Influence of Temperature and Pressure on the Lethality of Ultrasound

Abstract

A specially designed resistometer was constructed, and the lethal effect on Yersinia enterocolitica of ultrasonic waves (UW) at different static pressures (manosonication [MS]) and of combined heat-UW under pressure treatments (manothermosonication [MTS]) was investigated. During MS treatments at 30°C and 200 kPa, the increase in the amplitude of UW of 20 kHz from 21 to 150 μm exponentially decreased decimal reduction time values (D MS) from 4 to 0.37 min. When pressure was increased from 0 to 600 kPa at a constant amplitude (150 μm) and temperature (30°C),D MS values decreased from 1.52 to 0.20 min. The magnitude of this decrease in D MS declined progressively as pressure was increased. The influence of pressure onD MS values was greater with increased amplitude of UW. Pressure alone of as much as 600 kPa did not influence the heat resistance of Y. enterocolitica(D 60 = 0.094; z = 5.65). At temperatures of as much as 58°C, the lethality of UW under pressure was greater than that of heat treatment alone at the same temperature. At higher temperatures, this difference disappeared. Heat and UW under pressure seemed to act independently. The lethality of MTS treatments appeared to result from the added effects of UW under pressure and the lethal effect of heat. The individual contributions of heat and of UW under pressure to the total lethal effect of MTS depended on temperature. The inactivating effect of UW was not due to titanium particles eroded from the sonication horn. The addition to the MS media of cysteamine did not increase the resistance of Y. enterocolitica to MS treatment. MS treatment caused cell disruption.

Inactivation ofBacillus subtilisspores by combining ultrasonic waves under pressure and mild heat treatment

“…An exponential relationship was found between amplitude and the number of survivors of B. subtilis spores. The same relationship has been reported for death rate of Y. enterocolitica (Raso et al 1998) and erosion of different materials (Hobbs 1969).…”

“…Although the dependence of cavitation intensity on static pressure has been shown theoretically (Neppiras 1980), few studies have shown the effect of high static pressure on inactivation of vegetative cells by ultrasound (Neppiras and Hughes 1964;Raso et al 1998). Our results showed that at atmospheric pressure, ultrasound treatments barely affect viability of B. subtilis spores, but the lethality of treatments increased with increasing static pressure (Fig.…”

Variation in resistance of natural isolates of Staphylococcus aureus to heat, pulsed electric field and ultrasound under pressure

“…Whereas clear hypotheses have been put forward to explain how bacterial cells are inactivated by UW under pressure and PEF, still much is unknown about heat. Ultrasound under pressure is known to kill cells through mechanical disintegration of the envelopes (Raso et al 1998), and PEF leads to cell death by the formation of pores in the cytoplasmic membrane according to Zimmerman's electroporation theory (Zimmermann et al 1974). Heat, however, is known to affect many different structures within the bacterial cell, such as the membrane, the cell wall, ribosomes, RNA, etc., but the key target leading to cell death has yet to be identified.…”

“…Heat and MS treatments were carried out in a specially designed resistometer as already described (Raso et al 1998). Once temperature, and pressure and UW amplitude for MS determinations, had attained stability, 0AE2 ml of an adequately diluted cell suspension (to 10 7 cell ml )1 , approx.)…”

“…Although UW was initially discarded for food preservation because of its weak lethal action, the simultaneous application of an external hydrostatic pressure of up to 600 kPa (manosonication, MS) increases substantially the lethality of the treatment. Also, a combination of MS with temperature (manothermosonication, MTS) has been proposed (Raso et al 1998). In the last decade, a big research effort has been directed towards the investigation of microbial inactivation by these novel technologies, including aspects such as the influence of process parameters, relative resistance of different spoilage and pathogenic microbial species and, to a lesser extent, environmental factors affecting the resistance and mechanisms leading to cell death ).…”