The inactivation of lysozyme by ultrasonic waves under pressure has been investigated. The resistances to heat and to ultrasound have also been studied as references. Lysozyme thermal inactivation followed first-order kinetics. The enzyme was very heat stable in pH 6.2 phosphate buffer (D-100 degrees C = 11 min) but its heat resistance decreased by more than two orders of magnitude when ovalbumin was added to the buffer (D-70 degrees C = 2.4 min). Ultrasonic waves at room temperature and pressure hardly inactivated the enzyme. The application of an external pressure of 200 kPa and temperatures between 60 and 80 degrees C increased the inactivating effect of ultrasound. A manothermosonication treatment (117 mu m, 200 kPa, 70 degrees C) for 3.5 min decreased 10-fold the lysozyme activity. Inactivation curves of lysozyme in buffer by manothermosonication and thermoultrasonication were biphasic, and the loss of catalytic activity during the second step of the inactivation curves was coincident with the appearance of free sulfhydryl groups. (c) 2006 Elsevier Inc. All rights reserved.