Investigation of potential preharvest and postharvest treatments targeting Salmonella spp. risk reduction on peach in Australia

The goal of this exploratory rapid response research is to advance potential preventive controls of immediate and near-term use within the seasonal peach production period in Australia. Using an industry-research interactive process, preharvest and postharvest treatments are being prioritized to select the best practical options for risk reduction of Salmonella spp. on peach surfaces. Knowledge obtained in Australia will be disseminated to the peach industry, generating immediately actionable information for the United States and other stone fruit-producing countries in their coming season. Parameters chosen for these studies aim to address a limited set of preharvest options and, primarily, mimic current postharvest processing practices to provide a better understanding of how these factors impact Salmonella spp. survival in a large-scale handling and packing environment. The overall goal of the project is to provide data that the peach industry can use to support preventive controls that reduce Salmonella risk on peach surfaces.

This Rapid Response (RR) project was funded in the wake of the Salmonella enterica Enteritidis outbreak linked to peaches in the United States in 2020. The aim of this project was to assess several preharvest foliar treatments and postharvest sanitizer treatments for efficacy in reducing Salmonella risk in peach fruit. The purpose of the research was to take advantage of counterseason peach production in Australia to generate preliminary data that would help prioritize interventions for further research ahead of the next peach season in the United States. Several preharvest foliar treatments were investigated using bacterial inhibition assays and field trials to assess bactericidal activity and phytotoxicity. Peroxy Treat (hydrogen peroxide/peroxyacetic acid) demonstrated bactericidal activity but copper chelate and zinc sulfate did not. The treatments caused varying levels of foliar and fruit phytotoxicity when applied at high dose rates. Several postharvest sanitizer treatments were investigated in laboratory and packhouse challenge experiments. Nylate (bromo-chloro-dimethyl hydantoin) achieved more than 3.5 log reduction in Salmonella in the laboratory challenge, demonstrating that there are opportunities to effectively reduce risk through sanitizer optimization. The preharvest foliar treatments tested were not readily actionable interventions for peach orchards. However, we recommend that postharvest interventions be prioritized as an approach likely to yield actionable strategies in a relatively short time frame.

1. Identify optimum dose rates of Cu-EDTA, zinc sulfate heptahydrate (23% Zn), and Peroxy Treat™ applied pre-harvest to achieve potential Salmonella risk reduction and identify possible thresholds for phytotoxicity. 

2. Test and optimize the efficacy of different postharvest sanitizer treatments against Salmonella spp. on peach surfaces. 

3. * Determine whether interactions between postharvest sanitizers and fungicides reduce the efficacy of the sanitizers. 

4. To evaluate the impact of the selected treatments identified in Objectives 1, 2 and 3 on peach shelf-life and quality in comparison to standard industrial practices. *Objectives were reviewed at the start of the project in consultation with industry. Research of sanitizer-fungicide interactions was not considered a priority and was removed as an objective.

Preharvest foliar treatments (CuEDTA, ZnSO4, and Peroxy Treat) were not found to be readily applicable interventions for effective Salmonella risk reduction in this project. Peroxy Treat demonstrated efficacy as a bactericide. It had a MIC of 188-375 ppm and MLC of 375-750 ppm against S. Enteritidis in bacterial inhibition assays. Peroxy Treat (1-2%) decreased the total aerobic bacteria (0.3-0.4 log CFU/g) and total coliforms (0.6-1.8 log CFU/g) on fruit in Field Trial 2 but not in Field Trial 1. CuEDTA and ZnSO4 did not demonstrate bactericidal activity in the bacterial inhibition assays and treatments did not significantly affect pre-existing microflora in the orchard. Varying levels of foliar and fruit phytotoxicity were found to be a risk when products were sprayed at high application rates. Our recommendations regarding preharvest interventions include: 

• Separation and segregation of fruit from the buffer zone around a known or presumptive source of contamination. 

• Research to develop of evidence-based guidelines for appropriate demarcation and management of the buffer tree zone. Postharvest sanitizer treatments were found to offer opportunities for effective Salmonella risk reduction in peach fruit. We demonstrated that it is possible to achieve more than 3.5 log reduction in S. Enteritidis through selection of the postharvest sanitizer. Nylate was the most effective sanitizer tested in the laboratory challenge study. For all contact times (1, 30, and 60 s), it reduced the Salmonella count to below the limit of detection (100 CFU/g). Given an average initial inoculum concentration of 6.3 log CFU/g, this represented approximately 4 log reduction in S. Enteritidis. Lactic acid (2%) was the second-most effective sanitizer tested in the laboratory challenge study. It delivered a 3-4 log reduction in S. Enteritidis when applied for 1-30 s, and a 4 log reduction when applied for 60 s.