Pasteurization and changes of casein and free amino acid contents of bovine milk by low-pressure CO2 microbubbles

•Inactivation efficiency of MBCO2 on E. coli in CBM was less than that in PS.•A 3-log reduction in aerobic bacteria in UBM was occurred by MBCO2 for 1 min at 45 °C.•Casein in UBM treated with mixing vessel was easier to be decomposed by thermolysin.•Most of the free amino acid contents in UBM were decreased by the two-stage MBCO2.

The inactivation efficiency of a two-stage system with low-pressure CO2 microbubbles (two-stage MBCO2) on Escherichia coli in physiological saline (PS) and commercial sterilized bovine milk (CBM), and aerobic bacteria in unpasteurized bovine milk (UBM) was investigated. Furthermore, protease-resistance of casein and the free amino acid contents in the treated milk were measured. The number of surviving E. coli cells in PS and CBM by the two-stage MBCO2 decreased with increasing pressure in a mixing vessel or increasing temperature in a heating coil. A 3-log reduction in aerobic bacteria in UBM was achieved by two-stage MBCO2 after 1 min with the heating coil at 45 and 50 °C. Casein in UBM treated with only mixing vessel of two-stage MBCO2 was easier to be decomposed by thermolysin, although the casein warmed with the heating coil was difficult to be decomposed by thermolysin and papain. In addition, most of the free amino acid contents in UBM were decreased by the two-stage MBCO2. Therefore, it was suggested that limited inactivation of aerobic bacteria, and change of protease-resistance of casein and free amino acid contents in UBM were induced by two-stage MBCO2.