Effective control of Listeria on rockmelons through alternative post-harvest treatment methods (VM20007)

What was it all about?

From 2021 to 2023, this project examined strategies to optimise postharvest sanitation of melons and reduce the risk of listeria contamination.

The research confirmed that the current industry practice of washing plus brushing in run-to-waste sanitiser systems is an excellent way to ensure melons are free from bacteria. The type of sanitiser used, and the concentration, are less important. However, dwell time in commercial systems remains unclear, so it is recommended that growers measure dwell time and consider ways to manage melon speed through the sanitation step.

The research program began with industry consultation and packhouse visits to determine current commercial practice. While procedures differed between packhouses, all were using wash plus brush systems, with sanitiser sprayed over the melons from nozzles while the fruit was rolled forward on brushes.

To ensure that trial results mirrored industry practice, a laboratory scale washing plus brushing unit was developed. A single spray nozzle was mounted above spinning medium-firm bristle rollers the same as those used commercially. A pump was connected to a reservoir used for sanitisers, with the system adapted to run-to-waste instead of recirculating.

Trials then examined the effects of sanitiser type, concentration and dwell time, as well as whether efficacy was reduced when bacteria were in soil stuck to the melon skin instead of inoculated directly onto the rind. A final study compared the effects of brushing to simply immersing melons in sanitiser, trialed a short hot water treatment and tested removal of bacteria when the melons were contaminated one hour or 24 hours before washing.

The main mechanism by which bacteria were removed from the skin was the washing plus brushing process. While peroxyacetic acid (PAA) was consistently better than chlorine or chlorine dioxide, tap water was also effective at reducing bacterial populations. There was little or no impact of sanitiser concentration, further verifying the importance of the brushing treatment. Dwell time also had a significant effect, with longer brushing times reliably achieving log 3 reductions or greater, regardless of the sanitiser used. Results were not affected by presence of soil in these trials. This may be partly due to the run-to-waste system, which removed spent sanitiser rather than recycling it, but also to the effectiveness of the brushes at removing dirt from the skin.

The importance of sanitiser application method was confirmed in the last series of experiments, which demonstrated that washing plus brushing was more effective than simply immersing melons in water or sanitiser. Bacteria were more easily removed from the rind when they were contaminated immediately before washing compared to 24 hours prior. Inoculation 24 hours before washing therefore replicates pre-harvest contamination, while inoculation shortly before washing is more analogous to cross contamination postharvest.