Inactivation of Bacillus subtilis Spores in Soybean Milk by Radio-Frequency Flash Heating Treatment

Heat-resistant microorganisms such as Bacillus subtilis spores derived from soil may exist in soybean milk and can reduce the shelf life of tofu produced from conventional processing of soybean milk. Conventional high-temperature, long-time heat processing of soybean milk sufficiently inactivates spores of this organism. An apparatus to pasteurize soybean milk using radio-frequency flash heating (RF-FH) was developed. An electrode surface was covered with a 50μm-thick Teflon film, and 27 MHz RF-FH was applied to soybean milk flowing through the electrode unit. No difference was observed between the frequency of 20 kHz and that of 27 MHz regarding the effect of inactivation of B. subtilis in saline solution. No difference in inactivation speed of B. subtilis by RF-FH at a holding time (1.7 to 5.5 s) was observed between saline solution and soybean milk. However, the inactivation speed of B. subtilis in soybean milk by conventional heating was lower than in saline solution. RF-FH could inactivate B. subtilis without reference to surrounding soybean milk. Comparative studies revealed that trypsin inhibitor (TI) in soybean milk after RFFH processing had lower inactivation than TI after conventional heating. A four-logarithm-order reduction of B. subtilis spores was realized in the soybean milk by RF-FH at up to 107 °C for 5.5 s.