JIA-2018-09

2121 ZHANG Yi-min et al. Journal of Integrative Agriculture 2018, 17(9): 2118–2125 temperature decline and faster pH drop, compared with routine chilling (RC; Table 1). The temperature dropped from the initial temperature at around 39 to 16°C at 5 h, then muscles subjected to SC maintained the temperature of 14.6°C at 10 h post-mortem, while RC muscles showed a temperature below 12°C even at 8 h post-mortem. The pH of SC muscles was significantly ( P <0.05) lower than those subjected to RC at 8 h post-mortem as well as at 10 h post- mortem, but there was no effect on the ultimate pH ( P >0.05). The pH/temperature rate of decline during chilling has been reported to impact on meat quality (Hwang and Thompson 2001), with the ideal pH/temperature window established by MSA being described as when pH drops to 6.0 the temperature should be between 12–35°C (Thompson 2002). As such in the current study, the SC muscles hit the ideal “window”, whereas the pH of the RC muscles was around 6.0 at 8 h post-mortem, with a temperature lower than 12°C, thus “missing” the window. 3.2. Color and color-related traits Beef steaks subjected to SC exhibited higher color values and greater color stabililty. The initial lightness (L* values), redness (a* values), yellowness (b* values), and chroma of SC beef was higher than for RC samples, and these differences persisted during ageing, except for a*, b* values and chroma at 14 d (Table 2). L* values slightly increased at d 7 for steaks treated with both chilling methods, as well as a*, b* and chroma values, but most of the color values decreased at d 14, to lower levels than the initial values. The color differences between the two chilling procedures can be explained by the lower relative content of surface MetMb, higher MRA and NADH content in SC-treated beef steaks (Table 2), which resulted in the higher color stability of SC steaks. Additionally, the MetMb accumulation and NADH content decreased as storage time extended for steaks treated by both chilling procedures; MRA of RC steaks increased on d 7 and reduced on d 14. An improved beef color was found in previous studies, by setting the rigor temperature to 14°C (Li K et al . 2012), or 12–18°C (Liu et al . 2015), which showed relatively rapid pH declines. Mungure et al . (2016) reported beef L* and a* values increased as rigor temperature increased from 5 to 25°C. Sikes et al . (2017) also discovered a slower pH fall in hot-boned beef loin treated with very fast chilling resulted in lower L*, a* and b* values. Further it has been reported that the color of thawed meat was better when chilled at a higher temperature and showed a rapid pH decline during rigor (Farouk and Lovatt 2000), compared with a routine chilling procedure, and the authors assumed the improved color was related to the increased activity of MetMb-reducing Table 1 Effect of step-chilling on the temperature and pH declines of M . longissimus lumborum within 24 h post-mortem Time Routine chilling Step-chilling Temperature (°C) pH Temperature (°C) pH 45 min 39.03±0.77 ax 6.56±0.11 ax 39.03±0.77 ax 6.56±0.11 ax 5 h 15.73±1.58 bx 6.11±0.13 bx 15.57±0.50 bx 6.07±0.15 bx 8 h 10.54±1.43 bx 5.96±0.11 cx 14.78±0.62 by 5.75±0.11 cy 10 h 8.23±1.17 bx 5.79±0.12 dx 14.59±0.58 by 5.61±0.14 dy 24 h 2.99±0.21 cx 5.51±0.06 ex 2.82±0.04 cx 5.47±0.07 ex Means within columns with differing letters (a, b, c) are significantly different at P <0.05. Means within rows with differing letters (x, y) are significantly different at P <0.05. Table 2 Effect of chilling methods on the color and color-related traits of beef steaks from M. longissimus lumborum muscle during aging for 14 d Trait Chilling method 1) Storage time (d) SE 1 7 14 L* SC 40.7 bx 42.9 ax 42.6 ax 1.61 RC 38.8 by 41.0 ay 40.5 ay a* SC 21.8 bx 23.9 ax 19.1 cx 0.53 RC 19.9 by 21.9 ay 17.9 cx b* SC 15.6 bx 16.5 ax 14.8 bx 0.43 RC 14.4 aby 14.9 ay 13.7 bx Choma SC 26.2 bx 28.4 ax 23.5 cx 0.52 RC 23.7 by 25.4 ay 23.6 bx MetMb SC 0.58 cx 0.73 bx 0.87 ax 0.01 RC 0.63 cy 0.77 by 0.93 ay MRA (%) SC 80.6 ax 90.2 ax 84.2 ax 3.34 RC 58.7 by 78.4 ay 64.8 by NADH (mmol g –1 ) SC 59.3 ax 50.3 bx 42.4 cx 3.02 RC 52.0 ay 41.3 by 35.4 cy 1) SC, step-chilling; RC, routine chilling. Means within columns with differing letters (a, b, c) are significantly different at P <0.05. Means within rows with differing letters (x, y) are significantly different at P <0.05.