Purpose: We determined the synergistic effects of an ultrasound (US) and NaOCl (50–200 ppm) combination on C. sakazakii reduction in fresh head lettuce, in comparison with US or NaOCl alone, and its effects on the quality of the lettuce.
Methods: Fresh head lettuce was used for investigating the synergistic effects of US (37 kHz, 380 W for 5–100 min) and NaOCl (50–200 ppm) against C. sakazakii. The synergistic effect of combined US and NaOCl were calculated using the following equation: synergistic reduction value = A – (B + C), where A is log reduction by combined US and NaOCl, B is log reduction after single US, and C is log reduction after single NaOCl.
Results: US 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). The combination (4.44 log reduction) of 100 min US and 200 ppm NaOCl resulted in an additional 1.7 log reduction of C. sakazakii. Synergistic effects were observed in most combined treatments, although the most synergistic values were all < 1.0 log CFU/g. The highest synergistic values were 1.08 log CFU/g when treated with a combination of 100 min US and 200 ppm NaOCl. Moreover, the pH and °Brix after 5–100 min US and 50–200 ppm NaOCl did not differ significantly from that in head lettuce treated with US or NaOCl alone.
Significance: Combination of 100 min US and 200 ppm NaOCl could be a potential approach for the post-harvest leaf vegetable processing to enhance the microbial safety of leafy vegetables without any changes in food qualities.