Implications of step-chilling on meat color investigated using proteome analysis of the sarcoplasmic protein fraction of beef longissimus lumborum muscle

In order to improve beef color and color stability, step-chilling (SC) was applied on excised bovine longissimus lumborum muscle, with chilling starting at 0-4 degrees C for 5 h, then holding the temperature at 12-18 degrees C for 6 h, followed by 0-4 degrees C again until 24 h post-mortem. pH and temperature were measured during rigor on SC loins as well as those subjected to routine chilling (RC, 0-4 degrees C, till 24 h post-mortem). Color L*, a*, b* values, metmyoglobin (MetMb) content, MetMb reducing ability (MRA) and NADH content were determined on samples aged for 1,7, and 14 d. Sarcoplasmic proteome analysis was only conducted on d 1 samples. The results showed muscles subjected to SC maintained a temperature at around 15 degrees C for 5 to 10 h post-mortem, and exhibited a slow temperature decline, but rapid pH decline. Beef steaks treated with SC had higher L*, a*, b* and chroma values than those of RC samples at 1 and 7 d chilled storage (0-4 degrees C), while showing no significant difference for a*, b* and chroma values at d 14. The SC samples also exhibited a lower relative content of surface MetMb, higher MRA and NADH content, compared with RC beef steaks during storage, indicating the SC-treated beef showed an improved color stability. Eleven differential protein spots/nine proteins were identified by two-dimensional gel electrophoresis and mass spectrometry, and those proteins were mainly involved in redox, chaperone binding, metabolic and peroxidase activity. Oxidoreductases play a role in decreasing the oxidation-induced myoglobin oxidation and benefiting the production of NADH, and finally improving the colour of beef. Of these, pyruvate dehydrogenase E1 component subunit beta showed a positive correlation with color L*, a*, b* values and accounted for more than 60% of the variation in color values; this protein can be considered as a potential beef color biomarker. The present study provided valuable information for studies on the molecular mechanism of color improvement from step-chilling, as well as for identifying markers associated with beef color.