RESUMO
The contribution of major structural (myofibrillar fragmentation upon mechanical treatment) and ultra-structural (Z-line degradation, loss of electron density of M-line, transversal disruption of sarcomeres at N(2)-line level, longitudinal fissure of myofibrils, and loss of transversal alignments of Z- and M-lines) changes in myofibrils to rabbit (Oryctolagus cuniculus L.) meat tenderisation, during the ageing period (9 days at +4 °C), was studied for different types of muscle (type I, semimembranosus proprius; type IIB, semimembranosus accessorius; and type IID, psoas major). The results strongly suggest that myofibrillar structure weakening at N(2)-line level (evaluated by myofibrillar fragmentation upon mechanical homogenisation and observed by transversal disruption of sarcomeres), which is very likely mediated by cysteine endopeptidases, might be the major structural change responsible for rabbit meat tenderisation during ageing. Both myofibrillar fragmentation and transversal disruption of sarcomeres are good ageing indices for rabbit meat. The other major ultra-structural changes in myofibrils appear to have no major role in rabbit meat tenderisation at refrigeration temperatures. Finally, it is proposed that meat tenderisation during ageing depends mainly on the specific cleavage of titin molecules/filaments and nebulin molecules, at their susceptible sites located at or very close to the N(2)-line region (extensible segment and near C-terminus, respectively), mediated by cysteine endopeptidases (possibly calpains).