Relationship between early post-mortem muscle pH and shortening-induced toughness in the Pectoralis major muscle of processed broilers air-chilled at 0 degrees C and -12 degrees C

1. An experiment was conducted to investigate the development of shortening-induced toughness in the Pectoralis major (PM) muscles of commercially processed broilers, air-chilled at 0 degrees C and -12 degrees C, as a function of muscle pH early post-mortem. Electrical stimulation was used immediately after stunning and neck cutting to provide carcases with pH values 15 min post-mortem (pH15 min) ranging between 6.79 and 5.85. 2. The deep PM muscle temperatures of carcases chilled at -12 degrees C were lower (cooler) after primary chilling and at 215 min post-mortem than those chilled at 0 degrees C, although chilling regimen had no major effect on pH values over the 24 h post-mortem period. However, carcases chilled at -12 degrees C had longer sarcomeres, lower cooking losses and lower shear force values than those chilled at 0 degrees C. 3. Correlation analysis of the results for both chilling regimens clearly demonstrated that over the pH15min range 6.79 to 5.85, carcases with the lowest pH15min values had the shortest sarcomeres, the highest cooking losses and the toughest meat. In addition, there was no evidence to support the occurrence of cold shortening within this population. This suggests that an early onset of rigor at higher temperatures in broiler carcases, as well as inducing rigor shortening and toughness, might also induce greater protein denaturation and subsequent loss of water holding capacity as manifested in increased cooking losses. 4. Quadratic regression curves showed that over the pH15min range 6.80 to 6.30, only the fast chilling regimen at -12 degrees C could inhibit rigor shortening and minimise changes in cooking loss and shear force values. However, neither chilling regimen was effective in preventing severe rigor shortening, increased cooking losses and adverse toughness in carcases with pH15min values below 6.30. 5. The benefits of fast chilling carcases with pH15min values above 6.3 can also be quantified in terms of carcases exceeding a 4.00 kg/cm2 toughness threshold. Only 1.9% of these carcases chilled at -12 degrees C exceeded this limit (maximum shear force value of 4.72 kg/cm2) compared to 34.9% of the carcases chilled at 0 degrees C (maximum shear force value of 8.46 kg/cm2), further emphasising the considerable reduction in textural variability and improvement in tenderness gained by fast air-chilling at -12 degrees C.