Effect of Ultimate pH and Degree of Doneness on Fiber Degradation of Nellore Longissimus Lumborum Muscle

The Longissimus lumborum muscles of Nellore crossbred bulls (n = 9) were purchased from a commercial abattoir located in the state of São Paulo, Brazil. The muscles were collected at 24 h post mortem, according to the three pH groups using a pH meter coupled to a puncture electrode glass. At 48 h post mortem samples were roasted in an electric oven until these reached their corresponding final internal temperatures. Muscle fiber degradation was evaluated using scanning electron microscopy (SEM). Muscle portions (6.0 × 3.0 × 3.0 mm) were cut in the transversal direction of the muscle fibers and fixed for 1.5 h in Karnovsky solution (Karnovsky, 1965), prepared with small modifications (2.5% glutaraldhyde (v/v) and 2.5% formaldehyde in 0.05 M cacodylate buffer, pH 7; and 1 mM calcium chloride). The fixative solution was replaced by 30% glycerol (v/v), and then the samples were cryofractured in liquid nitrogen and post-fixed with 1% osmium tetroxide (w/v). After this, the samples were dehydrated at increasing concentrations of acetone (30, 50, 70, 90, 100%) and dried to the critical point of CO2. Then, the samples were sputtered with a 30 nm gold layer, and observed in a scanning electronic microscope (LEO 435 VP, Leo Electron Microscopy Ltd., Cambridge, England) at an acceleration voltage of 20 kV. The samples were photographed at 650 × magnification.