Antimicrobial effects of thymol-loaded phytoglycogen/zein nanocomplexes against foodborne pathogens on fresh produce

Authors: XUE, JINGYI - University Of Connecticut; Luo, Yaguang - Sunny; LI, BEIFANG - University Of Connecticut; WANG, XINHAO - University Of Connecticut; XIAO, ZHENLEI - Collaborator; LUO, YANGCHAO - University Of Connecticut

Submitted to: Food Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/14/2022
Publication Date: 4/26/2022
Citation: Xue, J., Luo, Y., Li, B., Wang, X., Xiao, Z., Luo, Y. 2022. Antimicrobial effects of thymol-loaded phytoglycogen/zein nanocomplexes against foodborne pathogens on fresh produce. Food Control. 209:1188-1196.

Interpretive Summary: Thymol, a plant essential oil, has shown strong antimicrobial efficacy. However, its application for food safety is limited due to the solubility issues. In this study, thymol was encapsulated via a novel phytoglycogen/zein nanocomplex and the efficacy of this combination on the inactivation of major foodborne human pathogens on fresh produce was assessed. Results showed significant improvement in antimicrobial activities of the nanocomplexes encapsulated thymol than the native form of thymol control (without encapsulation) and the antimicrobial efficacy remained stable at 4°C for over 60 days. In addition to pathogen reduction on both lettuce and cantaloupe rind, nanocomplexes encapsulated thymol also showed significant inhibition on spoilage and decay of strawberries. These findings will benefit growers, food processors, and the public for improved food safety and quality of fruits and vegetables.

Technical Abstract: Plant essential oils with excellent antimicrobial activity have been widely used as alternatives to chemical preservatives. In this study, thymol-loaded phytoglycogen/zein nanocomplexes with a particle size around 100 nm were developed for improving microbial safety of fresh produce. The antimicrobial activities, including the determination of MIC, MBC, growth kinetic curves, and inhibition zone of the nanocomplexes against foodborne pathogens and/or their suitable surrogates (Listeria monocytogenes, Salmonella Enteritidis, and Escherichia coli) were evaluated. The morphology of bacterial cells before and after treatment with nanocomplexes was examined by atomic force microscopy (AFM) and potential applications of these nanocomplexes as sanitizing agents in wash water were evaluated on different types of fresh produce (lettuce, cantaloupe, and strawberries). The antimicrobial activities of the nanocomplexes were significantly stronger than that of the free thymol control (without encapsulation) and the antimicrobial efficacy remained unchanged after storage at 4°C for 60 days. AFM results revealed that small micellar blebs were formed at the surface of bacteria after treatment with nanocomplexes and the gradual disappearance of the cell boundary indicated the occurrence of cytolysis. For lettuce artificially inoculated with Escherichia coli, the cell populations were reduced by 2.09 and 1.19 log CFU in the group treated with nanocomplexes (0.5 mg/mL thymol equivalent) after 2 h and 48 h storage, respectively, compared with water wash treatment. The nanocomplexes also demonstrated efficacy in biofilm removal, achieving 3.15 log CFU reduction on Listeria associated biofilm on the surface of cantaloupe. Furthermore, after washing with nanocomplex solution, fresh strawberries exhibited significantly stronger resistance against microbial spoilage during storage, compared with a conventional water wash treatment. Findings from this study demonstrated that the thymol-loaded nanocomplexes hold potential as an antimicrobial agent for improving microbial safety and quality of fresh produce.