Effect of dietary ractopamine on tenderness and postmortem protein degradation of pork muscle.

Twenty-four finishing pigs with a mean starting weight of 82kg were assigned to two dietary regimens: (1) a corn-soybean meal basal diet (control; n=12), and (2) the basal diet supplemented with 20ppm ractopamine HCl (RAC; n=12). After 28-30 days on the feeding trial, pigs were slaughtered, and the growth and carcass characteristics were measured. Furthermore, the 3rd-13th rib section of longissimus muscle was excised at 48h postmortem, sliced into 19-mm thick chops, vacuum packaged, stored at 2°C, and subjected to Warner-Bratzler shear force (WBSF) and electrophoretic tests after 2, 4, 7, 10, 14, and 21 days (postmortem). RAC feeding increased (P<0.01) pig carcass weight and percent lean, but it also increased the day-2 muscle WBSF by 20% (P<0.01). The shear force difference between control and RAC pig muscles gradually decreased and vanished by day 10 (P>0.05) when both muscle groups became more tender. The muscle from RAC-fed pigs exhibited a slower protein degradation rate than muscle from the control animals, notably for proteins in the 15-45kDa range. The results suggested that the tenderness difference between ractopamine-treated and control pig muscles was related to the proteolysis rate, and could be diminished with adequate postmortem ageing.