Characteristics of a Protease-Producing Strain of Bacillus cereus Isolated from Raw Milk

Eight Bacillus cereus strains were isolated from 10 batches of raw milk and identified through 16S rDNA sequencing and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Out of these strains, one was selected on the basis of higher protease production and identified as B. cereus LY-6. We inoculated this strain back into skim milk and observed the resulting spoilage process. It was found that the milk turned to yellow, and had a bitter taste and an unpleasant odor. Moreover, the pH decreased and large amounts of proteases were found, indicating that the spoilage process is closely related to microbial protease production. Strain LY-6 was intolerant to ultra-high temperature sterilization. A distribution map of B. cereus in the dairy was drawn to control its spread. It was found that this bacterium mainly existed in places where milk stains are difficult to clean such as pasture bedding, towels, milk trucks, workshop floors, personnel shoe soles, and equipment surfaces. Therefore, special attention should be paid to the cleaning effect of the backend pipeline after the sterilization process to prevent potential food safety hazards caused by secondary contamination of B. cereus.