| China is one of the biggest nations in beef production, but beef tenderness is poor. This is mainly attributed to, except for labor-force purpose, the lack of a systematic research on beef tenderness. Beef tenderness is influenced by the internal factors (myofibrillar components, intramuscular connective tissue, and intramuscular fat), which is, in turn, affected by many external factors by different ways. Meanwhile, there are still controversies on how to affect beef tenderness for a specific factor. Therefore, this study was designed to track the changes of beef tenderness and related characteristics with heating, carcass maturity, marbling, postmortem aging and anatomy, where more emphasis were placed upon the effects of IMCT on beef tenderness, and then to give some explanations for practical problems in beef production in China and also for some scientical problems.The paper includes the following five parts:Part I Effect of heating temperature on beef tendernessThe experiment was designed to explore the effect of heating on meat quality characteristics and IMCT of beef semitendinosus muscle, and then an explanation was given for how beef tenderness changed during heating. The semitendinosus steaks were cooked to 40, 50, 55, 60, 65, 70, 75, 80, 85, and 90℃respectively, and then were used for chemical determinations, mechanical and histological measurements. The results indicated that at the beginning of cooking, moisture and the secondary perimysial thickness decreased (P<0.05), whilst crude fat and filtering residues increased (P<0.05). Moisture content continued to decrease whilst WBSF increased (P<0.05) over 65℃. Through cooking, filtering residues increased continuously (P<0.05), concomitant with a gradual decline (P<0.05) for the fiber diameter. WBSF correlated (P<0.05) with percentages of moisture, crude fat, filtering residues, fiber diameter and perimysial thickness. The maximum transition temperature of endomysial components was lower than that of perimysial components (50.2℃vs. 65.2℃). 65℃is a critical cooking point to meat quality for beef semitendinosus muscle. WBSF is influenced by the natures of both myofibrillar components and intramuscular connective tissue. Below 65℃, the nature of myofibrillar components is the main contributor to beef tenderness, whilst that of intramuscular connective tissue is above 75℃.PartⅡEffect of carcass maturity on beef tendernessThe experiment was designed to explore the effects of carcass maturity on meat quality characteristics and IMCT of beef semitendinosus muscle, and then an explanation was given for the mechanism of tenderness with maturity. Chemical determinations, histological and mechanical measurements were performed on the raw and cooked meat with maturities A, C and E at days postmortem. The results indicated that WBSF, cooking losses, crude fat content and fiber diameter of cooked meat increased (P＜0.05) with the increase of carcass maturity. Great variations existed (P＜0.05) for most of the measurements among cores within the same steak, and among individual steaks within the same maturity group. WBSF was correlated significantly (P＜0.05) with moisture content, sarcomere length of raw meat, and cooking losses, total collagen content, sarcomere length and fiber diameter of cooked meat. Carcass maturity has a phase-in influence on beef tenderness, which is affected mainly by the traits of IMCT before body maturation, and then by the muscle fiber trait and IMF content.PartⅢEffects of marbling on beef tendernessThe experiment was designed to explore the effects of marbling on meat quality characteristics and IMCT of beef longissimus muscle. Chemical determinations, histological and mechanical measurements were performed on the raw and cooked meat at 4 days postmortem. The results showed that crude fat, collagen, fiber diameter and maximum transition temperature of IMCT increased (P＜0.05) with the increase of marbling score. The cooking losses, collagen solubility, WBSF and secondary perimysial thickness decreased (P＜0.05) with the increasing marbling. WBSF correlated (P＜0.05) with moisture, crude fat, collagen, cooking losses, sarcomere length and secondary perimysial thickness. The development of marbling results in the disorganization of the perimysia, the avoidance of sarcomere shortening during chilling, and the decline in cooking losses, which accounts for the improvement of beef tenderness. PartⅣEffect of postmortem aging on beef tendernessThis part was designed to find out the effect of postmortem aging on meat quality characteristics and IMCT of beef semitendinosus muscle. ST muscles were obtained at 4 days postmortem. Chemical determinations, histological and mechanical measurements were performed for the raw and cooked steaks at 4, 11, 14, 21, and 28 days postmortem, respectively. The results showed that postmortem aging resulted in an increase in percent moisture, collagen solubility, and cooking losses, and a decrease in filtering residues, primary perimysial thickness, fiber diameter, WBSF, and transition temperature of the endomysium. WBSF correlated with the quantity of filtering residues and the fiber diameter of cooked meat. Beef tenderness is improved within a whole period of postmortem aging (about 30 days), which is mainly attributed to the weakening of intramuscular connective tissue over 11 days.PartⅤA comparison of tenderness between beef longissimus and semitendinosusThis part was designed to compare meat quality characteristics and the traits of IMCT of beef longissimus (LM) and semitendinosus (ST) muscles at 4 and 21 days postmortem. Meat samples were obtained at 4 days and one halves were aged till 21 days. Chemical determinations, histological and mechanical measurements were performed on the raw and/or cooked meat. The results showed that LM had higher (P＜0.05) moisture content and WBSF, but lower (P＜0.05) collagen content and filtering residues than ST at 4 days. At 21 days, LM had lower (P＜0.05) moisture content, collagen content and fiber diameter, but higher (P＜0.05) perimysial thickness and collagen solubility than ST. The structure of LM IMCT was more liable to aging-induced destruction than ST. Both the nature of muscle fiber and the traits were the contributors to the differences in tenderness between the two muscles. Aging is more efficient to improve tenderness of LM than for ST.
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