Response surface methodology-based optimization of decontamination conditions for Escherichia coli O157:H7 and Salmonella Typhimurium on fresh-cut celery using thermoultrasound and calcium propionate.

A combination of thermoultrasound (temperature: 50, 55, 60°C; time: 10, 15, 20 min with the frequency of 40 KHz) and calcium propionate (concentration: 1, 2, 3%, w/v) treatment was applied to decontaminate Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium (S. Typhimurium) from fresh-cut celery. Using a Box-Behnken experimental design, predictive quadratic equations were developed for treatment-based population reductions of E. coli O157:H7 (R(2)=0.98, p<0.001) and S. Typhimurium (R(2)=0.96, p<0.001), and verified using 10 randomly selected treatment conditions. Among three factors (temperature, time, and calcium propionate concentration), temperature represented higher significance for inactivation of pathogenic bacteria. No significant changes (p>0.05) were observed in the color and shear force resistance of the treated celery. The optimum treatment conditions were 60°C thermoultrasound with 2% calcium propionate for 15 min (E. coli O157:H7) and 59°C thermoultrasound with 2% calcium propionate for 17 min (S. Typhimurium). Scanning electron microscopy was used to confirm membrane disruption in the treated microbial cells in each optimal condition. The combined treatment of thermoultrasound and calcium propionate contributes to the effective inactivation (more than 5 log reduction) of E. coli O157:H7 and S. Typhimurium on fresh-cut celery. Furthermore, this treatment extends fresh storability without physical quality deterioration.