99D-6

High-intensity ultrasonication is an emerging processing technology capable of deactivating pathogens and has considerable potential to improve food safety.

The objective of this study was to determine ultrasound-induced inactivation of Escherichia coli O157:H7 in the presence of six salts as a function of ultrasonic intensity, temperature and treatment time.

Test strains of Escherichia coli O157:H7 were cultured, washed, and resuspended (8-9 log CFU/mL) in 99mL sterile aqueous solutions containing 0, 0.5, 1.0, 2.0, or 5.0% of AlCl₃, CaCl₂, NaCl, NaNO₃, NaPO₄, or NH₄Cl.  Inoculated solutions were sonicated for 1, 5, or 10 minutes at intensity levels of 9.49, 21.83, or 49.22W/cm², at either 4°C or 40°C.  After treatment, samples were diluted, plated onto tryptic soy agar, and incubated for 24 hours at 35°C.

Studies to recover injured Escherichia coli O157:H7 cells showed that increase in sonication treatment time, intensity and temperature all attributed to enhanced cell death. However, the impact of solute concentration on cell lethality varied depending on the salt. CaCl₂, NaCl, NaNO₃, NaPO₄, and NH₄Cl had little or no effect on survival of E. coli O157:H7 (<1 log reduction) regardless of other treatment conditions at 9.49W/cm². Concentrations >0.5% of CaCl₂, NaNO₃, NaPO₄, and NH₄Cl adversely affected survival (1.0–1.6 log reduction) with treatment of 21.83W/cm² at 4°C, and showed greater inactivation (1.2-4.0 log reduction) at 40°C.  Treatment of 49.22W/cm² showed the greatest impact regardless of other parameters. At 40°C, treatment for 10 minutes at 49.22 W/cm² led to total inactivation for bacteria suspended in 0.5% NH₄Cl, 1% CaCl₃, 2% NaCl, 5% NaPO₄, and all concentrations of AlCl₃. Complete inactivation also occurred at 4°C for 5% AlCl₃, which was the most effective salt.

The results provide important insight into the mechanism of microbial deactivation by ultrasound and the practical aspect of inactivating pathogens in low water activity foods.