Abstract: This study aimed to compare the differences in myofibrillar protein aggregation, oxidation and structural properties among beef M. longissimus dorsi cooked to different degrees of doneness (control, rare, medium rare, medium, medium well, well done, and over cooked). The results showed that the cooking process significantly affected myofibrillar protein aggregation, oxidation and structural properties. As the degree of doneness increased, myofibrillar protein solutions gradually changed from an aggregated state to a cleaved one; the content of protein carbonyl increased gradually, and the surface hydrophobicity and sulfhydryl content increased first and then decreased; the fluorescence intensity of dityrosine and intrinsic fluorescence intensity increased first, then decreased and finally increased again. The results of attenuated total reflection Fourier transform infrared spectroscopy analysis showed that the degree of medium cooking was the turning point in the evolution of protein secondary structure. As the degree of doneness increased from control to medium, the β-sheet relative content decreased, while the α-helix relative content remained stable, the aggregation of protein was mainly through intramolecular interactions; as the degree of doneness further increased to over cooked, the β-sheet relative content increased gradually, the α-helix relative content increased first and then decreased, and the protein was mainly intermolecularly aggregated. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) studies indicated that myosin heavy chain, α-actin and actin were involved in the thermal aggregation of myofibrillar proteins. As the degree of doneness increased, the gray values of myosin heavy chain and actin increased first and then decreased, and the gray value of α-actin increased continuously. The exact mechanisms need further study. The results from this study can provide a theoretical basis for the optimization of the frying process and the quality control of steak.

Key words: myofibrillar protein; degree of doneness; aggregation properties; structural properties; oxidation properties