【作者】 李春保；

【导师】 周光宏；

【作者基本信息】 南京农业大学 ， 食品科学， 2006， 博士

【摘要】 我国是牛肉生产大国，但牛肉嫩度较差，除了历史原因(牛作为一种使役工具)外，关键是对我国黄牛肉品质的系统研究较晚。影响牛肉嫩度的外在因素很多，但所有外在因素的作用都是通过影响内在因素(肌原纤维、肌内结缔组织和肌内脂肪)而实现的，且各因素对牛肉嫩度的作用机制不尽相同，学术界还存在较多争议。本研究结合我国牛肉生产的实际情况，针对目前尚存在争议的科学问题，从生理成熟度、大理石花纹、分割部位、宰后成熟、加热等对牛肉嫩度及其相关指标影响的研究，旨在重点揭示肌内结缔组织变化对牛肉嫩度的影响。本研究主要围绕我国牛肉分级标准(生理成熟度、大理石花纹和分割部位)和屠宰加工工艺(宰后成熟和加热)开展系列研究。内容共包括五个部分：一、加热温度对牛肉嫩度影响的研究；二、生理成熟度对牛肉嫩度影响的研究；三、大理石花纹对牛肉嫩度影响的研究；四、宰后成熟对牛肉嫩度影响的研究；五、牛背最长肌和半腱肌嫩度的比较研究。具体研究内容和结果如下：1．加热温度对牛肉嫩度影响的研究通过对牛肉加热过程中嫩度及相关食用品质指标的变化研究，阐述加热对嫩度影响的可能机制。选择宰后96h牛半腱肌，分别加热到40，50，55，60，65，70，75，80，85和90℃。随后进行化学测定、嫩度测定和组织学观察。结果表明，加热初期，水分含量和次级肌束膜厚度降低(P＜0.05)，而粗脂肪和结缔组织滤渣含量增加(P＜0.05)。随着加热温度(65℃以上)进一步升高，水分含量进一步下降，而剪切力值增加(P＜0.05)。整个加热过程中，结缔组织滤渣持续增加，而肌纤维直径持续降低(P＜0.05)。剪切力值与水分含量、粗脂肪含量、结缔组织滤渣、肌纤维直径和肌束膜厚度显著相关(P＜0.05)。热示差扫描分析表明肌内膜的最大变性温度比肌束膜低(50.2℃vs．65.2℃)。对于牛半腱肌来说，65℃是保证牛肉嫩度和品质的关键加热温度。肌纤维和肌束膜的性质和状态决定着牛肉的嫩度。两者所起作用的大小因加热温度的变化而改变，65℃以下时，肌纤维起主要作用，胶原蛋白也起一定的作用；75℃以上，胶原蛋白起主要作用。2．生理成熟度对牛肉嫩度影响的研究通过对牛肉嫩度及相关指标随牛胴体生理成熟度变化的研究，阐述牛肉嫩度随生理成熟度变化的相关机制。对胴体生理成熟度为A、C和E、成熟96h的牛半腱肌进行化学测定、组织学观察和机械测定。结果表明，蒸煮损失、熟肉剪切力值、粗脂肪含量及肌纤维直径随生理成熟度的增加而增加(P＜0.05)。同一肉块内不同部位间、同一生理成熟度组内不同肉块之间各指标测定值之间存在很大差异。剪切力值与原料肉中水分含量和肌节长度、蒸煮损失、熟肉中总胶原蛋白含量、肌节长度和肌纤维直径显著相关(P＜0.05)。胴体生理成熟度对牛肉嫩度的影响具有阶段性，体成熟前，结缔组织特性的变化是导致嫩度下降的主要因素；体成熟后肌纤维特性和肌内脂肪含量的变化是引起嫩度变化的主要因素。3．大理石花纹对牛肉嫩度影响的研究比较研究了不同大理石花纹等级的牛肉嫩度及相关指标，并据此说明了牛肉嫩度变化的相关机制。对大理石花纹等级为少量(3)、丰富(2)和极丰富(1)、成熟96h的牛背最长肌进行化学测定、组织学观察和机械测定。结果表明，粗脂肪含量、胶原蛋白含量、肌纤维直径及肌内结缔组织的最大热变性温度随大理石花纹的增加而增加(P＜0.05)；蒸煮损失、胶原蛋白可溶性、剪切力值、肌束膜厚度随大理石花纹的增加而下降(P＜0.05)。同一肉块内不同部位间、同一大理石花纹组内不同肉块之间各指标测定值之间存在很大差异。剪切力值与水分含量、粗脂肪含量、胶原蛋白含量、肌节长度、蒸煮损失及肌束膜厚度显著相关(P＜0.05)。大理石花纹的沉积过程中使肌束膜内部胶原纤维排列发生改变、避免了宰后冷却过程中肌纤维收缩、降低了加热过程中蒸煮损失，从而使牛肉嫩度得到提高。4．宰后成熟对牛肉嫩度影响的研究跟踪研究了牛肉嫩度及相关指标宰后成熟过程中的变化，并据此阐明宰后成熟中结缔组织变化对牛肉嫩度的影响。选择宰后96h牛半腱肌，0-4℃条件下继续成熟7、10、17、24d。随后进行化学测定、嫩度测定和组织学观察。结果表明，成熟过程中，水分含量、胶原蛋白可溶性和蒸煮损失增加(P＜0.05)，而结缔组织滤渣含量、初级肌束膜厚度、肌纤维直径、肌内膜热变温度、剪切力值下降(P＜0.05)。剪切力值与熟肉的结缔组织滤和肌纤维直径显著相关(P＜0.05)。在一个完整的宰后成熟期(约30d)内，牛肉嫩度得到改善，中后期(11d以后)主要是肌内结缔组织结构弱化作用所致。5．牛背最长肌和半腱肌嫩度的比较研究比较研究了牛背最长肌和半腱肌在宰后成熟中嫩度和相关指标的变化。选择宰后4d牛背最长肌和半腱肌，一半直接用于相关指标测定，另一半在0-4℃条件下继续成熟17d，随后进行化学测定、嫩度测定和组织学观察。结果表明，宰后第4d，背最长肌的水分含量和剪切力值高于半腱肌(P＜0.05)，而胶原蛋白和结缔组织滤渣低于半腱肌(P＜0.05)，宰后第21d，背最长肌水分含量、胶原蛋白含量、肌纤维直径比半腱肌小(P＜0.05)，而肌束膜厚度和胶原蛋白可溶性高于半腱肌(P＜0.05)。成熟过程中，背最长肌肌内结缔组织结构更易遭到破坏。肌纤维和肌内结缔组织结构和性质的差异都是导致背最长肌和半腱肌嫩度差异的重要原因。宰后成熟可更有效的改善背最长肌的嫩度。更多还原

【Abstract】 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.更多还原