Inactivation of Cronobacter sakazakii in head lettuce by using a combination of ultrasound and sodium hypochlorite

The present study investigated the synergistic effects of combined ultrasound (37 kHz, 380 W for 5-100 min) and sodium hypochlorite (NaOCl) (50-200 ppm) on reducing Cronobacter sakazakii KCTC 2949 in head lettuce. Ultrasound was not enough to inactivate C. sakazakii (0.01-0.58 log-reduction), whereas NaOCl significantly (P < 0.05) reduced C. sakazakii (0.58-2.77 log-reduction). Despite the significant reduction in C. sakazakii with NaOCl treatment (200 ppm), the combination of 100 min ultrasound and 200 ppm NaOCl resulted in an additional 1.67 log-reduction of C. sakazakii (4.44 log-reduction = 2.77 + 1.67 log). Synergistic reductions on C. sakazakii were observed in most combined treatments, although the most synergistic reduction values were <1.0 log10 CFU/g. The highest synergistic value of reduction in head lettuce was 1.08 log10 CFU/g when treated with a combination of 100 min ultrasound and 200 ppm NaOCl. Moreover, the pH and °Brix after combined treatment with 5-100 min ultrasound and 50-200 ppm NaOCl did not significantly differ from any single treated lettuce. These results indicate that combined treatment with 100 min ultrasound and 200 ppm NaOCl could be a potential approach to reduce C. sakazakii on post-harvest leaf vegetable during processing and enhance the shelf-life during distribution and storage.