**Changes in Physical Meat Traits, Protein Solubility, and the Microstructure of Di** ff**erent Beef Muscles during Post-Mortem Aging**

#### **Yong-Hong Feng** †**, Song-Shan Zhang** †**, Bao-Zhong Sun, Peng Xie, Kai-Xin Wen and Chen-Chen Xu \***

Institute of Animal Sciences, Chinese Academy of Agricultural Sciences, Beijing 100093, China; yike556@163.com (Y.-H.F.); zhangsongshan@caas.cn (S.-S.Z.); sunbaozhong@caas.cn (B.-Z.S.); seulbird@163.com (P.X.); wenkaixin215@126.com (K.-X.W.)

**\*** Correspondence: chenxu30510@cau.edu.cn; Tel.: +86-010-62816010

† These authors contributed equally to this work.

Received: 19 April 2020; Accepted: 17 June 2020; Published: 19 June 2020

**Abstract:** This study was performed to compare the di fferences in pH, myofibril fragmentation index (MFI), total protein solubility (TPS), sarcoplasmic protein solubility (SPS), myofibrillar protein solubility (MPS), and the microstructure of seven beef muscles during aging. From the six beef carcasses of Xinjiang brown cattle, a total of 252 samples from *semitendinosus* (ST), *longissimus thoracis* (LT), *rhomboideus* (RH), *gastrocnemius* (GN), *infraspinatus* (IN), *psoas major* (PM), and *biceps femoris* (BF) muscles were collected, portioned, and assigned to six aging periods (1, 3, 7, 9, 11, and 14 day/s) and 42 samples were used per storage period. IN muscle showed the highest pH (*p* < 0.05) from 1 to 14 days and the lowest TPS (*p* < 0.01) from 9 to 14 days with respect to the other muscles. Moreover, the changes in IN were further supported by transmission electron microscopy due to the destruction of the myofibril structure. The highest value of MFI was tested in ST muscle from 7 to 14 days. The total protein solubility in PM, RH, and GN muscles were not a ffected (*p* > 0.05) as the aging period increased. The lowest TPS was found in the RH muscle on day 1, 3, and 7 and in the IN muscle on day 9, 11, and 14. The pH showed negative correlations with the MFI, TPS, and MPS (*p* < 0.01). The results sugges<sup>t</sup> that changes in protein solubility and muscle fiber structure are related to muscle location in the carcass during aging. These results provide new insights to optimize the processing and storage of di fferent beef muscles and enhance our understanding of the biological characteristics of Xinjiang brown cattle muscles.

**Keywords:** beef muscle; protein solubility; myofibril fragmentation; microstructure; aging
