Effects of double-muscling on carcass quality, beef tenderness and myofibrillar protein degradation in Belgian Blue White bulls

Carcass properties and meat quality characteristics of 32 Belgian Blue White double-muscled bulls (DM) were compared with those of 59 bulls of the same breed with normal conformation (N). DM showed superior carcass quality as revealed in increased dressing percentage, meat production yield, conformation grade, muscle~fat and muscle~bone ratios (all P < 0.001). Longissimus dorsi (LD) shear force values, drip and cooking losses at 8 days post mortem (pm) were significantly higher (P < 0.001) for DM, whereas sarcomere lengths were not different. Calpain 1 and calpastatin levels at l h and 24 hpm were tremendously decreased in DM as were also cathepsin B and L levels at 1 and 8 days pm (P < 0.001). As evident from semi-quantitative SDS-PAGE, these differences were accompanied by higher titin and lower 30 kDa levels (P < 0"001) in DM. Troponin-t levels were not different, but very low. Intramuscular collagen content was significantly lower in DM (P<O'O01). This suggests that
lower background toughness in DM was compensated for by reduced postmortem proteolytic tenderization. Discrepancy with literature reports
regarding tenderness of DM might be related to the extreme muscularity
of the Belgian Blue White breed, compared to other DM breeds. The results also suggest that reduced protein turnover might be involved in the muscle hypertrophy phenomenon within this breed, because of likely reduced levels of calpains and cathepsins in living DM animals.