Contents lists available at ScienceDirect Food Control journal homepage: www.elsevier.com/locate/foodcont Efcacy of low concentration acidic electrolysed water and levulinic acid combination on fresh organic lettuce (Lactuca sativa Var. Crispa L.) and its antimicrobial mechanism Lin Zhao a,b , Mitchie Yiqi Zhao a , Chin Ping Phey a , Hongshun Yang a,b,∗ a Food Science and Technology Programme, c/o Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore b National University of Singapore (Suzhou) Research Institute, 377 Lin Quan Street, Suzhou Industrial Park, Suzhou, Jiangsu, 215123, PR China ARTICLEINFO Keywords: Electrolyzed water Organic acid Modeling Epifuorescence microscopy Atomic force microscopy Nanostructure Listeria Escherichia coli Protein Organic produce Sanitization Mechanism Bacterium Microbial safety SDS-PAGE Physicochemical property ABSTRACT The sanitising efect of low concentration acidic electrolysed water (AEW, free available chlorine (FAC): 4 mg/L) combined with levulinic acid (LA, 3% v/v) on fresh organic lettuce during 7-day storage was evaluated. The combined sanitising method showed additional bactericidal efcacy against naturally existing microbiota, while LA alone and combined with AEW could reduce survival population of Escherichia coli ATCC 25922 and Listeria innocua Seeliger ATCC 33090 inoculated on lettuce surface efectively, with 3.5–4.0 log CFU/g reduction for both during storage. Moreover, the modifed Gompertz model provided a good ftness to the sanitising results during storage, with highest R 2 in AEW group for E. coli (0.99) and in combination group for L. innocua (1.00), respectively. In addition, the physicochemical properties of organic lettuce treated by each sanitising treatment were not changed signifcantly during storage. Epifuorescence microscopy and atomic force microscopy (AFM) revealed that the cell permeability and morphology of E. coli and L. innocua were changed after sanitising treatments, with damaged cell membrane and disordered cellular structure in diferent degrees. Besides, the size of cells became smaller after combined sanitising treatment, with 2.12 and 1.24 μm 2 for E. coli and L. innocua, respectively, indicating some cytoplasm leakage. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) showed the number and intensities of the protein bands of E. coli were reduced, while those of L. innocua remained similar after sanitising treatments. The results suggest that low concentration AEW combined with LA is a potential efective approach to sanitise organic produce. 1. Introduction The purchases of organic food have been dramatically increased over the past decades. For instance, organic sales in the U.S. have in- creased rapidly from $3.6 × 10 9 in1997to$49.4×10 9 in 2017 (OTA, 2018; Sow, Tirtawinata, Yang, Shao, & Wang, 2017). However, as a primary part of organic food, organic produce is vulnerable to be contaminated by pathogens due to using organic fertilisers, bringing microbiological safety concerns to consumers (Ilic et al., 2012). Among them, organic lettuce (Lactuca sativa) is most likely to become a major source of spreading pathogens, considering it is often consumed as ready-to-eat salad and responsible for many outbreaks of foodborne illnesses (Zhang & Yang, 2017). Therefore, appropriate sanitisation before consumption is required to enhance microbiological safety of organic lettuce. When only limited numbers of synthetic sanitisers have been allowed for organic food processing due to strict regulations (Chen et al., 2019), acidic electrolysed water (AEW), which is generated by electrolysing the dilute sodium chloride solution, can act as an efcient sanitiser against various kinds of pathogenic bacteria, like Escherichia coli O157:H7, Salmonella spp. and Listeria monocytogenes. The anti- microbial mechanisms of AEW could be mainly attributed to three factors, free available chlorine (FAC), oxidation reduction potential (ORP) and pH (Hati et al., 2012). There exist interaction efects among them, for example, the FAC and ORP values were found to decrease signifcantly when the pH increased (Rahman, Ding, & Oh, 2010). Be- sides, high ORP played an important role in the bactericidal efects of AEW, as it could damage cell membrane and increase membrane pe- netrability to make intracellular components released, leading to ne- crosis of bacteria (Liao, Chen, & Xiao, 2007). In addition to strong antimicrobial activity, environmentally-friendly nature, safe char- acteristic, and economical running expense also account for the https://doi.org/10.1016/j.foodcont.2019.02.039 Received 28 December 2018; Received in revised form 26 February 2019; Accepted 28 February 2019 ∗ Corresponding author. Food Science and Technology Programme, c/o Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore. E-mail address: chmynghs@nus.edu.sg (H. Yang). Food Control 101 (2019) 241–250 Available online 05 March 2019 0956-7135/ © 2019 Elsevier Ltd. All rights reserved. T