and Objectives: In this study, the proteolysis and lipolysis of Poosti cheese produced from raw sheep milk in mountainous eastern regions of Iran were investigated during 90 days of ripening. Materials and Methods: Sodium dodecyl sulfate polyacrylamide gel electrophoresis for proteolysis (SDS-PAGE) and gas chromatography (GC) for free fatty acids (FFAs) were applied to investigate the intensity of lipid degradation. To evaluate the Poosti cheese microstructural changes, the area fraction

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... er of the scanning electron microscopy (SEM) micrographs was also calculated by the Image J software. Results: The most alteration in protein profile was occurred in the first month of aging for high activity of the proteolytic microorganisms in this period. The amount of free fatty acids was depended on their length due to the variety of involved mechanisms. In addition, the microstructural parameter was considerably affected by the aging as a consequence of the effect of salt on the activity of raw milk and skin micro flora. Conclusions: The decline in proteolysis rate during the last stage of aging could be correlated with the inhibitory effects of salt on the engaged microorganisms, and increase in the pore fraction of the microstructure during the first month of Poosti cheese aging could be due to casein rearrangement and gas release by the fermentative activity of microorganisms. Keywords: Proteolysis, Lipolysis, Poosti cheese, Raw sheep milk.