The Effect of Osmotic Pressure on Rigor Mortis in Salmonids
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Rigor mortis is a natural phenomenon caused by a series of complicated chemical changes in the muscles after the death of an animal. This concept is important especially in meat technology since it affects texture, yield and meat palatability. However, the understanding of rigor mortis in animals is currently not fully discovered, and research is still ongoing. The rigor process is frequently explained by the sliding filament theory, where actomyosin bridges form and cause stiffness during rigor. Yet adenosine triphosphate (ATP) concentration gradually depletes as the muscle enters rigor, so little or no ATP is available for rigor resolution and muscles remain stiff. Hence, this theory does not explain the rigor resolution process, where muscles eventually become soft and limp. An alternative hypothesis of rigor mortis is postulated describing the rigor process via changes in osmotic pressure in muscle cells. Since catabolic reactions (glucose to lactate) predominate after death, the number of molecules inside muscle cells increase. This induces an increase in osmotic pressure and water flows into the cells to achieve osmotic equilibrium between compartments, thus creating stiffness. Therefore, this thesis investigates the hypothesis by analysing the movement of water in muscles immersed in solutes of varying salinities, together with texture and morphological analysis. It was found that mass changes in salmonid muscles were significantly affected by the movement of water in different salinities and were independent with the contraction rates. Rigor index measurements showed that the individual fish reached 63.2% of their maximum rigor within 4 to 6 hours post mortem. Shear force was expected to be highest at this time point but texture analysis did not follow the same trend and gave a general decrease in breaking force. Even so, there were high variations observed with the individual texture measurements explained by multiple reasons. The effect of formalin fixation also revealed that prolonged fixation of muscle tissues can lead to cell shrinkage. Morphological analysis of muscle cells by image processing in Matlab further showed that there were distinct differences in the amount of intra and extracellular spaces and inter-fibre distances from the tissue surface and center in the various solutions.