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.
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