PLG and mastitis: When mastitis occurs, on the one hand, plasminogen is catalyzed by a plasmin plasminogen activator secreted by somatic cells to plasmin at an increased rate, plasmin binds to casein micelle substrates, and preferentially hydrolyzes β-CN into γ-casein, and small peptides are released into the whey [36,37], resulting in decreased β-CN and total CN contents; on the other hand, mammary epithelial cells are damaged due to inflammation and pathogen invasion, resulting in decreased protein synthesis capacity.