Reduction of Escherichia coli O157:H7, Listeria monocytogenes, and Naturally Present Microbe Counts on Lettuce using an Acid Mixture of Acetic and Lactic Acid.

Lactic acid (LA) and acetic acid (AA) are independently used to disinfect fresh leaf vegetables. LA has a higher efficacy but costs more than AA. Herein, we compared the disinfection efficacy of LA, AA, and their mixture on lettuce to determine whether the cheaper acid mixture shows similar or more efficacy than LA. Quality analysis indicated that the acid mixture and individual acids did not cause additional loss of instrument color and polyphenolic content compared with that of the control; however, visible defects were observed at AA concentrations exceeding 0.8%. Analysis of Escherichia coli O157:H7, Listeria monocytogenes, and naturally present microbes (aerobic mesophilic and psychrotrophic bacteria, coliforms, molds, and yeasts) showed that the acid mixture led to the highest reduction in microbial count during storage. 16S rRNA sequencing was further employed to understand the effects of the acid mixture and individual acids on lettuce microbial ecology. During storage, the acid mixture and individual acids significantly decreased the abundance of Massilia spp. and Alkanindiges spp. but there was a marked increase in Escherichia-Shigella abundance (LA: 0.003-58.82%; AA: 0.01-55.34%; acid mixture: undetected to 50.71%; control: 0.007-33.09%), indicating that acid disinfection altered the microbial ecology to stimulate Escherichia-Shigella growth. These results enhance our understanding of the relationship between lettuce disinfection and ecological changes.

Disinfection of lettuce using organic acids: an ecological analysis using 16S rRNA sequencing.

Organic acid disinfection efficacy has been previously estimated by analyzing microbial reduction on fresh produce. However, the effects of organic acids on the fresh produce microbiome are not considered for the evaluation of disinfection efficacy. Here, we studied the effects of seven generally recognized as safe organic acids (lactic, tartaric, acetic, propionic, malic, succinic, and citric acid), on the microbial counts and community on the surface of lettuce. The community was dominated by the following genera: Xanthomonas (24.73%), Sphingomonas (15.85%), Massilia (10.23%), Alkanindiges (9.00%), Acinetobacter (7.57%), and Pseudomonas (6.02%). Organic acid washing did not affect microbial diversity. Lactic acid was the most effective agent causing aerobic plate count reduction of 0.97 log CFU g-1; additionally, it increased the Escherichia-Shigella abundance from 0.77% to 3.29%. The relative abundance of Xanthomonas, a plant pathogen, was significantly increased by malic and propionic acid-propionic acid caused an increase from 24.73% to 47.53%. Microbial interaction analyses revealed the co-exclusion of Xanthomonas with the other core taxa, suggesting that the microbial distribution on the lettuce surface after disinfection carries a higher risk of quality loss. Therefore, the difference in disinfection efficacy of sanitizers was reflected in both microbial counts and bacterial community changes. We also propose a potential solution to control fresh produce safety and the rational use of sanitizers by collecting microbial diversity, composition, and count data from planting, transport, minimal processing, shelf and consumer storage, and gut digestion, and then using big data technology to develop a model to provide recommendations for sanitizer selection.