JIA-2018-09

2119 ZHANG Yi-min et al. Journal of Integrative Agriculture 2018, 17(9): 2118–2125 Meat color is mainly determined by the content and the status of myoglobin, and it is affected by lots of factors, including intrinsic factors such as animal species/breed, age, muscle fibre types, glycogen storage; extrinsic factors such as dietary regime, animal management, packaging methods, aging time, storage temperature, meat processing additives (Mancini and Hunt 2005; Suman et al . 2014). Among these factors, muscle chilling rate/pH-temperature decline is seldommentioned, as controlling the rigor process has been mainly aimed at improving beef tenderness. For example, the Meat Standards Australia (MSA) beef grading system established an ideal pH/temperature window to improve beef tenderness, so that the temperature should be between 12 and 35°C when the pH drops to 6.0 (Thompson 2002). Having said this, several previous studies did report an improved beef color concurrently with better tenderness via controlling pH/temperature decline during rigor. Li K et al . (2012) tested several different pre-rigor temperatures to improve beef quality, and found beef samples held at 14°C within 10 h of animal death, exhibited the highest a* and b* values during the early aging time. Liu et al . (2015) also found that beef steaks from carcasses subjected to step- chilling (SC) ((–11±1)°C, for 2 h; then held at a carcass temperature of 12–18°C till 10 h post-mortem, and followed by (1±1)°C to 48 h post-mortem), showed higher a* and b* values, compared with steaks subjected to a routine chilling procedure. So it appears that there may be some benefits for colour by applying step-chilling, but this remains to be elucidated. With rapid development of “omics” techniques, proteomics has been applied in meat color research successfully, to discover differential expression of proteins among different animal species or muscle types, or color biomarkers during different storage times. The color-related differential proteins are mainly grouped into metabolic enzymes, oxidoreductases, peroxidase and chaperone proteins. Of these, Yu et al . (2017) found four oxidoreductases and two metabolic enzymes that were positively correlated with beef a* values. Canto et al . (2015) found three glycolytic enzymes that were over-abundant in color stable beef and significantly positively correlated to color stability and a* values (redness). Nair et al . (2016) found a differential abundance of glycolytic enzymes related to intramuscular color stability variations in beef semimembranosus , where the outside (OSM) and inside (ISM) of semimembranosus muscle displayed different chilling rates during rigor. We hypothesize that the sarcoplasmic proteome profiles of beef muscles subjected to step-chilling (starting at 0–4°C for 5 h, then holding the temperature at 12–18°C till 11 h post- mortem, followed by 0–4°C again until 24 h post-mortem) and routine chilling will be different, and impact via the variation in pH decline on color stability. Thus, the objective of current study was to explore the possible underlying mechanisms of improved color which resulted from the step-chilling procedure by using proteomics analysis combined with meat color biochemistry analysis. 2. Materials and methods 2.1. Sample collection Six carcasses of Chinese yellow cattle (Luxi×Simmental, 22monthsold)were randomlyselectedon theslaughtering line. The M . longissimus lumborum (LL, the part of striploin) from both sides were removed immediately, commercially vacuum packaged and then transferred (0–4°C) to the lab immediately. The left loins of each carcass were subjected to step-chilling (stored in an incubator with the temperature at 0–4°C till 5 h post-mortem, and then transferred to another incubator at 14°C, holding for 6 h, followed by 0–4°C chilling again to 24 h post-mortem). The right side LL loins were chilled by a routine procedure (kept in 0–4°C till 24 h post-mortem). Six beef steaks (2.54 cm thick) were cut from each loin at 24 h post-mortem, vacuum packaged and then stored at 4°C. Two steaks (as replicates) were selected randomly to determine color values, metmyoglobin (MetMb) content, NADH at each of three ageing periods, 1, 7 and 14 d. Samples for protemics analysis (2 g) were removed from d 1 samples ( n =3) without replication, frozen in liquid nitrogen immediately and stored at –80°C. 2.2. Temperature and pH Temperature and pH were measured at 1, 5, 8, 10 and 24 h after slaughter using a digital thermometer (DM6801A, Shenzhen Victor Hi-Tech Co., Ltd., China) and a portable pH probe with temperature compensation (Senven2Go-S2, Mettler-Toledo, Switzerland) which was calibrated in buffers with pH 4.0 and 7.0 before use. The probes were inserted into the geometrical center (about 3 cm) of the LL muscles. Measurements were taken three times for each loin and averaged. 2.3. Color The meat color L*, a* and b* values, and reflectance on the surface of each beef steak were determined by an X-Rite spectrophotometer-SP62 (Grand Rapids, USA, 8 mm diameter aperture, Illuminant A, 10° observer), after blooming for 30 min at 4°C. The color meter was calibrated by a black and a white plate before use. Each steak was read 3 times, and the reflectance values were recorded in the range of 400 to 700 nm at 10-nm intervals. The Kubelka-