Effect of heat and ultrasonic waves on the survival of two strains of Bacillus subtilis.

The combined effect of ultrasonic (20 KHz, 150 W) and heat treatment on the survival of two strains of Bacillus subtilis in three suspending media (distilled water, glycerol and milk) has been studied. When spores suspended in water or milk were subjected to ultrasonic waves before heat treatments a little or no decrease of the heat resistance was observed. However, both sporicidal agents applied simultaneously (thermo-ultrasonication) decreased by 63% (B. subtilis, var. niger-40) and 74% (B. subtilis ATCC 6051) the decimal reduction times for the heat treatment when the spores were suspended in glycerol and by 79% and 40%, respectively when suspended in milk. The thermo-ultrasonication of spores in water markedly reduced the heat resistance of them (between 99.9% and 70%) in the range 70-95 degrees C but the effect of the thermo-ultrasonication significantly diminished as the temperature of the treatment was approached to the boiling point of the water.