Relationship between early post-mortem muscle pH and shortening-induced toughness in the Pectoralis major muscle of processed broilers air-chilled at0 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 pro cessed broilers, air-chilled at 0 degrees C and -12 degrees C, as a functio n of muscle pH early post-mortem. Electrical stimulation was used immediate ly after stunning and neck cutting to provide carcases with pH values 15 mi n post-mortem (pH(15 min)) ranging between 6.79 and 5.85. 2. The deep PM muscle temperatures of carcases chilled at -12 degrees C wer e lower (cooler) after primary chilling and at 215 min post-mortem than tho se 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 fo rce values than those chilled at 0 degrees C. 3. Correlation analysis of the results for both chilling regimens clearly d emonstrated that over the pH(15 min) range 6.79 to 5.85, carcases with the lowest pH(15 min) values had the shortest sarcomeres, the highest cooking l osses and the toughest meat. In addition, there was no evidence to support the occurrence of cold shortening within this population. This suggests tha t an early onset of rigor at higher temperatures in broiler carcases, as we ll as inducing rigor shortening and toughness, might also induce greater pr otein denaturation and subsequent loss of water holding capacity as manifes ted in increased cooking losses. 4. Quadratic regression curves showed that over the pH(15 min) range 6.80 t o 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. Ho wever, neither chilling regimen was effective in preventing severe rigor sh ortening, increased cooking losses and adverse toughness in carcases with p H(15 min) values below 6.30. 5. The benefits of fast chilling carcases with pH(15 min) values above 6.3 can also be quantified in terms of carcases exceeding a 4.00 kg/cm(2) tough ness threshold. Only 1.9% of these carcases chilled at -12 degrees C exceed ed this limit (maximum shear force value of 4.72 kg/cm(2)) compared to 34.9 % of the carcases chilled at 0 degrees C (maximum shear force value of 8.46 kg/cm(2)), further emphasising the considerable reduction in textural vari ability and improvement in tenderness gained by fast air-chilling at -12 de grees C.