Inactivation of Salmonella, Listeria monocytogenes, Aspergillus and Penicillium on lemons using advanced oxidation process optimized through response surface methodology

Abstract

An advanced Oxidation process (AOP) based on using hydrogen peroxide, ozone and ultraviolet light (UV-C) was developed for inactivating mould spores (Penicillium expansum and Aspergillus niger) and bacterial pathogens (Salmonella Typhimurium and Listeria monocytogenes) inoculated onto lemons. Response Surface Methodology (RSM) with the central rotatable composite design was applied to study the combined effects of the percent of H2O2 (1–6% v/v), UV-C intensity (0.6–3.6 mW/cm2) and treatment time (20–60 s) as independent variables. The dependent variables were the colour change of lemons induced by AOP and log count reductions (LCR) of A. niger spores. Hydrogen peroxide concentration was the key parameter for both antimicrobial efficacy and discoloration of lemons. The optimal conditions for the AOP treatment were 1.88% v/v H2O2, 3.26 mW/cm2, UV-C intensity and 65.2 mJ/cm2 UV-C dose. From an initial loading between 7.23 and 7.62 log CFU/lemon of test microbes, the optimized treatment resulted in a LCR of 1.54 log CFU/lemon for Aspergillus spores and >5 log CFU/lemon for Salmonella and Listeria without changing the sensory characteristics of lemons. Over 30% of citrus fruit is lost to post-harvest spoilage and consequently, decontamination methods are required to reduce the carriage of moulds such as Aspergillus, in addition to inactivating potential human pathogens (e.g. Salmonella and L. monocytogenes). Currently, there is a reliance on post-harvest washing to remove contamination although has low efficacy and potentially can spread contamination between batches. Advanced Oxidation Process (AOP) is an alternative decontamination method that generates antimicrobial free radicals from the UV-C degradation of hydrogen peroxide and ozone. Although antimicrobial, the free radicals also discolor lemons and hence a balance needs to be made between decontamination efficacy and fruit quality. In this study, RSM was applied to identify the optimum treatment that was subsequently verified. Although the current study was limited to lemons, the same approach could be applied to other fresh produce when designing AOP-based treatments.