Kinetics of inactivation of Bacillus subtilis spores by continuous or intermittent ohmic and conventional heating

Bacillus subtilis spores were suspended in 0.1% NaCl solution (ca. 10(7) CF U/mL) and treated by conventional or ohmic heating under identical temperat ure histories. Temperatures tested were in the range of 88 to 99 degrees C. Survival curves and calculated D values showed significantly higher lethal ity for spores by ohmic than conventional heating. The z or E-a values corr esponding to the two heating methods, however, were not significantly diffe rent. Spores of B. subtilis were suspended in nutrient broth and treated wi th conventional and ohmic heating through a single- or a double-stage treat ment. In case of double-stage treatment, heating was interrupted by a 20 mi n of incubation at 37 degrees C to induce a Tyndallization effect. Spore in activation during double-stage treatment was greater for ohmic than convent ional heating. The enhanced spore inactivation by ohmic, compared with conv entional, heating resulted from a greater rate of spore death during the fi rst stage of heating and greater decrease in count of viable spores immedia tely after the incubation period that intervened the heating process. Thus it is concluded that spore inactivation during ohmic heating was primarily due to the thermal effect but there was an additional killing effect caused by the electric current. (C) 1999 John Wiley & Sons, Inc.