Inactivation of Bacillus subtilis spores in MISO by continuous joule heating

Miso products, such as instant miso paste, are seasonings made from fermented soybeans, and must be subjected to the inactivation of microbial spores to ensure a long shelf life. To inactivate spores in miso by conventional heating, it is necessary to heat the miso at a high temperature for a long time. However, heating can degrade the color and flavor of miso. In this study, continuous joule heating under pressure was employed to inactivate Bacillus subtilis spores in miso. A 2.7 log reduction in B. subtilis spores was achieved by applying joule heating, in which the temperature was raised to 98°C in 4 s, and then maintained for 4 s at 0.75 MPa. No change in color was observed after joule heating, in contrast to the darkened color after conventional heating.