Influence of the season on the relationships between NMR transverse relaxation data and water‐holding capacity of turkey breast meat

In the last few years the poultry industry has seen a significant deterioration in meat quality properties during the summer season. The objective of this study was to evaluate the seasonal effect (summer and winter) on turkey meat quality assessed by both conventional and low‐resolution nuclear magnetic resonance (LR‐NMR) analysis. Eighty‐eight breast muscle samples (35 winter and 53 summer) from BUT‐Big 6 turkeys belonging to 16 different flocks, were randomly collected from a commercial processing plant. The samples were analysed for transverse relaxation times (T₂) by LR‐NMR and for initial pH (15 min post mortem), ultimate pH (24 h post mortem) and pH after cooking, temperature at 15 min post mortem, water‐holding capacity (WHC, drip loss, filter paper press wetness and cooking loss) at 24 h post mortem, colour of raw and cooked meat and chemical composition (moisture, lipids and proteins). The results indicate that, during the summer season, turkey breast meat undergoes a relevant WHC decrease. Cluster analysis of the raw LR‐NMR data evidenced the presence of two groups corresponding to samples harvested in each different season. Correlations between the LR‐NMR signal and the conventional parameters measuring WHC were obtained by a recently proposed type of principal component regression (PCR) termed relative standard deviation PCR. Copyright © 2004 Society of Chemical Industry     

Post-mortem changes in myofibrillar proteins of breast and leg muscles from broilers,spent hens and Taiwanese Country Chickens

Post mortem ageing at 4°C was studied in the breast and leg muscles from broilers, White Leghorn spent hens and Taiwanese Country Chickens (TCC). Purified myofibrils were prepared from muscles after 0, 1, 3, 5, and 7 days of post mortem storage at 4°C. SDS-PAGE was used to examine the changes in myofibrillar proteins of the muscles. Results showed that 30 kDa components appealed earlier in the breast muscles than in the leg muscles and in the order: broiler, TCC, and spent hen. The intensity of 30 kDa band increased with post mortem time. In the breast muscles, a decrease in the intensity of the α-actinin band could be observed at 1 day post mortem in broilers and TCC and at 3 days post mortem in spent hens. This decrease, however, could not be found in the leg samples until 5 or 7 day post mortem. Titin 1 band disappeared within 3 day post mortem in the breast samples but within 5 days in the leg samples. Similar results could be observed in the degradation of nebulin although traces of nebulin remained in the teg muscles of TCC after 7 days post mortem.     

The mechanism of drip production: Formation of two compartments of extracellular space in muscle Post mortem

The structural changes occurring post mortem at 10°C in beef sternomandibularis muscle have been studied by fixing muscles at various times post mortem and examining transverse sections by light microscopy, or transversely cut surfaces by scanning electron microscopy. At 2 h post mortem, the muscle fibres filled the endomysial network and the fibre bundles filled the perimysial network. At 4 to 6 h post mortem, presumably as a result of fibre shrinkage, gaps appeared between the fibre bundles and the perimysial network but at this stage the fibres still filled the endomysial network. Gaps between fibres did not appear until about 24 to 48 h post mortem. It is concluded that at rigor onset, when myofibrils shrink owing to pH fall and the attachment of myosin heads to actin filaments, the fluid expelled accumulates in these two types of extracellular space. Such fluid, especially that accumulating between fibre bundles, is the likely source of drip.     

Effect of Postmortem Storage on Cold-Shortened Bovine Muscle: Analysis by SDS-Polyacrylamide Gel Electrophoresis

Myofibrils were isolated from the longissimus (L) muscle of control (CON) and cold-shortened (CS) muscles after 0, 1, 3, 7, and 10 days of postmortem storage at 2°C. Isolated myofibrils were then examined by SDS-polyacrylamide gel electrophoresis to monitor the changes in the myofibrillar proteins during postmortem storage. The main changes in CS muscles were the gradual appearance of 110,000-, 95,000-, and 30,000d-dalton components and the disappearance of desmin and troponin-T components of myofibrils. In addition, there was a gradual increase in the intensity of a protein around 55,000-daltons. CON samples showed similar changes to those of CS samples. It appears that myofibrillar proteins of cold-shortened muscles are affected by postmortem aging in a manner similar to that of the normally chilled muscles.     

The use of simultaneous 1H & 31P magic angle spinning nuclear magnetic resonance measurements to characterize energy metabolism during the conversion of muscle to meat

In order to investigate the potential of magic angle spinning nuclear magnetic resonance (MAS NMR) in the elucidation of post‐mortem metabolism in muscle biopsies, simultaneous ¹H and ³¹P MAS NMR measurements were made continuously on post‐mortem (20 min to 24 h) muscle longissimus samples from rabbits. The animals had either been or not been given adrenaline (0.5 mg kg^?1 4 h pre‐slaughter) to deplete stores of muscle glycogen. The intracellular pH was calculated from ¹H spectra, and the post‐mortem rate of formation of lactate was followed and quantified. Comparison of measurements made on muscle samples from rabbits treated with adrenaline with measurements made on muscle samples from untreated rabbits revealed significant effects of adrenaline treatment on both pH (pH24 h = 6.42 vs. pH24 h = 5.60) and formation of lactate (16 mmol g^?1 vs. 65 mmol g^?1). The ³¹P NMR spectra were used to follow the rate of degradation of ATP and phosphocreatine. The present study clearly shows that MAS NMR has potential for the study of post‐mortem energy metabolism.     

Influence of dietary conjugated linoleic acid on the fatty acid composition and volatile compounds profile of heavy pig loin muscle

This study evaluated the effects of dietary conjugated linoleic acid (CLA) on fatty acid composition, chemical composition and volatile compounds profile of the longissimus dorsi muscle in Italian heavy pigs. The animals (97 kg) were randomly assigned to three diets varying in supplemental CLA (CON = 0 CLA, T1 = 2.5 g CLA kg⁻¹ feed and T2 = 5.0 g CLA kg⁻¹ feed) till the slaughtering at 172 kg. Samples of longissimus dorsi were analysed for chemical composition (moisture, protein and lipid content), fatty acid composition and volatile compounds. No significant differences were observed for proximate chemical composition. Dietary CLA showed limited effects on fatty acid composition of longissimus dorsi, with higher, but not significantly, amounts of saturated fatty acids in the treated groups than in the control group; both the cis‐9, trans‐11 and the trans‐10, cis‐12 isomers of CLA were increased in longissimus dorsi from pigs fed CLA. T1 and T2 pigs had a greater concentration of C16:0 and of C16:1 (P < 0.01) than CON. CLA diets tended to reduce C20:2 (P = 0.077) and C20:4 (P = 0.065) content in longissimus dorsi muscle. Diets containing higher amount of CLA were responsible for increased levels of volatile compounds in meat, but not at a significant level. Copyright © 2005 Society of Chemical Industry     

Cryoprotective additives and cryostabilisation effects on muscle fillets of the freshwater teleost fish Rohu carp (Labeo rohita) during prolonged frozen storage

The effects of various cryoprotective additives separately and in combination were studied on the myofibrillar protein integrity, biochemical enzyme activity levels and muscle ultrastructure in the freshwater teleost fish Rohu carp (Labeo rohita). Fish muscle samples were divided into eight groups and immersed in different mixtures of cryoprotective additives (S1–S8), then frozen at ? 20 or ? 30 °C for 24 months. Electrophoretic studies revealed early (within 6 months) alteration of the myofibrillar proteins myosin light chain, α‐actinin and tropomyosin. Reduction of the storage temperature from ? 20 to ? 30 °C slowed down the degradative processes. Sodium dodecyl sulfate polyacrylamide gel electrophoresis indicated that fish muscle treated with cryoprotective mixture S8 (40 g L^?1 sorbitol/3 g L^?1 sodium tripolyphosphate/4 g L^?1 sodium alginate) showed minimal post mortem changes in myofibrillar proteins. Ultrastructural results also revealed post mortem damage to the muscle, seen earliest (within 6 months) in the sample frozen‐stored without additives (S2), as compared with the normal, unfrozen muscle (S1). The influence of cryoprotectants alone and in combination on fish muscle structural proteins, myosin and actin filaments (A and I bands), during prolonged frozen storage was investigated. After 12 months, samples frozen‐stored with various cryoprotective additives (S2‐S7), except S8, showed signs of myofibrillar disintegration. Beyond that time the degradative processes started showing up in all samples, with minimal muscle ultrastructural damage in sample S8. Again, reducing the storage temperature from ? 20 to ? 30 °C slowed down the degradative processes. Ultrastructural results correlated well with levels of biochemical enzymes (Ca²⁺ myofibrillar ATPase and succinic dehydrogenase) during frozen storage. This is the first report of the cryoprotective effects of these additives on this popular edible fish species. Of the various combinations of additives tested, cryoprotective mixture S8 was found to preserve the muscle structure longest under frozen storage conditions. However, even this mixture was only effective for 18 months at ? 30 °C. Beyond that time the myofibrillar degradative processes were apparent with correlative electrophoretic, biochemical and ultrastructural studies. Copyright © 2006 Society of Chemical Industry     

The effect of hot-boning on glycolysis in beef muscle.

The M. semimembranosus (SM) was excised from one side of six beef carcasses within 1 h of slaughter 3(hot-boned) and held at 10°C until 24 h post mortem and then at 3°C. The contralateral muscle underwent normal chilling in the intact side. Changes in energy phosphates, temperature and pH were followed in the intact SM at depths of 1.5, 5 and 8 cm from the carcass surface and at corresponding locations in the hot-boned SM. Hot-boning had a major effect on the rate of metabolism in the muscle post mortem. A relatively uniform rate of glycolysis was observed throughout the hot-boned muscle compared to the carcass muscle in which the rate of glycolysis increased with depth. The ATP content in the hot-boned muscle fell to 50% at 8 h post mortem and rigo^r was completed within 24 h.     

Immunolocalisation of intermediate filament proteins in porcine meat. Fibre type and muscle-specific variations during conditioning

Two intermediate filament proteins, desmin and vinculin, were immunofluorescently localised in porcine longissimus dorsi(??) and iliocostalis muscles up to 7 days post mortem. Changes in tenderness of the m. longissimus and the water-holding capacity of both muscles were also monitored throughout this storage period. Longissimus muscle shows a rapid decline in the overall intensity of labelling for both desmin and vinculin. In contrast to the vinculin labelling, desmin labelling is preferentially lost from type IIB muscle fibres in the longissimus muscle. In the redder iliocostalis muscle, the loss of desmin and vinculin labelling was less rapid and did not show an obvious relation to muscle fibre type. In one sample with very high water loss, there were indications of greater extracellular space development and delayed loss of intermediate filament labelling. The time course of diminishing intermediate filament labelling is consistent with previous suggestions that degradation of these proteins is involved in post-mortem conditioning. The variations within and between muscles shown here may explain some of the variability in their mechanical properties. Additionally, it is suggested that intermediate filament integrity is necessary for the expulsion of water from the muscle cells during drip formation.     

Identification of biomarkers of meat tenderisation and its use for early classification of Asturian beef into fast and late tenderising meat

BACKGROUND: The objective of this work was to study the post‐mortem evolution of potential biomarkers (µ‐calpain activity and proteolytic profile) of meat tenderisation in bovine longissimus dorsi (LD) muscle from several biotypes coming from two beef breeds (‘Asturiana de los Valles’ and ‘Asturiana de la Montaña’) and showing different levels of muscular hypertrophy (mh/mh, mh/+, + /+). RESULTS: LD samples were taken at 2, 12, 24 and 48 h and 3, 7, 14 and 21 days post‐mortem. The presence of muscular hypertrophy produced a faster rate of pH decline, faster exhaustion of µ‐calpain activity and earlier occurrence of proteolytic changes. Changes in the electrophoretic pattern of some peptides from sarcoplasmic (glyceraldehyde‐3‐phosphate dehydrogenase) and myofibrillar (troponin T and troponin I) muscle extracts within the first 24 h significantly correlated with meat toughness and allowed accurate discrimination of meat products into two groups: (1) fast tenderising meat, coming from mh‐biotypes, and (2) late tenderising meat, from normal (+/+) biotypes. CONCLUSION: Early monitoring (within 24 h after slaughter) of selected biomarkers in LD muscle allowed accurate prediction of ultimate meat toughness and could be used in the meat industry as a tool for early classification of beef into fast and late tenderising meat. Copyright © 2012 Society of Chemical Industry     

Post-mortem Changes in Breast Muscles of Mule Duck

Post-mortem changes in myofibril fragmentation index and degradation of myofibrillar proteins of duck breast muscles at 5°C was investigated. Muscle samples were collected immediately after killing and from the stored carcasses at 5°C for 1, 3, 7, and 14 days post mortem . The results showed that the MFI of the breast muscles increased with time post mortem . SDS-PAGE of myofibrils indicated that the disappearance of troponin-T accompanied concurrently with the appearance of 32–30 kDa components. After 7 days of storage, α-actinin was degraded, and a 98 kDa component appeared. Titin 1 and nebulin also disappeared after post-mortem storage for 3 days.     

Influence of caspase3 selective inhibitor on proteolysis of chicken skeletal muscle proteins during post mortem aging

The objective of this study was to investigate the potential contribution of apoptosis related downstream executioner caspase3 to post mortem skeletal muscle proteolysis by use of caspase3 selective inhibitor DEVD–CHO (N-acetyl–Asp–Glu–Val–Asp–CHO). After slaughter, four chicken breast muscles were removed and cut into small pieces, then marinated in treatment solution containing DEVD–CHO, or in control solution, and stored at 4 °C for 1, 3 or 7 d. Meat samples were obtained and used for detecting muscle protein degradation or calpain activity. Results showed that DEVD–CHO had inhibited the degradation of muscle skeletal proteins (titin, nebulin, desmin and troponin-T) significantly, whereas the activity of calpains had not been influenced. Therefore, the degradation of muscle proteins should not been exclusively attributed to the calpain system, and the effector caspase3 may be a new protease involved in meat post mortem tenderization.     

Ultramicroscopical Structures and Liquid Loss in Heated Cod ( Gadus morhuaL) and Salmon ( Salmo salar) Muscle

This study was performed in order to assess the effect of heating in pre- and post-rigor muscle of fed cod, wild cod and farmed salmon harvested at different times of the year. The structural changes in muscle samples pre-heated from 5 to 60°C were qualitatively evaluated using both light and transmission electron microscopy techniques. The microstructural changes are discussed in relation to the liquid loss measured by a low-speed centrifugation test. The heat-induced structural changes varied between the fish tested, reflecting different degrees of post mortem degradation prior to heating, the muscle-pH and species-specific structural properties. The fed fish, both cod and salmon, underwent the most severe structural degradation. This reflected both the low muscle pH and the more severe post mortem degradation observed in these fish prior to heating, compared with the wild cod. Heating caused extensive shrinkage of the myofibrils and hence, widened intermyofibrillar and extracellular spaces in both the fed cod and the salmon muscle. In the sample of wild cod muscle, the extracellular spaces were narrow and the myofibrils were closely packed. The difference in heat-induced liquid loss of the fed compared with the wild cod muscle coincides with their different structural features, as observed both by LM and TEM. The better liquid-holding properties of the salmon muscle than the cod muscle are attributed to the species-specific ultrastructural features as observed with TEM. In addition to the denser appearance of the salmon myofibres, it is suggested that both fat droplets and aggregated sarcoplasmic proteins filling the intermyofibrillar and extracellular spaces are important in preventing release of liquid upon heating.     

Structural and ultrastructural changes of full-cycle cultured Pacific bluefin tuna (Thunnus orientalis) muscle slices during chilled storage

Background: This study examined the structural and ultrastructural changes of dorsal and ventral muscle tissues of full‐cycle cultured Pacific bluefin tuna (PBT), Thunnus orientalis Temminck & Schlegel 1844, cut into slices simulating sashimi and placed in chilled storage for varying periods. Structural and ultrastructural changes were determined in order to understand the physical texture by breaking strength measurement. Results: Progressive deterioration of myofibril structure was observed during chilled storage (4 °C) of PBT muscle slices over 5 days post mortem . Muscle degradation included detachment between myofibres, detachment of the plasmalemma, disruption of mitochondria, loss of Z‐line density and alignment, cementation of myofibrils, loss of the hexagonal arrangement of thick versus thin myofilaments and migration of subsarcolemmal nuclei to intermyofibrillar spaces. Conclusion: Loss of myofibre‐myofibre adhesion, detachment of the plasmalemma and disruption of other components did not lower the breaking strength of PBT muscle. This provides evidence that the muscle breaking strength of PBT is not only associated with the detachment of myofibres or detachment of the plasmalemma. Other factors that produce cement‐like substances, such as cementation of the myofibrillar components and degradation of the sarcoplasmic reticulum, may also increase breaking strength. Copyright © 2011 Society of Chemical Industry     

Effect of Calcium Lactate, 4-Hexyl Resorcinol and Vacuum Packing on Physico-chemical, Sensory and Microbiological Qualities of Minimally Processed Litchi (Litchi chinensis Sonn.)

Litchi (Litchi chinensis Sonn.) fruits are very susceptible to pericarp browning which adversely affects consumer acceptability even though the aril portion remains in excellent condition. Litchi arils (litchis) were treated with a solution containing 0–2% (w/v) calcium lactate (CL), 0–0.02% (w/v) 4‐hexyl resorcinol (4‐HR) and 1% potassium sorbate. The pH of solution was adjusted to 4.0 with citric acid. Treated litchis were packed in polystyrene trays, over‐wrapped with polypropylene film, vacuum‐packed (0, 47409.3, 94831.9 Pa) and stored at 4 ± 2 °C. Drip losses, pH, total soluble solids (TSS), sensory attributes and microbiological quality of stored samples were estimated. A four‐factor, three‐level experimental design (D₆ Hokes design) with 19 experiments was chosen. Mathematical models were developed to analyse and predict the effect of CL, 4‐HR, in‐package vacuum and storage time on the responses. TSS, pH and sensory scores decreased significantly (P 0.01), whereas drip losses and microbial count increased significantly (P 0.01) with time. Drip loss was significantly (P 0.1) reduced by addition of CL. 4‐HR prevented browning and changes in colour score during storage were significantly less. Vacuum in packages exerted significant (P 0.01) effect over pH, TSS, sensory and microbiological qualities of minimally processed litchis.     

Effect of calcium lactate on m-calpain activity and protein degradation under oxidising conditions

In two experiments, the effects of calcium lactate (CAL) on calpain activity were determined. In the model system, purified porcine skeletal muscle m-calpain was pre-incubated with various combinations of hydrogen peroxide (H₂O₂), calcium chloride, and/or different CAL concentrations. The m-calpain was activated by CAL, and the extent of m-calpain oxidation by H₂O₂ was significantly decreased with increasing CAL concentrations. In the muscle system, a beef longissimus lumborum (1 day postmortem) from each carcass (n = 6) was cut in half, randomly assigned to either 0.2 M CAL or water injection (WAT), and then packaged in a high-oxygen modified atmosphere. The CAL injected steaks resulted in less intact desmin and greater production of a 30-kDa troponin-T compared to the steaks in the WAT group. The CAL injection did not affect colour and lipid oxidation of steaks during display. These results suggest that CAL addition may improve tenderness of meat by enhancing activation of endogenous calpain and by protecting against calpain oxidation under oxidative conditions.     

Aktivit?t und subcellul?re Verteilung einiger Mitochondrien-Enzyme im Skeletmuskel. II. Ver?nderungen im Rindermuskel post mortem

Zusammenfassung Proben desLongissimus-dorsi-Muskels des Rindes wurden bis zu 15 Tage nach dem. Schlachten bei 4° C gelagert. Die Änderung der extrahierbaren absoluten und spezifischen Aktivität der Mitochondrien-Enzyme Aconitase (AC), Fumarase (FU), Glutamatdehydrogenase (GLDH), Malatdehydrogenase (MDH) and Succinodehydrogenase (SDH) während der Lagerung wurde studiert. Es wurde ferner der Einfluß der Lagerung auf die Aktivität der SDH im nicht zentrifugierten Muskelhomogenat ermittelt. Während die Gesamtaktivität von FU und ACpost mortem abnahm, blieb die Aktivität von SDH und MDH nahezu unverändert. Die extrahierbare GLDH-Aktivität wies im Zustand desRigor mortis ein Minimum auf.Durch Bestimmung der Enzymaktivitäten im Preßsaft des unzerkleinerten Gewebes wurde die Änderung der subcellulären Verteilung der Mitochondrien-Enzymepost mortem verfolgt. AC, MDH und SDH wurden während der Lagerung des Muskelspost mortem nicht aus ihrer Bindung an die Mitochondrien freigesetzt. FU wurde nach Eintritt desRigor mortis in geringem Maße freigesetzt. GLDH trat innerhalb von 24 Stdpost mortem in erheblichem Umfang aus ihrer Bindung an Mitochondrien in das Sarkoplasma über.Die Resultate lassen den Schluß zu, daß die Mitochondrien des Rindermuskels im Verlauf desRigor mortis eine Schädigung erfahren, die zu einem partiellen Austritt der Matrix in das Sarkoplasma ohne wesentliche Desintegration der Mitochondrienmembranen führt. Die Möglichkeit einer Öffnung der intramitochondrialen Räume durch Quellung der Mitochondrien als Folge des glykolytisch bedingten pH-Abfalls wird diskutiert.

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Activity and subcellular distribution of some mitochondria) enzymes in skeletal muscle. II. Post-mortem changes in bovine muscle

Summary Samples of bovineM. longissimus dorsi were stored for up to 15 days post mortemat at 4° C. The changes in the extractable absolute and specific activities of the mitochondria) enzymes aconitase (AC), fumarase (FU), glutamic dehydrogenase (GLDH), malate dehydrogenase (MDH) and succinic dehydrogenase (SDH) were determined. The influence of storage on the activity of SDH in the muscle homogenate was also studied. There was a decrease in the total activity of AC and FU post mortem, but no remarkable changes in the activity of SDH and MDH. The GLDH activity showed a minimum at the time of rigor mortis.The changes in the subcellular distribution of the mitochondria) enzymes post mortem were determined by studying the enzyme activities in the press juice of the intact muscle tissue. AC, MDH and SDH were not released from mitochondria) structures during storage of muscle. There was a small release of FU after development of rigor mortis. However, a great part of GLDH was released from the mitochondria into the sarcoplasma within 24 h post mortem.The results show that the mitochondria of bovine muscle undergo a certain damage during development of rigor mortis which results in a partial efflux of the mitochondria) matrix into the sarcoplasma without a remarkable desintegration of the mitochondria) membranes. The possibility of an opening of the intramitochondrial spaces by swelling of mitochondria caused by the decrease of pH post mortem is discussed.

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Stipendiat der Alexander-von-Humboldt-Stiftung. Jetzige Adresse: University of Assiut, Faculty of Agriculture, Assiut, VAR.     

Relationship between Ca(2+) release, sarcoplasmic Ca(2+), glycolysis and meat quality in halothane-sensitive and halothane-insensitive pigs

Post-mortem assessment of porcine stress susceptibility and meat quality by measurements of mitochondrial calcium (content and rate of release), water-holding capacity, pH at 1h post mortem and colour on M. longissimus dorsi confirm the usefulness of the halothane test for detecting stress susceptibility. Good correlations were observed between halothane sensitivity and meat quality. Post-mortem samples of M. longissimus dorsi of halothane-sensitive pigs showed significantly higher levels of sarcoplasmic calcium than similar muscle from halothane-insensitive pigs. This strongly suggests that elevated sarcoplasmic calcium levels are linked to the formation of PSE meat.     

Non-invasive mobile monitoring of meat quality

Detailed investigations into the time-dependent changes of pork meat quality and composition have been carried out using standard analytical techniques and Raman and fluorescence spectroscopy. Characteristic changes in soluble protein content, microbial load and biogenic amine content have been measured in the time period from slaughter up to 20 to 30 days post mortem on samples of porcine musculus longissimus dorsi. Characteristic behaviour indicative of a deterioration of meat quality and an increase in microorganism population on the meat surface has been observed. Fluorescence and Raman optical signals have been identified which change over this time period, correlating with the standard measurements and these can be used as indicators of meat properties. These signals can be measured rapidly and without the destruction of the meat sample allowing non-invasive testing. The combination of these two powerful measurement techniques provides a pathway to comprehensive meat quality monitoring. Furthermore we have developed a hardware concept for carrying out these spectroscopic measurements using a compact and light handheld device and we present concepts for the use of such a device in the logistic chain in combination with a transponder chip and integrated sensors which provide product identification and information about storage conditions along the entire logistic chain to the supermarket shelf. Received: September 16, 2008; accepted: October 5, 2008     

Relationship between rate of post mortem pH fall and ageing of Longissimus muscle in pietrain pigs

Twenty-six pietrain pigs exhibiting wide variability in pH₄₅ (pH value 45 min post mortem) values were used to assess the mechanical behaviour of cooked and raw Longissimus muscle at 24 h and 8 days post mortem. Rheological behaviour was studied in compression under strain conditions which allow specific determination of myofibrillar resistance. The muscles were divided into three groups on the basis of pH₄₅ values: (Severe pale, soft and exudative (PSE), pH₄₅ = 5.49 ± 0.03 (n = 9); PSE pH₄₅ = 5.78 ± 0.04 (n = 10); Normal, pH₄₅ = 6.35 ± 0.05 (n = 7). The three groups did not differ significantly for ultimate pH (pH_u = 5.57 ± 0.02 for the 26 muscles). A lower mechanical resistance of raw samples was found in the Normal group, regardless of the time post mortem. Although this difference was not significant, a significant relationship was found between this trait and pH₄₅ at both times post mortem (r = ?0.43, P < 0.05, at 24 h and r = ?0.44, P < 0.05, at 8 days post mortem). However, no significant effect of pH₄₅ was found on the mechanical resistance of cooked muscles. The relative decrease in myofibrillar strength after an 8-day ageing period was not affected by pH₄₅ but this was due to the fact that the three groups exhibited different starting points. Mechanical resistance of raw and cooked muscles at 24 h post mortem were significantly related to pH_u (r = ?0.55, P < 0.01, and r = ?0.49, P < 0.05, respectively). The present results show that after an 8-day ageing period, PSE muscles are not fully aged.