Microbiological Safety in Fresh Produce: Optimising Sanitiser Processes and Understanding the Pathogen Response

Abstract

Foodborne disease outbreaks resulting from fresh produce contamination pose significant challenges to horticultural industries and public health. While postharvest treatments increase the safety and shelf life of fresh produce, innovation is needed to improve their effectiveness, cost- efficiency, and environmental sustainability. The objective of this thesis was to advance the safety and sanitisation practices of fresh produce to improve food security. To achieve this, the aims were to determine the safety of fresh produce in Australia, to asses interactions between postharvest treatment chemicals, to evaluate the efficacy of novel sanitiser systems, and to analyse stress and virulence responses of pathogenic bacteria to sanitisers. Examination of 16 years of data on foodborne disease outbreaks confirmed that fresh produce is an important source of outbreaks in Australia, with 102 incidents and 3,657 illnesses reported. Experiments combining industrial sanitisers and fungicides revealed their antimicrobial capacity to change in unpredictable ways, confirming industry advice that this should be avoided. Novel sanitiser systems based on plasma- activated water (PAW) generated by bubble spark discharge (BSD) and dielectric barrier discharge- diffuser (DBDD) reactors demonstrated acute antimicrobial power, particularly for the DBDD-PAW system. DBDD-PAW was further tested in a fresh produce model and produced effects comparable to the established sanitiser, sodium hypochlorite. Modelling and scavenger assays found the antimicrobial activity of DBDD-PAW to be due to the reactive species superoxide, in combination with the effects of electrical forces. Sublethal treatment with sodium hypochlorite or PAW did not increase the expression of virulence and stressrelated genes in pathogenic E. coli. Collectively, these findings indicate that the novel sanitisation methods presented in this thesis have the potential to help improve fresh produce safety in Australia.