Optical characterization of skeletal muscles
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The optical absorption and reduced scattering coefficients of bulk skeleton muscle were measured by fitting the diffusion model to spatially resolved diffuse reflectance measurements. Muscle pigments were characterized using optical absorption spectra and muscle sarcomere structures were characterized using optical scattering coefficients. The sarcomere is the fundamental functional unit in muscle. Using a heating model, we found that muscle pigments and their denaturation under heating could be characterized by the changes of the specific absorption spectra. The structural changes by the denaturation of collagen and sarcomere proteins could be characterized by the scattering changes. By studying the correlation between the reduced scattering coefficient and sarcomere length, we found that the reduced scattering coefficient increased with sarcomere length. Using pre-rigor muscles as experimental models, we investigated the possibility of optical characterization of myosin-actin interaction within sarcomere by comparing optical scattering changes, pH value changes, muscle tension changes and structure changes viewed by microscopy. We found that the temporal changes of optical scattering during proceeding rigor closely reflected the changes of pH and muscle tension. It was concluded that the change of optical scattering during rigor reflected the metabolism-related functional and structural changes and optical scattering had the ability to monitor the muscle rigor process. The capability of characterizing sarcomere structures, especially sarcomere length, suggests the potential of using optical scattering in beef tenderness prediction. The correlation between optical scattering and Warner-Bratzler shear force (WBSF) in muscles at different anatomic locations was studied. The correlation coefficient for M. psoas major was r = - 0.44 (n = 15), for M. semimembranosus r = 0.45 (n = 10) and for M. longissimus dorsi r = 0.82 (n = 19). These results indicated that the correlation was muscle dependent. This is because beef tenderness is determined by different contributing factors related to anatomical locations within the body. The negative correlation in M. psoas major supported the general belief that sarcomere length was the significant factor in determining its tenderness. On the contrary, the positive correlation in M. semimembranosus and M. longissimus dorsi indicated that sarcomere length was not the dominant factor contributing to tenderness in these samples. The majority of research supports that collagen content is the major determinant to tenderness in M. semimembranosus and the extent of protein proteolysis in sarcomere determines the tenderness in M. longissimus dorsi. Our detailed analysis suggested that sarcomere length alone was not sufficient in determining tenderness. For a better prediction, the effects from other factors such as the collagen content and the extent of proteolysis should also be counted for a thorough characterization of meat tenderness.
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