结缔组织作为肌肉的重要组成部分之一，对肉的食用品质具有非常重要的影响。在宰后成熟过程中，结缔组织的成分会发生一系列的变化进而影响肉的嫩度，其中肌内膜与肌束膜中胶原蛋白相关特性的变化起到关键的作用。目前超声波嫩化技术主要处于理论阶段，在实际的生产和应用中尚且不够成熟，仍然需进一步的研究与探索。本论文主要研究了超声波处理对牛肉在宰后成熟期间品质的相关影响，并通过观察结缔组织微观结构的变化、测定相关内源酶的活性以及胶原蛋白特性的变化情况等，重点从结缔组织的变化探究其作用机理。为超声波技术应用于牛肉的生产与加工，通过对牛肉结缔组织进行合理的调控从而改善牛肉嫩度提供一定理论与技术支持。具体的研究内容与结果如下：

1. 超声波处理对牛肉宰后成熟期间品质的影响

本章主要探究了经不同超声功率处理后的牛肉在宰后成熟期间品质的变化情况，其中未经超声处理的牛肉样品为对照组。选用体重相近且性别一致的西门塔尔杂交牛的半腱肌作为实验材料，并进行随机分组，分别为对照组、300 W超声处理组和600 W超声处理组，每个处理组均为5个生物学重复。于宰后48 h对牛肉样品进行超声波处理，而后在4 ℃的条件下进行宰后成熟0、4和8 d处理。通过测定各组牛肉样品的pH、色泽、保水性（水分含量、蒸煮损失、低场核磁分析）、剪切力以及质构等指标的变化，来探究超声波处理对宰后成熟期间牛肉品质的影响。本章研究结果表明：在第0、4 d，两个超声功率处理组牛肉样品的pH值较对照组均有显著性增大（P <0.05）；超声处理提高了牛肉在宰后3个成熟时间点的亮度值以及红度值（P <0.05）；此外，超声波处理的牛肉中水分含量较对照组显著提高，而蒸煮损失显著降低（P <0.05），部分牛肉样品在经超声处理后不易流动水的比例增大；同时超声处理还使牛肉的剪切力值、硬度值、凝聚性以及咀嚼性等指标显著降低（P <0.05），改善了牛肉在宰后成熟期间的嫩度。

2. 超声波处理对牛肉宰后成熟期间结缔组织及相关酶活性的影响

本章主要通过探究超声波处理对牛肉在宰后成熟期间结缔组织的微观结构以及相关内源酶活性的影响，进而分析酶活性与结缔组织结构变化之间的关系及其对牛肉品质的影响。扫描电镜结果表明，在3个宰后成熟时间点，超声处理后的牛肉样品胶原纤维网络变得松散且紊乱，肌束膜的结构明显受到破坏，而对照组的肌束膜整体上仍然保持着相对较为完整的致密结构。同时，在第0 d，经超声处理后的牛肉样品中β-半乳糖苷酶的活性以及经600 W处理的牛肉样品中β-葡糖醛酸酶的活性较对照组均有显著性提高（P <0.05），这有助于结缔组织中蛋白多糖的降解以及胶原蛋白的暴露；并且在宰后的3个成熟时间点，经超声处理的牛肉样品中基质金属蛋白酶的活性均显著高于对照组（P <0.05），在第0、4 d，超声处理还使牛肉样品中组织蛋白酶B+L的活性显著提高（P <0.05），这对于牛肉在宰后成熟过程中胶原蛋白的降解以及结缔组织结构的破坏有促进作用，有利于牛肉嫩度的改善。

3. 超声波处理对牛肉宰后成熟期间胶原蛋白特性的影响

为探讨超声波处理对牛肉中胶原蛋白特性的影响，本章主要分析了超声对牛肉宰后成熟期间胶原蛋白二级结构、内源荧光、表面疏水性、共价交联含量、热稳定性以及溶解度等指标的影响。通过对胶原蛋白的二级结构变化进行分析，发现经超声处理之后酰胺I带的主要吸收峰向着短波方向进行迁移，这表明胶原蛋白的三股螺旋结构受到了一定程度的破坏并变得无序。在第0、8 d，超声处理后的牛肉样品中胶原蛋白的最大荧光强度与对照组相比有显著性增大（P <0.05）；在3个宰后成熟时间点，经超声处理后胶原蛋白的表面疏水性同样较对照组有显著性增加（P <0.05）。此外，胶原蛋白中的共价交联含量以及热稳定性降低，同时热溶解度有显著性提高（P <0.05）。这表明了超声波通过空化效应、机械效应等作用破坏了胶原蛋白的分子结构使其展开并变得疏松，有助于牛肉在宰后成熟过程中嫩度的改善。

Connective tissue is one of the important components of muscle and plays a very important role in meat quality. The components of connective tissue would change during post-mortem aging which could affect the meat tenderness, wherein the changes in collagen-related characteristics in the endomysium and perimysium play a key role. At present, ultrasonic tenderization technology is mainly in the theoretical stage, and it is not mature enough for the practical production and application, which still needs further research and exploration. This study mainly explored the related effects and mechanism of ultrasonic treatment on the quality of beef during postmortem aging mainly from the aspect of connective tissues by observing their microstructure changes, and measuring the activities of related endogenous enzymes and the changes of collagen properties. It was expected that meat tenderness could be effectively improved by reasonably regulating the intramuscular connective tissue through ultrasonic technology, so as to provide some theoretical and technical support for the production and processing of beef in the future. The specific research contents and results were as follows:

1. Effect of ultrasound treatment on the quality of beef during post-mortem aging

This chapter mainly explored the changes of beef quality treated with different ultrasonic power during post-mortem aging and the beef samples without ultrasonic treatment were used as the control. The semitendinosus muscles of Simmental crossbred beef cattle with similar weight and same gender were selected as the experimental material which were randomly divided into the control group, and ultrasound treatment with 300 W and 600 W groups. There were 5 biological repeats in each treatment group. The beef samples were treated with ultrasound at 48 h after slaughter, and then were aged at 4 ℃ for 0, 4 and 8 d, respectively. The effects of ultrasound treatment on beef quality during post-mortem aging were explored by measuring the pH values, meat color, water-holding capacity (moisture content, cooking loss, Low-Field Nuclear Magnetic Resonance), shear force and texture profile analysis indexes of beef. The results showed that the pH values of beef in the two ultrasonic treatment groups were significantly higher than that in the control at 0 and 4 d (P <0.05). Ultrasound treatment increased the L* of beef at 3 post-mortem aging times and improved a* at 0 and 4 d (P < 0.05). After ultrasound treatment, the moisture content in beef was higher than that in the control, the cooking loss was decreased (P <0.05), and the proportion of immobilized water in beef samples increased. At the same time, ultrasound treatment also significantly reduced the shear force values, hardness, cohesiveness and chewiness of beef (P < 0.05), improving the tenderness of beef during post-mortem aging.

2. Effects of ultrasound treatment on connective tissue and related enzyme activities of beef during post-mortem aging

This chapter mainly explored the effects of ultrasound treatment on the microstructure of connective tissue and the activities of the above five enzymes during post-mortem aging, and then analyzed the relationship between enzyme activity and the changes of connective tissue structure and its impact on beef quality. The results observed by scanning electron microscope showed that at three post-mortem aging time points, the collagen fiber network of beef samples treated by ultrasound became disorganized and visibly loosened, and the structure of perimysium was obviously damaged, while the overall dense structure of the perimysium in the control was still relatively complete. At the same time, the β-galactosidase activities of beef samples treated with ultrasound and the β-glucuronidase activities of beef samples treated with 600 W were significantly higher than that of the control (P <0.05) at 0 d, which was conducive to the degradation of proteoglycan and the exposure of collagen. At the three post-mortem aging time points, the matrix metalloproteinases activity of beef samples treated with ultrasound was significantly higher than the control (P <0.05). Ultrasound treatment significantly increased the activities of cathepsin B and L in beef samples at 0 and 4 d (P <0.05). The increased enzyme activity was conducive to the degradation of collagen and the destruction of connective tissue structure during post-mortem aging, and promoted beef tenderness.

3. Effect of ultrasound treatment on collagen properties of beef during post-mortem aging

In order to analyze the effects of ultrasound treatment on the characteristics of collagen in beef, this chapter mainly explored the effects of ultrasound on the secondary structure, intrinsic fluorescence, surface hydrophobicity, intramolecular crosslinks content, thermal stability and solubility of beef collagen during post-mortem aging. The analysis of the secondary structure of collagen showed that the main absorption peak of amide I band was shifted to short wave after ultrasound treatment, which indicated that the triple helix structure of collagen was damaged and became disordered. At 0 and 8 d, the intrinsic fluorescence of collagen in beef treated with ultrasound treatment was significantly higher than the control (P <0.05). The surface hydrophobicity of collagen in ultrasound treatment groups was also significantly higher than the control at three post-mortem aging time points (P <0.05). The covalent cross-linking content and thermal stability of collagen decreased, while the thermal solubility increased significantly (P <0.05). These results showed that ultrasound destroyed the molecular structure of collagen through cavitation effect, making it expand and become loose which was instrumental for the improvement of beef tenderness during post-mortem aging.

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