STRUCTURAL AND MECHANICAL CHANGES IN RAW AND COOKED SINGLE PORCINE MUSCLE-FIBERS EXTENDED TO FRACTURE

Tensile tests on single muscle fibres from raw and cooked porcine long issimus thoracis muscle were performed to explore the structural mecha nisms responsible for their deformation and fracture properties. Measu rements of load and deformation were made simultaneously with light mi croscopy observations of the structural changes which occur on extensi on. On extending the fibres to fracture, an r-shaped stress-strain cur ve was observed and the structural changes which occurred during this process could be divided into three phases. Phase one, was characteris ed by a rapid increase in stress with little change in strain and ende d at the yield point. Sarcomere length was uniform along the fibre in this initial phase. Raw fibres yielded at strains of between 2 and 5% of their resting lengths and cooked fibres at strains of between 10 an d 20%. In phase two, there was vapid increase in strain with minimal c hanges in stress. In most fibres this phase was characterised by multi ple cracks on the fibre surface and unequal sarcomere stretching. Sarc omeres in the regions where the surface had ruptured extended faster t han those in areas still covered by the surface membrane, where sarcom ere length remained relatively unchanged. In some cooked fibres, there was little or no surface cracking and all the sarcomeres in these fib res extended almost uniformly. Phase three was characterised by a rise in stress as strain increased and then a final fall in stress at the breaking point. This was accompanied by myofibrillar failure and final ly breakage of the whole fibre. The myofibrils did not always fail as one unit, a progressive snapping of small bundles of myofibrils was se en in some raw fibres. Muscle fibres could be stretched to 10.9 +/- 1. 45% of their resting length before breaking when raw, but to 130 +/- 4 2% of their rest lengths after they were cooked for 1 h at 80 degrees C. Where multiple surface cracking was observed in phase two, sarcomer es in some cracked areas lengthened faster than others and the cracked areas which extended fastest were usually the focus of the eventual f ailure of the fibre. In raw fibres, sarcomeres in the areas where the fibre surface had ruptured could be stretched up to 107.7% before fail ure, while those in areas of the fibre with an intact surface remained relatively unchanged. In cracked areas of cooked fibres the sarcomere s were more extensible and could be stretched to 169.7% before breakin g. The order-of-magnitude increase in overall extension to failure of fibres resulting from cooking is only partially due to this increase i n sarcomere extensibility in cracked areas. Mechanically demembranatin g raw fibres depressed the stress at which yielding occurred and doubl ed their breaking strain. However, this process had no effect on the s tress at which the fibres fractured. The results show that deformation is not uniform along individual fibres, especially in the raw case an d that the endomysium has an important contribution to this non-unifor m deformation.