JIA-2018-09

2124 ZHANG Yi-min et al. Journal of Integrative Agriculture 2018, 17(9): 2118–2125 it was over-expressed in beef carcasses with a lower pH decline (dark cutting), while the expression of this enzyme was reported to have no correlation with bovine and swine meat color parameters (Kwasiborski et al . 2008; Canto et al . 2015), but was positively related ( P <0.05) with a* value and negatively with surface MetMb content of ovine meat (Li et al . 2018). The mechanisms through which AK1 affects color stability are not clear, and in this study no relationship was found between the expression of ADK, AK1 and beef color traits. However, the higher expression of both enzymes in RC-treated steaks could probably reflect the lower metabolism rate due to the slower pH decline during rigor, resulting in relatively more energy in the early post- rigor period, which further induced higher enzyme activity in ATP conversion at 24 h post-mortem. Further study, such as the comparison energy level and the activity of other enzymes involved in energy metabolism between SC- and RC-treated beef steaks should be explored. FKBP4 is a chaperon binding protein, which interacts with heat shock factor protein 1 in the HSP90 complex, assisting the proper folding, stabilization and activation of target proteins. It promotes the recruitment of ATP, and was over-abundant in RC samples consistent with the expression of AK 1 and ADK. FKBP4 was found to be related to color values in our study, and could explain 74.8, 66.4 and 75.7% of the variation of L*, a*, b* values, respectively when treatment was in the model (Table 4). However, the direct pathway of how this protein is involved in meat color remains to be discovered in future studies. PRDX 1 belongs to the peroxiredoxin family and is able to detoxify peroxides, protect cells against oxidation and act as a sensor of hydrogen peroxide-mediated signaling events. Our results showed that this protein accounted for 72.2, 71.4, and 73.0% of the variance of L*, a*, b* values when treatment was in the model (Table 4), and was negatively correlated ( P <0.05) with these color traits. In previous proteomics studies, researchers found other members of the peroxiredoxin family, such as peroxiredoxin-6 (PRDX 6) exhibited a negative relationship with beef a* values (Wu et al . 2015), while peroxiredoxin-2 was correlated with R630/580 (Joseph et al . 2012). Glutathione peroxidase 1 has also been reported to be negatively related with beef color (a*) (Yu et al . 2017). These peroxiredoxins were all reported to eliminate hydrogen peroxide and served as antioxidants to protect meat against discoloration. A previous study stated that the role of peroxiredoxin eliminating peroxides could inhibit lipid oxidation, and then improve meat color (Joseph et al . 2012). PRDX 1 interacts with PRDX5 and catalase (CAT), involved in the hydrogen peroxide catabolic process, as a response to reactive oxygen species and a response to oxygen-containing compounds. It seems it could benefit the formation of a reduction environment in the meat, however, its function on decreasing the meat color and color stability, especially after SC treatment remains to be clarified. 4. Conclusion The step-chilling procedure we applied successfully promoted the color improvement of bovine M . longissimus lumborum muscle. This improvement was a result of a lower MetMb content; higher MRA and accumulation of NADH in SC-treated beef, due to the fast pH decline. The proteomics results further explain the reason for the improvement. It is mainly because three oxidoreductases (ADI1, PDHB, ALDH7A1), were over-abundant in SC beef, which can minimize lipid oxidation-induced myoglobin oxidation and benefit the production of NADH, and then facilitate color improvement. Of these PDHB is positively correlated with color values, and is proposed as a potential biomarker for beef color. Several other proteins were also found related with color traits, such as PRDX1 and FKBP4 and their roles in color development are worthy of exploration in future studies. Ackonwledgements This work was supported by the earmarked fund for the China Agriculture Research System (beef) (CARS-37), the Special Fund for Innovation Team of Modern Agricultural Industrial Technology System in Shandong Province, China (SDAIT-09-09) and the Funds of Shandong “Double Tops” Program, China (SYL2017XTTD12). References AMSA (American Meat Science Association). 2012. Meat Color Measurement Guidelines . [2017-01-08]. https:// www.meatscience.org/docs/default-source/publications- resources/Hot-Topics/download-the-ebook-format- pdf-of-the-meat-color-measurement-guidelines. pdf?sfvrsn=a218b8b3_0 Canto A C, Suman S P, Nair M N, Li S, Rentfrow G, Beach C M, Silva T J, Wheeler T L, Shackelford S D, Grayson A, McKeith R O, King D A. 2015. Differential abundance of sarcoplasmic proteome explains animal effect on beef Longissimus lumborum color stability. Meat Science , 102 , 90–98. Cornforth D. 1994. Color: Its basis and importance. In: Pearson A M, Dutson T R, eds., Quality Attributes and Their Measurement in Meat, Poultry and Fish Products . Advances in Meat Research Series Glasgow . Blackie Academic & Professional, London, England. pp. 35–77. Farouk M M, Lovatt S J. 2000. Initial chilling rate of pre-rigor beef muscles as an indicator of colour of thawed meat. Meat Science , 56 , 139–144.