| Meat palatability consists of tenderness, juiciness, and flavor, ordered by the theirimportance for red meat and poultry meat. Of the three attributes, meat tenderness isthe most important, which determines the commercial value of meat products.Ante-mortem treatments and technological treatments within the first 24h postmortem have a great effect on meat tenderness. Meat has the similar tenderness, if theanimals have the same ante-mortem biological traits and the same treatments beforeslaughter. Chilling procedure and electrical stimulation within the first 24h affect therates of carcass temperature and pH decline, which determines structural andbiological changes during the transition from muscle to meat, and further leads to thedifferences in tenderness.Beef tenderness decreases within 24h postmortem and the largest decreaseoccurs between 9h and 24h. After that time, tenderness increases. Generally,it takes18 to 40h for beef pH to decline from 7.0 to 5.4. The application of electricalstimulation, low-temperature aging and chilling regime changes muscle ultrastructuresand the temperature/pH/time window, which in turn affects the starting time, rate, andextent of postmortem aging and ultimately affects beef palatability. The objective ofthis study was to explore the effects of electrical stimulation and delay-chilling onbeef ultrastructures and palatability. The study consists of the following two parts:Part 1. Effects of electrical stimulation and delay-chilling on beef palatabilityTwenty-four carcasses of crossbred Luxi cattle were randomly divided into twogroups: 12 carcasses (ES) were electrically stimulated just after bleeding with anelectrical stimulator (EST—601, Freund, Germany) for 60s (voltage 42V, electricalflow 0.7A, frequency, 50Hz); the other 12 carcasses were not stimulated (Control).The right side of each carcass (CC, conventional chilling) was chilled at 0 to 4℃for24h with the wind speed of 0.5m.s-1, whilst the left side (DC, delay chilling) was chilled at 15±2℃for 3h and then for 21h at 0 to 4℃. During chilling, carcasstemperature and pH were determined at intervals. Meanwhile, carcass evaporativeloss, purge loss, cooking loss, water-holding capacity, .meat color, fat color, shearforce and MFI were determined.The results indicated that DC decelerated the rate of carcass temperature decline,whilst electrical stimulation had no significant effect on the rate of carcasstemperature decline. Both DC and ES changed the rate of pH decline, decreased thewater-holding capacity, increased purge loss and cooking loss. Both the treatments didnot affect evaporative losses and fat color. ES improved meat color that is moreacceptable to consumers. At 0,3,7,10d postmortem, the WBSF values of DC beeflongissimus were lower than those from CC group by 18.25%,12.11%,13.47% and11.03%; the WBSF values of ES beef longissimus were lower than Control by 23.15%,14.0%,.21.38%, 18.01%. ES, low temperature aging and DC decreased WBSF valuesof beef longissimus. The longissimus treated with ES/DC had lower WBSF values,which indicates that there is an interaction between ES and DC. A high correlationcoefficient existed between MFI and WBSF value, which further indicates thatmyofibrillar degradation during postmortem aging contributes to the improvement ofbeef tenderness. Therefore, MFI value is an important predictor of beef tenderness.Several definitions of cold shortening have been hold by different meatscientists. The differences among these definitions were the scopes of temperature andpH that lead to cold shortening. Our results showed that under CC, beef muscles are more liable to cold shortening, resulting in smaller sarcomere length and higherWBSF values. Cold shortening is suggested to be defined as a phenomenon ofextreme shortening when rigor begins (pH5.8) below 12℃.Part 2 Effects of ES and DC on ultrastructure of beef longissimusBeef samples were obtained at 1h, 24h, 3d, 7d and 10d. The ultrastructuralchanges were observed under TEM and the sarcomere length was measured. Theresults showed that ES led to the occurrence of contractile zone, where sarcomerelength decreased but muscle fiber increased. Sarcomeres around contractile zone weremechanically stretched. At 24 postmortem, the average sarcomere length of ES sample was 1.36μm, and 0.50μm for sarcomeres within contractile zone, whilst1.73μm for sarcomeres around contractile zone. Ultracontracting proportion andstretching proportion were 63.24% and 27.21% respectively. Of 100 eye fields underTEM, 44.9% of sarcomeres formed contractile zone, 16.7% were stretched,correspondingly. WBSF values were decreased by 23%. Contractile zone formedduring ES is, in fact, attributed to partial ultracontraction of myofibrils, whichimproves beef tenderness. With the increase of aging time, length of sarcomere withincontractile zone and MFI value increased, and thus beef tenderness increased.DC avoided the occurrence of cold shortening, loosened ultrastructure of musclefiber and improved beef tenderness. With the increase of aging time, sarcomere lengthof DC sample increased and was similar to that of control group at 7d. This indicatedthat delay chilling affects meat ultrastructure, and also affects the temperature/pHdecline, which alters the action of calpains and speed up aging rate.
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