**1. Introduction**

Meat and meat products are important nutrient-intensive foods that are widely consumed worldwide [1]. For consumers, tenderness is a critical indicator that drives their willingness to repurchase and acceptability [2,3]. There are still grea<sup>t</sup> variations in the tenderness between di fferent parts of beef muscles [4,5]. In the beef industry, aging is often considered to be one of the important factors determining the ultimate tenderness of the meat [6,7]. Due to the di fferences between the biochemical characteristics of these muscles, they may cause di fferent degrees of response to aging [4]. Therefore, the inconsistency of the tenderness of di fferent muscles has become the main problem that currently exists in the meat processing industry during the aging process.

Reducing the intake of fat in meat is essential for maintaining consumer health and reducing the risk of illness [8]. In fact, the type of muscle fiber is one of the factors that a ffects the lean meat rate [9]. The muscle fiber type di ffers greatly among various body parts and is generally divided into

slow-oxidized fiber (type I), fast oxido-glycolytic fiber (type IIA), and fast glycolytic fibers (type IIB and type IIX) [10,11]. Studies indicated that muscle fiber type was closely associated with muscle quality and more type IIB fiber might lead to poorer muscle quality and lower lean meat productivity [12,13]. Our previous unpublished study has shown that s*emitendinosus* (ST) and *longissimus thoracis* (LT) muscles of Xinjiang brown cattle have large percentages of type IIB fibers (approximately 47.89% and 40.85%, respectively), *rhomboideus* (RH) and *gastrocnemius* (GN) muscles have high percentages of type IIA fibers (approximately 61.43% and 42.10%, respectively), and *infraspinatus* (IN), *psoas major* (PM), and *biceps femoris* (BF) muscles have large percentages of type I fibers (79.69%, 43.06%, and 39.75%, respectively). In the current study, we focused on these seven muscles, representing the characteristics of the three muscle fiber types.

Protein in skeletal muscle is a key factor in determining the type of muscle fiber. Establishing the relationship between muscle protein structure and functional properties will contribute to the understanding of the modification mechanism of meat products during processing and storage [14]. According to the solubility and location of protein, it can be divided into myofibrillar protein, sarcoplasmic protein, and matrix protein, which maintain the structural integrity of myofibrillar fiber [15,16]. These proteins are susceptible to oxidative degradation during aging, which may explain the meat tenderization after muscle fiber breakage [17]. Oxidation a ffects the chemical properties of proteins and changes in protein solubility can reflect the degree of protein denaturation [18,19]. Sarcoplasmic protein solubility (SPS) is sometimes used as a measure of muscle quality [20]. It has been well documented that precipitated or denatured sarcoplasmic proteins may bind to myofibrils, leading to a decrease in water holding capacity [21–23]. It is vital to the production of Xinjiang brown beef, for the lives of Xinjiang people, and for local meat production. However, there are still limited studies examining the changes in protein solubility between various parts of muscles of Xinjiang brown cattle during post-mortem aging.

Therefore, the aim of this research was to evaluate the changes in pH, myofibril fragmentation index (MFI), total protein solubility (TPS), SPS and myofibrillar protein solubility (MPS), and the microstructure of seven beef muscles (ST, LT, RH, GN, IN, PM, BF) during aging from 1 to 14 days.

#### **2. Materials and Methods**

#### *2.1. Animals and Muscle Samples Preparation*

Six Xinjiang brown bull calves of approximately 30 months of age were slaughtered at a local slaughterhouse (Yining, Xinjiang, China) according to the commercial procedures. The mean weight at slaughter was 566 ± 32 kg. All corn-fed calves were raised on the same farm to ensure background consistency and then slaughtered using electrical stunning on the same day. The carcasses were overhung in a cold room (3 ± 1 ◦C) for 24 h. All procedures were undertaken following the guidelines given by the Animal Care and Ethics Committee for animal experiments, Institute of Animal Science, Chinese Academy of Agricultural Sciences.

The *semitendinosus*(ST),l*ongissimus thoracis*(LT),*rhomboideus*(RH), *gastrocnemius*(GN), *infraspinatus*(IN), *psoas major* (PM), and *biceps femoris* (BF) muscles were collected from each carcass. The samples were kept on ice and transported to the laboratory after 50 min and all visible intermuscular and subcutaneous fat was removed. Prior to packaging, each sample was stamped with a date mark. Each muscle was further separated into three equal-length sections, resulting in six muscle Sections (5 cm × 3 cm × 2 cm, 50 ± 0.05 g) per carcass, with 42 slices per aging period (six replications per muscle and per aging period). Afterwards, the samples were individually vacuum-packaged in a polyolefin bag and randomly assigned to aging at 3 ± 1 ◦C (relative humidity: 70–80%) for 1, 3, 7, 9, 11, and 14 days (within muscles, samples from the same carcass were not aged for the same time). Upon completion of each postmortem aging period, samples were taken to determine pH and transmission electron microscopy (TEM). The remaining samples were stored at −30 ◦C for further analysis within 2 weeks.
