Proteinase (Cathepsin B, D, L and Calpains) levels and conditioning rates in normal, electrically stimulated and high-ultimate-pH chicken muscle.

Control, electrically stimulated (ES) and glycogen-depleted (GD) chicken muscles were conditioned at 15°C with continuous mechanical testing for extensibility. The ES and GD muscles went into rigor 3·6 and 2·8 h earlier, respectively, than control muscle. At 24h post-rigor the extensibility of control muscle (11·2%) was markedly less than ES (19·2%) and GD (27·3%) muscles indicating that these latter two treatments should provide more tender meat. Measurement of sarcomere lengths showed no significant differences between control and GD muscle and thus, the greater extensibility in the high pH condition may be restricted to a wider separation of myofibriller fragments at the intermittent fracture zones when under load. Examination of muscle proteinase (cathepsins B, D and L, calpains I and II) and glycosidase (β-d-glucuronidase, N-acetyl-β-d-glucosaminidase) levels at 0 and 48h post-slaughter revealed changes in some key enzymes between the different treatments. Calpain I activity declined markedly during 48h storage of ES muscle (83%) compared to control (58%) and GD (63%) muscles. Cathepsin B and L activities did not decline during storage of ES muscle but there was a slight fall in control and GD muscles. Dosing of chicken shortly before slaughter with inhibitors of cysteine proteinases had a negligible effect on conditioning rate, apparently due to lack of inhibition of these proteinases during this short time period in the intact muscle.