Combined treatment of pulsed light and nisin-organic acid based antimicrobial wash for inactivation of Escherichia coli O157:H7 in Romaine lettuce, reduction of microbial loads, and retention of quality ¿

Authors: Mukhopadhyay, Sudarsan; Ukuku, Dike; Olanya, Modesto; Niemira, Brendan; Jin, Zhonglin; Fan, Xuetong

Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/14/2023
Publication Date: 10/16/2023
Citation: Mukhopadhyay, S., Ukuku, D.O., Olanya, O.M., Niemira, B.A., Jin, Z.T., Fan, X. 2023. Combined treatment of pulsed light and nisin-organic acid based antimicrobial wash for inactivation of Escherichia coli O157:H7 in Romaine lettuce, reduction of microbial loads, and retention of quality. Food Microbiology. https://doi.org/10.1016/j.fm.2023.104402.
DOI: https://doi.org/10.1016/j.fm.2023.104402

Interpretive Summary: Leafy greens like Romaine lettuce provide an attractive ecological niche for the proliferation of numerous microorganisms including Escherichia coli O157:H7. To minimize risk of pathogen contamination, industry uses chlorine-based sanitizer which has limited efficacy but has health and safety concerns. New methods are warranted. Pulsed light (PL) is a sustainable, nonthermal technology which can effectively inactivate foodborne pathogens. Regulations limit PL to relatively short treatment times and hence lower doses, primarily to limit thermal effects which can damage quality. These short treatment times hinder PL's antimicrobial efficacy. For this reason, a low dose PL treatment was combined with a safe and effective antimicrobial wash. The combination treatment reduced E. coli O157:H7 by greater than 99.999% on Romaine lettuce, and kept the pathogen from regrowing in subsequent storage. This PL + sanitizer treatment hold promise as a useful tool to enhance the safety and quality of fresh produce.

Technical Abstract: Microbial safety of fresh produce continues to be a major concern. Novel antimicrobial methods are needed to minimize the risk of contamination. This study investigated the antimicrobial efficacy of pulsed light (PL), a novel antimicrobial wash (AW) and the synergy thereof in inactivating Escherichia coli O157:H7 on Romaine lettuce. Treatment effects on background microbiota and quality during storage at 4 degree C for 7 days was also investigated. A bacterial cocktail containing three outbreak strains of E. coli O157:H7 was used as inoculum. Lettuce leaves were spot inoculated on the surface before treating with PL (1 - 60 s), AW (2 min) or combinations of PL with AW. PL treatment for 10 s, equivalent to fluence dose of 10.5 J per square cm, was found optimal which resulted in a 2.3 log CFU/g reduction of E. coli O157:H7 while a rapid 2 min AW treatment, provided a comparable 2.2 log CFU/g, reduction of the pathogen. Two possible sequences of PL and AW treatment combinations were explored. For PL-AW combination, inoculated lettuce leaves were initially exposed to optimum PL dose followed by 2 min AW treatment whereas for AW-PL combination, inoculated lettuce leaves were subjected to 2 min AW treatment prior to 10 s PL treatment. Both combination treatments, PL-AW and AW-PL, indicated strong synergistic inactivation as E. coli cells were not detectable after treatment indicating greater than 5 log reduction of the pathogen. Combination treatments not only significantly reduced spoilage microbial populations on Romaine lettuce but also hindered their growth while in storage for 7 days. The firmness and visual appearance of lettuce were not significantly influenced due to combination treatments. Overall, the results reveal that PL and AW combination treatment can be implemented as a novel approach to enhance microbial safety, quality and shelf life of Romaine lettuce.