The effect of chilling, electrical stimulation and conditioning on pork eating quality

The effect of three different post-slaughter treatments and subsequent conditioning times on the eating quality of pork was studied, using a total of 72 pigs (80–90 kg live wt). The treatments were: (A) holding in air at > 10°C for 3 h, followed by chilling in air at 1°C; (B) chilling in air at 1°C; (C) high voltage electrical stimulation (ES) at 20 min post-slaughter, followed by Treatment B. The quality attributes were measured in M. longissimus thoracis et lumborum (LTL) and in M. semimembranosus (Sm).

There was little difference in cooling rate between the three treatments; the major effect on quality came from the use of ES in Treatment C. ES reduced pH at 45 min by approximately 0·3 units, and achieved pH values at 3 h post-slaughter of 5·64 (LTL) and 5·87 (Sm) but did not produce PSE meat. Drip losses were generally low, but were slightly higher with Treatment C. By all three instrumental texture parameters, LTL from Treatment C was significantly more tender than from A and B at 4, 7 and 12 days post-slaughter, suggesting that either some cold toughening with A and B was overcome by ES in treatment C or that ES had some other beneficial action. Conditioning at 1°C improved the tenderness of LTL from 4 to 7 days and further to 12 days. Taste panelling of loin chops and Sm roasts confirmed that Treatment C gave significantly more tender meat than A and B, and that ageing from 4 to 7 days and further to 12 days significantly improved tenderness. High voltage electrical stimulation at 20 min post-slaughter followed by cooling in air at 1°C (Treatment C) produced loin muscle which was more tender at 4 days than at 12 days with the other treatments.     

Effect of different electrical stimulation and chilling treatments on pork quality

The effect of different chilling and electrical stimulation (ES) treatments on the carcass and eating quality of pork has been examined. Preliminary trials (Study I), with a variety of ES treatments, indicated that, on the basis of pH fall, drip loss and tenderness, the most effective was that using 700 V peak at 12·5 Hz for 90 s applied at 20 min post-slaughter. These ES parameters were used throughout the main study (II) using 80 pigs, one side of which was stimulated and the other not, before being chilled rapidly (10°C in deep loin within 2–3 h of slaughter) or conventionally (10°C in deep loin within 5 h of slaughter).

Changes in pH and temperature during chilling were observed and, after chilling, quality assessments on LD and Sm muscles included drip loss, muscle opacity and instrumental toughness at 3 days post-slaughter.

Electrical stimulation, 20 min post-slaughter, improved tenderness of the LD and, to a lesser extent, the Sm of rapidly chilled pig sides. This advantage was gained without producing PSE pork. In fact, drip loss from the LD was consistently less from the sides which had been stimulated.     

低压电刺激和冷却方式对猪肉品质特性的影响

采用2×2析因设计研究了低压电刺激和冷却方式对猪肉品质特性,包括背最长肌温度、pH、肉汁损失、煮制损失和剪切力的影响。结果表明,低压电刺激加快猪背最长肌pH下降,增大剪切力,对持水性有不利影响;二阶段冷却加快猪背最长肌温度下降,减缓pH的下降速率,显著降低肉汁损失,对剪切力没有显著影响。回归分析表明,煮制损失最能反映猪肉剪切力。采用二阶段冷却或低压电刺激配合二阶段冷却工艺可以保持良好的猪肉品质。     

Effects of electrical stimulation, chilling temperature and hot-boning on the tenderness of bovine muscles

The objective of this study was to determine the effects of hot-boning, low voltage electrical stimulation (ES) and chilling temperature on the tenderness of bovine M. longissimus dorsi (LD) and M. semimembranosus (SM) muscles. LD (n=32) and SM (n=32) muscles were subjected to different post-mortem treatments; hot-boning (before 90min post-mortem), cold-boning (at 48h post-mortem), low voltage ES and rapid or slow chilling. Hot-boned muscles which were not electrically stimulated (NES) had higher Warner Bratzler shear force (WBSF) values (P<0.001) and shorter sarcomeres (P<0.001) than cold-boned muscles. Hot-boned muscles subjected to ES had lower pH values (P<0.001) post-stimulation and up to 8h post-mortem than NES muscles. At both chilling temperatures WBSF values were lower in ES hot-boned LD and SM muscles at days 2, 7 and 14 post-mortem than NES muscles. Hot-boned muscles subjected to slow chilling had longer sarcomeres (P<0.001) than those subjected to fast chilling. In hot-boned SM muscles, ES resulted in longer sarcomere lengths (P<0.001). However, ES did not have a significant effect on the sarcomere length of LD muscles. As indicated by WBSF values all muscles tenderised during ageing, including muscles which were 'cold shortened'. Proteolysis was not the main reason for differences in WBSF values between ES and NES muscles as judged by qualitative sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). A combination of slow chilling and ES had a positive effect on hot-boned muscles with respect to WBSF values.     

High voltage electrical stimulation enhances muscle tenderness, increases aging response, and improves muscle color from cabrito carcasses

Our study examined high and low voltage electrical stimulation and postmortem storage as strategies to improve tenderness and lean color in cabrito carcasses. Boer cross (n=60) kids were assigned to either high (550 V), low (20 V), or no electrical stimulation treatments. No differences in muscle temperature were observed between treatments at any time measured. Muscle pH declined fastest in high voltage treated carcasses. High voltage electrical stimulation slightly increased (P<0.05) b^* and a^* in the M. gluteus medius and tended to increase L^* and b^* (P=0.06 and 0.11, respectively) values in the M. longissimus thoracis. Electrical stimulation had no effect on myofibril fragmentation at 1-, 3-, or 14-d postmortem or sarcomere length. High voltage electrical stimulation increased (P<0.05) tenderness at 1- and 3-d postmortem, but not at 14-d postmortem. Aging for 3 d did not affect tenderness regardless of stimulation treatment, but aging time for 14 d improved (P<0.05) tenderness. These data indicate that high voltage electrical stimulation and 14 d aging were effective in improving the tenderness of meat from cabrito carcasses.     

Effect of electrical stimulation, high temperature conditions and ageing on muscle myofibrillar proteins

Electrical stimulation, when accompanied by high temperature incubation, was found to lower the extractability of myofibrillar proteins and increase their degradation, particularly that of the myosin component. Troponin T and myosin light chain 2 were similarly affected. Troponin T and troponins I and C were degraded earlier during ageing in electrically stimulated muscles incubated at high temperature than in conventionally chilled muscles.     

Electrical stimulation and ultra-rapid chilling of pork

Six pigs were stimulated at 5 min post mortem and six remained unstimulated. All the pigs were split hot and one side from each pig was rapidly chilled in two stages (air at -40°C and 1 m/s for 80 min followed by 0°C and 0·5 m/s for 130 min) and the other side was conventionally chilled (air at 0°C and 1 m/s for 24 h). The weight loss from rapidly chilled sides was approximately 1% less than that from conventionally chilled controls. Cooked samples of Longissimus dorsi were tougher from unstimulated rapidly chilled sides (0·23 J) than from unstimulated conventionally chilled sides (0·18 J), whilst samples from both stimulated treatments were significantly more tender (0·15 J). These differences in toughness are thought to be due to a combination of cold shortening and lack of conditioning. The average pH in the longissimus dorsi of both rapidly and conventionally chilled stimulated sides at 50 min post mortem was 5·57 and samples from these muscles were slightly paler and more watery than the unstimulated controls.     

Responses of different types of ox muscle to electrical stimulation

The effects of electrical stimulation on fall in pH upon stimulation (ΔpH), rate of pH fall (dpH/dt at 35°C, cold shortening and muscle ultrastructure were investigated for the Cutaneus trunci (predominantly fast-twitch glycolytic fibres), the Masseter and Diaphragm (predominantly slow-twitch oxidative fibres) and the Sternomandibularis and Longissimus dorsi (bot fast- and slow-twitch fibres) of the ox. The Masseter and Diaphragm showed a small ΔpH and no increase in dpH/dt upon stimulation. Stimulation produced supercontracture but no tearing of the fibres throughout all of the Masseter. Stimulation of the Cutaneus trunci resulted in a significantly increased ΔpH and dpH/dt, loss of glycogen, mitochondrial swelling but no gross sarcomere changes. The Longissimus dorsi and Sternomandibularis had a moderate ΔpH and an increase in dpH/dt intermediate between that of the Masseter and the Cutaneus trunci. The Longissimus dorsi showed supercontracture, but the Sternomandibularis did not. Cold shortening responses at 2°C and 0°C were virtually unaffected by stimulation, being greatest in the Masseter and Diaphragm and least in the Cutaneus trunci. All muscles showed significantly greater shortenings at 0°C than 2°C. Stimulation of the Cutaneus trunci did not affect the tenderness of the cooked meat, but the toughness increased dramatically in cold-shortened Cutaneus muscle, regardless of stimulation. The Cutaneus trunci least requires stimulation as it does not cold shorten appreciably and therefore early rigor would confer no advantage. The Masseter and Diaphragm have a fast dpH/dt and therefore would enter rigor early. Mixed muscles apparently have the combined, least desirable characteristics of the muscle fibre types-i.e. their rate of rigor development is slow and they cold shorten. Electrical stimulation confers a significant advantage by mitigating these mixed-muscle characteristics in carcass muscles.     

Effect of low-voltage electrical stimulation after dressing on color stability and water holding capacity of bovine longissimus muscle

The effect of low voltage electrical stimulation after dressing (ES) on color stability and water holding capacity (WHC) of beef was investigated. Nine Swedish red cattle were slaughtered and the left side was electrically stimulated (80 V, 35 s) approximately 30 min after stunning, whereas the other side was not treated and used as control. Color and its stability, WHC, and protein solubility were evaluated on longissimus lumborum muscles from the two sides. ES produced a brighter red color at 24h mainly by increasing the oxygenation capacity of myoglobin (P<0.01), which was attenuated by postmortem aging. ES did not affect WHC, protein solubility and color stability (P>0.05). Therefore, this technology could accelerate meat tenderization without any negative effect on commercial attributes, such as color or drip of bovine longissimus muscle.     

Cold shortening capacity and beef muscle growth

Cold shortening capacity is related to animal growth. For unloaded strips of beef sternomandibularis muscles the degree of cold shortening developed at 2 °C during the pre-rigor period increased from approximately 0.2 for small young calves to 0.6 for large old bulls. This growth-dependent capacity to cold shorten is even more pronounced in muscle strips shortening isometrically. Thus in terms of power development, it increases forty-fold with the growth of beef sternomandibularis muscles to their mature sizes. In contrast, temperature sensitivity of the phenomenon is virtually the same for both small and large animals with the tendency to cold shorten declining in both to zero at 35 °C. These findings are important to meat chilling since they imply that cold shortening will be more pronounced the larger the carcass. This is offset to a degree by the fact that large carcasses will chill more slowly than smaller carcasses.     

Proteome changes in the insoluble protein fraction of bovine Longissimus dorsi muscle as a result of low-voltage electrical stimulation

Changes induced by low-voltage electrical stimulation (ES; 0-95 V for 8 s; 95 V for 32 s) in the insoluble protein fraction of bovine longissimus dorsi (LD) muscle at 1 and 24h post-ES were investigated by proteomics. Protein abundance patterns from ten Norwegian Red (NRF) young bulls were compared, and significant changes due to ES were found by rotation test and partial least square (PLS) regression analyses. Five protein spots showed lower abundance in ES samples at both sampling times, and in addition, 10 proteins at 1 h post-ES and 13 proteins at 24 h post-ES changed significantly in abundance due to ES. Reduced abundance of full-length structural proteins in ES samples indicates an accelerated proteolysis due to ES. Moreover, increased abundance of small heat shock proteins indicates earlier initiation of stress responses due to ES. These findings provide a better understanding of the biochemical processes taking place as a result of ES during post mortem storage of meat.     

Effect of electrical stimulation on the rheological properties of rabbit skeletal muscle

The effect of electrical stimulation on the rheological properties of rabbit skeletal muscle after death was investigated. The extensibility of electrically stimulated psoas muscles decreased more rapidly than that of non-stimulated muscles. For raw non-stimulated longissimus thoracis muscles excised from the carcasses immediately after slaughter, the penetration force required was greatest 24 h after slaughter and then decreased slightly after 168 h. The corresponding force for stimulated longissimus thoracis muscles increased to a maximum in 12 h and then decreased to values less than non-stimulated muscles. However, in the case of raw longissimus thoracis muscles whichhad been attached to the skeleton until measurement, there was no significant difference in penetration force between stimulated and non-stimulated muscles. In cooked muscles, electrical stimulation resulted in lower penetration forces at 24 h post mortem, but on further storage the differences decreased.     

乾州板鸭加工中肌肉蛋白质及肌纤维的降解规律分析

乾州板鸭是湘西吉首地区的一种传统腌腊肉制品。为了解乾州板鸭加工过程中蛋白质的降解规律,文中采用SDS-PAGE电泳分析不同加工阶段鸭胸肉中蛋白质的降解情况,并使用扫描电子显微镜观察肌肉纤维的变化。结果发现:在乾州板鸭腌制第14天,全肌肉蛋白、肌浆蛋白和肌原纤维蛋白发生了明显的降解,而腌制第21天至风干第14天期间,肌肉蛋白质降解不明显。肌肉纤维在腌制第14天时排列较紧密,而当风干至第7天时,结构变得松散,肌纤维数量减少,出现断裂的肌纤维,至风干第14 d时,肌纤维间的孔隙较大,肌纤维数量进一步减少。     

An immunological method to assess protein degradation in post-mortem muscle

A method for determining proteolysis of any specific protein in muscle is demonstrated. The protocol involves the preparation of a specific antibody which is used in immunoblotting total protein extracts from post-mortem muscle. In the present study an antibody to bovine myosin heavy chain was prepared that reacts with intact myosin heavy chain as well as proteolytic fragments that are generated by proteases. We show that little, if any, myosin heavy chain fragments are generated during prolonged aging of muscle at 4°C. In contrast, storage of muscle at 37°C results in the rapid breakdwon of myosin heavy chain to immunologically detectable peptides. Using a monoclonal antibody to titin, we demonstrate that this protein is degraded at 4°C during the aging period, and that, between 2 and 3 weeks following slaughter, no undergraded titin is detectable. This method is suitable for the analysis of any protein that can be separated by SDS polyacrylamide gel electrophoresis (SDS-PAGE).     

Cryogenic chilling of pork carcasses: Effects on muscle quality, bacterial populations and palatability

Two experiments were conducted to determine the effects of cryogenic chilling on the carcass shrinkage, meat quality, bacterial condition and palatibility of pork. In experiment I, pork sides were chilled at 1°C (n = 20), or immersed in liquid nitrogen (LN) for 1 or 3 min prior to placement in a 1°C cooler. Muscle temperature in the loin was significantly lower at 2 and 6 h post mortem in treated compared to control sides, and loin muscle pH was higher (P < 0·05) at 6 h post mortem in sides immersed for 3 min in LN. Carcass side shrinkage was reduced from 29·3 g kg(-1) in control sides to 20·9 and 13·5 g kg(-1) in sides dipped in LN for 1 and 3 min. Chilling treatment had no significant effect on the survival of mesophilic bacteria on carcass sides, on meat colour, drip loss, protein solubility or sarcomere length, but sides dipped for 1 min in LN has a higher muscle shear value than control sides. In experiment II, carcass sides from halothane positive (H+) and negative (H-) pigs were conventionally chilled (n = 49), immersed in LN for 3 min (n = 23), or electrically stimulated and chilled in LN for 3 min (n = 26). Similar results for temperature, pH, colour, protein solubility and drip loss in loin muscle were found to those in experiment I. Laboratory taste panel results showed that chilling treatment had no effect on palatability. Genotype produced meaningful differences in most palatability attributes with H+ pigs having less tender, less juicy and less desirable flavour than pork from H- pigs. Laboratory studies with inoculated fresh muscle slices showed that a 3 min immersion in LN resulted in a 10-fold reduction in the aerobic spoilage pseudomonads, but effects upon other spoilage bacteria and potential human pathogens were less pronounced. It was concluded that cryogenic chilling using LN reduced carcass shrinkage during cooling, but had no consistent effects on meat quality, palatability or bacterial numbers on the carcass. In contrast, genotype had a significant effect on most pork quality and palatability attributes.     

Effect of dietary ractopamine on tenderness and postmortem protein degradation of pork muscle

Twenty-four finishing pigs with a mean starting weight of 82 kg were assigned to two dietary regimens: (1) a corn–soybean meal basal diet (control; n = 12), and (2) the basal diet supplemented with 20 ppm ractopamine HCl (RAC; n = 12). After 28–30 days on the feeding trial, pigs were slaughtered, and the growth and carcass characteristics were measured. Furthermore, the 3rd–13th rib section of longissimus muscle was excised at 48 h postmortem, sliced into 19-mm thick chops, vacuum packaged, stored at 2 °C, and subjected to Warner–Bratzler shear force (WBSF) and electrophoretic tests after 2, 4, 7, 10, 14, and 21 days (postmortem). RAC feeding increased (P < 0.01) pig carcass weight and percent lean, but it also increased the day-2 muscle WBSF by 20% (P < 0.01). The shear force difference between control and RAC pig muscles gradually decreased and vanished by day 10 (P > 0.05) when both muscle groups became more tender. The muscle from RAC-fed pigs exhibited a slower protein degradation rate than muscle from the control animals, notably for proteins in the 15–45 kDa range. The results suggested that the tenderness difference between ractopamine-treated and control pig muscles was related to the proteolysis rate, and could be diminished with adequate postmortem ageing.     

Effect of heating rate on shortening, ultrastructure, and fracture behavior of prerigor beef muscle

Rapidly heated prerigor beef is tender because of incompletely described myofibrillar disruption and tissue fracture. This study was designed to evaluate the effects of heating rate on heat-induced myofibrillar shortening, ultrastructural changes, and fracture behavior in prerigor triceps brachii muscle. Rapid heating (2°C/2 min) to 53°C caused (P < 0·05) more severe myofibrillar shortening in a shorter time and at higher muscle pH and temperature, less muscle weight loss, and shorter sarcomeres than slow heating (2°C/12 min) to 47 or 53°C. Rapid heating caused more extensive degradation of A and I bands, greater loss of the tridimensional pattern of myofibrils, more fragmentation and melting of myofibrils, widened intermyofibrillar spaces, and maximum separation of fiber bundles as compared to slow heating. Slow heating caused extensive shortening but not extensive degradation and disruption of myofibrils. Muscles slowly heated to 53°C sustained greater loss of structural integrity than those slowly heated to 47°C, but fracture behavior was similar. Separation and fracture occurred near the perimysial/endomysial junction in all heated samples, but the perimysium remained affixed to the endomysium at one side of the interface in many rapidly heated samples. Longitudinal fractures showed a granular endomysium and large numbers of supercontraction nodes alternating with areas of sarcolemmal membrane fragmentation and fiber tearing in rapidly heated samples. Alterations of myofibrillar ultrastructure and fiber structure, and separation of bundles, may account for enhanced tenderness of rapidly heated prerigor muscle.     

Colour stability of bovine Longissimus and Psoas major muscle as affected by electrical stimulation and hot boning

From eight electrically stimulated and eight non-stimulated cows the righthand-side longissimus and psoas major muscles were hot boned within 1 1 2 h post mortem, vacuum packaged and chilled and storred at 1±1°C. Immediately after slaughter, the lefthand carcass-sides were blast-chilled for 1 1 2 h and subsequently chilled at 1±1°C until the following day. After cold boning, the longissimus and psoas major muscle were packaged, chilled and stored as the hot boned muscles. After 12 days of storage, steaks, cut from the primals, were displayed at 1±1°C under continuous illumination (300-400 lx). Colour measurements after 0, 2 and 4 days of display revealed a significant (p<0·10) effect of time of boning on non-stimulated psoas major muscle (lower values for a (?), b (?) values, chroma and %R630-%R580). Significant effects of electrical stimulation were not observed. Changes in hue tended to be more pronounced when the meat had been stimulated. Changes in chroma were largest (p<0·10) is non-stimulated, hot boned psoas muscle. Analysis of variances showed that in the longissimus muscle significant effects (p<0·10) of time boning and electrical stimulation were present. The effect of time of boning was often influenced by the use of electrical stimulation. Changes in hue and chroma indicated that hot boned samples had a higher colour stability than cold boned controls, especially when the carcasses had not been stimulated electrically. The observed differences in colour stability were rather small in all treatment groups and are not expected to present any practical merchandising problem.