSCLES WHICH CONTRIBUTE TO CHANGES IN MEAT TENDERNESS

The tensile and adhesive characteristics of acetic acid marinaded bovine Ms sternomandibularis (StM), infraspinatus (IS), triceps brachii caput longum (TB), and psoas major (PM) were studied over the meat pH range 5.80 to 4.10 on both raw and cooked strips of muscle. In the raw muscle samples, tensile and adhesive characteristics were greatest around pH 5.0, progressively decreasing as muscle pH values approached 4.1. Cooking increased tensile strength, particularly around pH 5.0 where peak force values virtually doubled. Cooking had less influence on the adhesive characteristics of all muscle samples which tended to decrease below pH 4.5. In both raw and cooked muscle samples, intermuscular differences in adhesive characteristics were also much greater than those obtained for tensile characteristics. Irrespective of intermuscular differences in collagen content or its degree of maturity, the gradual decreases in tensile and adhesive characteristics below pH 5.0 could be related to the effect of acetic acid induced swelling on both muscle fibres and the connective tissue network. The influence of acetic acid marinading in promoting meat tenderness is due more to its effect on the behaviour of the muscle fibres during cooking than to its effect on the connective tissue network of different muscles.     

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).     

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.     

Intramuscular connective tissue and its role in meat quality

The amount, spatial distribution and composition of the connective tissue within muscle vary with muscle position in the carcase and with animal age. This has long been recognised to influence the tenderness of cooked meat. This paper builds upon some historical perspectives with a review of some recent clarifications of the biological function of intramuscular connective tissue (IMCT) and of its contribution to meat texture, which is clearly multifactorial. The perimysial component of IMCT varies most in amount between muscles and is also the IMCT structure most involved in defining the mechanical integrity of cooked meat. The distribution of perimysium defines muscle fascicle size (muscle "grain" size), which is also still regarded as an indicator of tenderness. Postmortem conditioning of meat has consistently been shown to reduce the strength of intramuscular connective tissue in the raw state, but with equal consistency, this has been shown not to affect the toughness of cooked meat. Cooking increases IMCT strength in the range 20-50 °C and decreases its contribution at higher temperatures and longer cooking times. Crosslinking of collagen in older animals is generally considered to result in tougher meat, although definitive links between mature crosslink content and cooked meat toughness have been difficult to prove. In the last quarter-century, IMCT has been increasingly viewed as a "background" contributor to meat texture, which is difficult to change. However, the large variation in perimysial content of muscles in one animal represents an incredible range of expression. This appears to be firmly fixed to the functional properties of different muscles. In particular, it is hypothesised that definition of muscle fascicle size and shape by the bounding perimysium is related to the need for sub-sections of the whole muscle to slip past each other in the normal contractile function of the tissue. Despite this, the amounts and composition of IMCT can be manipulated by animal nutrition and exercise, and factors affecting the turnover of IMCT may especially be a future target for manipulation of meat texture.     

Muscle glycogen depletion pattern in halothane-gene-free pigs at slaughter and its relation to meat quality

Muscle fibre type composition and glycogen depletion pattern at slaughter as related to meat quality, were studied in M. longissimus dorsi of halothane-gene-free Swedish Yorkshire pigs fed a high (n=19; 10 entire males and 9 gilts) or a low (n=18; 10 entire males and 8 gilts) protein diet. The muscle consisted of, on average, 8% type I, 9% type IIA and 83% type IIB fibres. Muscle fibre areas were significantly smaller in entire male pigs than in gilts irrespective of fibre type. There were no marked differences in muscle characteristics between pigs fed the low and high protein diets. Low glycogen concentrations were found in most type I and type IIA muscle fibres, while a greater variation in glycogen content was evident in type IIB fibres. The pigs were divided into two groups according to the proportion of glycogen depleted IIB fibres (more or less than 30% depleted IIB fibres). In the group where 30% or more of the type IIB fibres were glycogen depleted, a tendency toward DFD meat (dry, firm, dark) was seen, as the meat had higher ultimate pH (5·62 vs 5·52; p=0·02), lower drip loss (2·7% vs 4·4%; p=0·007) and lower reflectance value (darker meat; 16·5 vs 20·6 EEL units; p=0·0005), compared with the group with less than 30% depleted IIB fibres. No difference in total glycogen content was found between these two groups. This indicates that the distribution of glycogen in different fibre types seems to be of importance for the ultimate meat quality.     

Elastin in bovine Semitendinosus and Longissimus dorsi muscles

The structural organization of elastin present in bovine semitendinosus and longissimus dorsi muscles was examined using light and scanning electron microscopy. Qualitatively, both muscles were found to have the same organization of the two structural forms of elastin present.

Coarse elastin fibres (5–10 μm diameter) were found in the epimysium and perimysium, aligned with the long axis of the muscle fibres. In addition, finer elastin fibres (1–2 μm diameter) were found in the epimysial and perimysial sheets of collagen. These finer elastin fibres approximated the course of the collagen fibres, i.e. at an angle to the muscle fibre long axis.

Quantitatively, the semitendinosus muscle epimysium and perimysium contained very heavy deposits of the coarse elastin fibres and nearly every sheet of perimysium, even the thinest, contained the fine elastin fibres. By comparison, less of the coarse elastin was present in the longissimus dorsi epimysium and it was quite rare in the perimysium. The finer elastin fibres were, however, reasonably common.

The significance of this structural arrangement is discussed in relation to muscle function and to meat toughness assessment.