Structural and mechanical changes in raw and cooked single porcine muscle fibres extended to fracture

Tensile tests on single muscle fibres from raw and cooked porcine longissimus thoracis muscle were performed to explore the structural mechanisms responsible for their deformation and fracture properties. Measurements of load and deformation were made simultaneously with light microscopy observations of the structural changes which occur on extension. On extending the fibres to fracture, an r-shaped stress-strain curve was observed and the structural changes which occurred during this process could be divided into three phases. Phase one, was characterised by a rapid increase in stress with little change in strain and ended 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 and 20%. In phase two, there was rapid increase in strain with minimal changes in stress. In most fibres this phase was characterised by multiple cracks on the fibre surface and unequal sarcomere stretching. Sarcomeres in the regions where the surface had ruptured extended faster than those in areas still covered by the surface membrane, where sarcomere length remained relatively unchanged. In some cooked fibres, there was little or no surface cracking and all the sarcomeres in these fibres 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 finally breakage of the whole fibre. The myofibrils did not always fail as one unit; a progressive snapping of small bundles of myofibrils was seen 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 ± 42% of their rest lengths after they were cooked for 1 h at 80°C. Where multiple surface cracking was observed in phase two, sarcomeres in some cracked areas lengthened faster than others and the cracked areas which extended fastest were usually the focus of the eventual failure of the fibre. In raw fibres, sarcomeres in the areas where the fibre surface had ruptured could be stretched up to 107·7% before failure, while those in areas of the fibre with an intact surface remained relatively unchanged. In cracked areas of cooked fibres the sarcomeres were more extensible and could be stretched to 169·7% before breaking. The order-of-magnitude increase in overall extension to failure of fibres resulting from cooking is only partially due to this increase in sarcomere extensibility in cracked areas. Mechanically demembranating raw fibres depressed the stress at which yielding occurred and doubled their breaking strain. However, this process had no effect on the stress at which the fibres fractured. The results show that deformation is not uniform along individual fibres, especially in the raw case and that the endomysium has an important contribution to this non-uniform deformation.