Possible mechanism for the effect of the RN(-) allele on pork tenderness

The effect of the dominant RN⁻ allele on rigor development, ageing and tenderness was studied in M. longissimus dorsi (LD) from 11 heterozygous carriers and five non-carriers of the RN⁻ allele. Rigor development was followed by measurements of muscle shortening, isometric tension, pH and FOP. During ageing the myofibrillar length and Warner–Bratzler shear force were measured in the meat. Sensory analysis was performed at 4 days post-mortem using a trained expert panel. It was found that the decrease in pH was faster for RN⁻ carriers than non-carriers during the first 5 h post-mortem, after which the pH-time slope was similar for the two groups. This resulted in a significantly lower mean ultimate pH in LD from RN⁻ carriers than non-carriers. During rigor development the isometric tension was lower in RN⁻ carriers than in non-carriers, while contraction (shortening and sarcomere length) did not differ significantly between the two genotypes. The myofibrillar length, which is an indirect measure of the proteolytic activity that has occurred in the meat, was shorter for the RN⁻ carriers than for the non-carriers. The difference in myofibrillar lengths between the genotypes was significant at 1 and 4 days post-mortem but not at 7 days post-mortem, which indicates that the RN⁻ carriers have a higher proteolytic activity earlier post-mortem. The results from the Warner–Bratzler shear force measurements showed that the meat from the RN⁻ carriers was significantly more tender, 1 and 4 days post-mortem, than the meat from the non-carriers. The meat from non-carriers needed 7 days to reach the tenderness attained by that from the RN⁻ carriers 4 days post-mortem. The greater tenderness in LD from RN⁻ carriers than that from non-carriers was also confirmed by a sensory panel at 4 days post-mortem. In conclusion, differences observed in the course of rigor and ageing in muscle from carriers and non-carriers of the RN⁻ allele suggest that proteolytic action, as initiated by a more rapid fall in pH, is the most important factor governing the variation in tenderness of the two genotypes.