Antimicrobial efficacy of nisin, oregano and ultrasound against Escherichia coli O157:H7 and Listeria monocytogenes on lettuce

Ready-to-eat vegetables such as lettuce are prone to bacterial contamination, predominantly from Escherichia coli and Listeria monocytogenes, the leading causes of foodborne illnesses globally. Chlorine is widely used as a decontaminant in the fresh produce industry, however, potential carcinogenic risks limit its usage. As an alternative, green methods and hurdle technology are gaining interest. In this paper, the efficacy of a combination of nisin, oregano and ultrasound on the reduction of E. coli and L. monocytogenes on lettuce was studied using response surface methodology/Box-Behnken model design, and was found to be reliable (p < 0.05). A combination of 771.2 IU/g nisin, 0.185% v/v oregano and 14.65 min ultrasound was the most effective treatment on both pathogens, showing log reductions of 3.43 and 9.20 CFU/mL for E. coli and L. monocytogenes, respectively. The treated lettuce samples did not show any significant differences in textural properties; however, mild colour changes and a slight increase in the electrolyte leakage rate was observed, within permissible limits. Interestingly this is the first report on the combination of nisin, oregano and ultrasound, as a promising alternative to chemical treatments for the reduction of E. coli and L. monocytogenes on lettuce, without compromising its appearance and quality.