Jacques Lepetit

Relationships between muscle characteristics and meat quality traits of young Charolais bulls

Charolais bull calves (106) were used to study the variability in meat quality attributes in relation to the variability in muscle characteristics in the Longissimus thoracis (LT) muscle. The variability in traits was adjusted either to constant age or constant weight at slaughter and thus originated only from differences between animals born, reared and fattened in the same location. The following meat quality attributes were measured: the strength of the myofibrillar resistance to a 20% compression strain measured on the raw meat 2, 7 and 21 days post mortem; and taste panel scores of tenderness (initial and overall), flavour and juiciness of steaks grilled to a 55°C core temperature 6 or 15 days post-mortem. The following muscle characteristics were measured 24 h after slaughter: pH, dry matter, protein, lipid, heme iron and collagen contents, collagen solubility, LDH and ICDH activity, the proportion of slow twitch myosin heavy chain, the mean muscle fibre area and the mean sarcomere length. One fourth to one third of the variability of 2 day mechanical strength and 15 day tenderness or flavour scores were related to the variability in muscle characteristics. Tenderness and strength measurements were predominantly related to the muscle fibre area, collagen characteristics and energetic metabolic activity. Dry matter content was the principal muscle characteristic related to flavour.

Mechanical properties of meat

Determination of meat mechanical properties is still usually performed under empirical conditions, using tests which have been developed many years ago. This paper discusses the limitations of these methods. It also reviews recently developed research which has been focused on a more analytical approach to mechanical tests performed in closely controlled conditions. The objectives of these investigations are on the one hand to obtain specific properties of the myofibre and connective tissue structures involved in meat toughness and on the other hand to understand the fracture mechanisms involved in the perception of texture. Due to the anisotropic and composite nature of meat, attention has been focused on the influence of the direction and amplitude of strains to be used for the characterisation of myofibres and connective tissue, respectively. These strain conditions are linked to the contraction and stretching state of each of these structures. Indeed, whatever the type of deformation (shear, compression or tension), myofibre resistance can be determined under low strain conditions as long as collagen fibres in the connective tissue remain in a more crimped state than myofibres, i.e. in raw or slightly cooked meat of normal sarcomere length. On the contrary, determination of connective tissue resistance has, up to now, only been obtained using large strain tests. Contraction of collagen fibres induced by cooking makes the separate analysis of meat structures more difficult, but this can partially be overcome by applying mechanical tests in different strain directions. However, the role of the spatial distribution of connective network on the mechanical properties of meat still remains largely unknown. Image analysis and ultrasonic methods, currently developed to study this problem, are briefly presented.

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.

Assessment of breed type and ageing time effects on beef meat quality using two different texture devices

Forty-two male yearlings were used to assess the influence of breed type and ageing time on beef texture. Samples of the M. longissimus dorsi of four breed types [double muscled (DM), dual purpose (Brown Swiss, BS), fast growth (FG) and unimproved type (UT)] were aged for 1, 3, 7, 10, 14 or 21 days at 4°C and frozen at -18°C until analysed. Cooked samples (to end-point of 70°C) were assessed using a Warner-Bratzler (WB) device. Raw samples were assessed using a compression device in which transverse elongation was prevented. There were no significant differences in WB values of cooked meat due to breed type, but ageing had a significant (P<0.05) on maximum load. Ageing, but not breed type, had a significant effect on the compression values of raw meat at low compression rates (P<0.001). Compression values, of raw samples, at 80% compression differed significantly (P<0.001) between breed types, but were not affected by ageing. Compression values of raw samples, at 80% compression, were affected by breed type, probably because of genotype differences in the contribution of connective tissue.

A theoretical approach of the relationships between collagen content, collagen cross-links and meat tenderness

This work concerns the relationship between meat tenderness and the rubber-like properties, i.e. pressure and elastic modulus, that endomysium and perimysium connective tissues develop when meat has been heated to a temperature above which collagen contracts. For rest length meats with similar intramuscular connective tissue morphology, and which are at the same ageing state and pH, the elastic modulus of the collagenous fraction of connective tissues is approximately proportional to the total number of collagen cross-links present per volume of meat. Calculations from various published experiments concerned with the effect on tenderness of muscle type, animal age, type, and sex from different species show that this modulus follows most of the variations of meat toughness. Moreover, the proportionality between the increase in this elastic modulus and the increase in meat toughness approaches unity in situations where toughness mainly depends on connective tissues. This work demonstrates the decisive role of rubber-like properties of connective tissues in meat tenderness variations.

Predicting variability of ageing and toughness in beef M. Longissimus lumborum et thoracis.

The object of this study was to determine muscle characteristics which might predict meat toughness. Eleven Charolais cattle were slaughtered at approximately 26 months of age and the Longissimus lumborum et thoracis muscle was taken 1 hr post mortem and stored at 12 °C for 24 hr and then at 4 °C. The average half-life for ageing in these raw muscles was 4.6 days but the toughness varied widely between the animals. Toughness varied 3-fold and the rate of ageing varied 20-fold between animals. Correlations were done to determine which characteristics might explain this variability. Toughness was correlated positively with increase in oxidative status of muscle and the initial levels of calpastatin. Toughness was correlated negatively with the initial levels of μ- and m-calpains and cysteine and serine proteinase inhibitors, the initial pH values and the rates of their decline. The rates of ageing were highly correlated positively with the initial levels of proteinase inhibitors and the rates of decline of calpastatin and negatively with the ultimate amounts of expressible juice. There was a wide variability in tenderness in M. Longissimus lumborum et thoracis from similar animals. Variations in metabolism and enzyme activity controlled by inhibitors and calpains appear to be largely responsible for this variability.

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.

Dielectric behavior of beef meat in the 1–1500 kHz range: Simulation with the Fricke/Cole–Cole model

The electrical properties of biological tissues have been researched for many years. Impedance measurements observed with increasing frequencies are mainly attributed to changes in membrane conductivity and ion and charged-molecule mobility (mainly Na(+), K(+), CL(-) ions). Equivalent circuits with passive electrical components are frequently used as a support model for presentation and analyses of the behavior of tissues submitted to electrical fields. Fricke proposed an electrical model where the elements are resistive and capacitive. The model is composed of a resistive element (Rp) representing extracellular fluids (ECF) placed in parallel with a capacitive element (Cs) representing insulating membranes in series and a resistive element (Rs) representing intracellular fluids (ICF). This model is able to describe impedance measurements: at lower frequencies, most of the current flows around the cells without being able to penetrate them, while at higher frequencies the membranes lose their insulating properties and the current flows through both the extracellular and intracellular compartments. Since meat ageing induces structural change, particularly in membrane integrity, the insulating properties of membranes decrease, and intracellular and extracellular electrolytes mix, thus driving changes in their electrical properties. We report a method combining the Fricke and Cole-Cole models that was developed to monitor and explain tissues conductivity changes in preferential directions during beef meat ageing.

Post-mortem evolution of rheological properties of the myofibrillar structure

The post-mortem evolution of the rheological behaviour of beef muscle has been studied with the SATA compressive device. The resistance and elasticity of the muscle tissue, measured at low strain, refer to the myofibrillar structure and provide an indication of the advance of the rigor maturation process. These parameters have been used to quantify the electrical stimulation effect on Longissimus dorsi and Semi-membranosus muscles. An empirical model is proposed for the post-mortem evolution of the resistance and provides three parameters-the initial rate of rigor onset, the time of rigor and the residual stress after ageing. On the other hand, parameters obtained at high strain refer mainly to the connective tissue. The particularly high stress developed by the pre-rigor muscle, however, is presumed to be associated with the myofibrillar component.

Meat quality traits in the emu (Dromaius novaehollandiae) as affected by muscle type and animal age

Meat quality traits were determined in the major muscles of the emu (Dromaius novaehollandiae) at different slaughter ages (6, 10, 14, 17 or ≥20 months). A mean ultimate pH value of 5.5 was reached within around 3 h post mortem, but this value was 6.1 in animals that had suffered a preslaughter stress (transportation and fasting). The collagen and pigment contents varied widely among the muscles. The protein and pigment contents increased with animal age, but this effect was perceptible only between 6 and 14 months. The other chemical constituents were little affected by muscle type or animal age. The intense red colour of emu meat, due to a high pigment content, was very sensitive to oxidation, thus limiting the storage of fresh meat under aerobic conditions to short periods of time. Despite a rapid post-mortem tenderization (≤24 h), the residual myofibrillar strength obtained after extended ageing remained intermediate between those reported for chicken and beef. The tenderness of meat, cooked to 60 °C, differed between muscles and decreased with increasing age, thus reflecting the changes occuring in the concentration and in the heat stability of the intramuscular connective tissue.