Waxing of whole produce and its involvement in and impact on microbial food safety

Waxing has been widely used by the produce industry. Unfortunately, there is very limited information about its impact on microbial food safety. To address this knowledge gap, this proposal aims to deliver the following objectives by using citrus as the model fruit. First, the team will artificially inoculate different storage and finishing waxes and determine the survivability of human pathogens in these waxes under simulated storage conditions. Secondly, the team will evaluate the impact of the application of storage waxes on the behavior of pathogens on fruit surfaces. The evaluation will be conducted under both degreening and long-term storage conditions. Thirdly, the team will characterize the bactericidal efficacy of the application of finishing waxes and the following heated drying steps. The observed bactericidal efficacies will then be evaluated in pilot scale studies at the pilot packinghouse operated by the University of California’s Lindcove Research and Extension Center. Results of this study will be summarized in CPS reports, presented at CPS meetings and published in peer-reviewed journals. The outcomes of this study will assist the produce industry with risk assessment and management and provide the information needed in developing control strategies to reduce potential risks associated with application of waxes and to take advantage of potential risk reduction benefits of appropriate selection and application of finishing waxes.

After harvest many fresh fruits and vegetables are washed and waxed in order to prevent premature rotting and to extend shelf life. The point at which wax is applied differs from one produce type to another. However, for the majority of produce, such as apples and pears, wax is applied a single time just before packing and shipping. In contrast, citrus fruits may be waxed twice after harvest; one time after initial washing and before storage (storage wax) and a second time just before packing and shipping (finishing wax). For lemons, storage waxes are applied and fruit is stored without drying. In contrast, finishing waxes are typically dried with application of hot air. Over the past decade, the influence of commercially-available waxes on fruit and vegetable quality has been investigated and reviewed, however, only a few studies have investigated the impact of waxes on microbial food safety. Recently, this proposal team evaluated the survival of Salmonella and Listeria monocytogenes (LM) in a range of citrus storage and finishing waxes. The composition, pH, and microbial loads of these waxes differed significantly as did the behavior of pathogens inoculated into the waxes. No significant population reductions were observed in some storage waxes while significant population reductions were observed within 30 min in some finishing waxes. These observations highlight the importance of conducting a systematic evaluation of waxes used by the produce industry. To address this need, we are proposing to use citrus fruit (oranges and lemons) as model commodities. Microbial and chemical properties of a wide range of citrus storage and finishing waxes available from industry collaborators will be evaluated. Salmonella and LM will be separately inoculated into the waxes to mimic potential contamination of the wax at a packinghouse and the survival of pathogens will be monitored at ambient temperature and 4 °C. The behavior of pathogens will also be evaluated on fruit surfaces during lemon storage after the application of storage wax and after application and dry heating of finishing waxes. Wax applications that demonstrate significant bactericidal efficacy under laboratory conditions will then be evaluated via pilot scale studies at the pilot packinghouse operated by the University of California Lindcove Research and Extension Center. The outcomes of this study will bridge the knowledge gaps associated with the microbial safety risks of waxing by providing information about pathogen behavior in a range of storage and finished waxes and how the application of storage and finishing wax impacts the survival of pathogens on fruit surfaces.

1. Evaluate the microbial and chemical properties of currently available citrus waxes, and determine the potential for human pathogens to survive in these waxes under simulated packinghouse storage conditions. 

2. Investigate the behavior of Listeria monocytogenes (LM) and Salmonella inoculated onto citrus fruit surfaces when storage waxes are subsequently applied and fruit is stored. 

3. Characterize and evaluate the pathogen control efficacy of different finishing waxes, the heated drying steps, and subsequent storage by conducting in-lab and pilot-scale packinghouse trials.

This project has generated significant new findings, relative to factors affecting pathogen reduction in citrus waxes and during the waxing process. Key findings and recommendations: 

• Common foodborne pathogens (LM and Salmonella) survive in storage and finishing waxes. 

• The survival of common foodborne pathogens in full-strength storage and finishing waxes was influenced by wax type and storage temperature. 

• LM survived better than Salmonella in storage waxes, with greater survival observed at 4 °C (39 °F) than at 22 °C (72 °F) and better in diluted than in undiluted storage waxes. 

• The application of storage waxes led to LM reduction during storage. The differences in LM reduction between waxed and unwaxed lemons were influenced by wax type and dilution. When compared to unwaxed lemons, the application of select diluted storage wax (1:9 diluted S1) facilitated reduction in LM populations, whereas other waxes did not. 

• The application of finishing waxes and a heated drying step led to higher reduction of LM than Salmonella. 

• There was no significant difference in the fate of Salmonella on waxed lemons and oranges. 

• Greater reductions of Salmonella were observed at 4 °C (39 °F) than at 22 °C (72 °F) when inoculated fruits were waxed and stored for 30 days. 

• Greater reductions of Salmonella were observed on waxed lemon and orange surfaces when compared to unwaxed fruit surfaces, regardless of fruit type and storage temperature. 

• Although the application of finishing waxes coupled with a heated drying step reduced the number of pathogens on fruit surfaces, if the initial contamination level is high, cross contamination between contaminated and uncontaminated fruits can happen during the waxing step.