Preservation of stone fruits by spray application of edible coatings with antimicrobial properties

Listeria monocytogenes is an important foodborne pathogen commonly found in the environment. Recent Listeria foodborne outbreaks have been linked to fresh produce including stone fruits. Contamination of stone fruits is problematic since these products are usually consumed without heating. In addition, some surfaces associated with packing operations (brushes, peach rollers) are inherently difficult to sanitize. In the packing house, these fruits are covered (brushed) with a wax-­‐based coating, containing antifungal agents to prevent moisture loss and fungal infection. We propose to develop and compare alternative coatings based on edible components that have antilisterial properties in addition to their physical barrier and antifungal role. The coatings will be formulated to contain safe antimicrobial agents such as nisin, Listex P100, organic acids and or their combinations and could be applied as a spray reducing the risk of cross-­‐contamination in the packing house. Experiments will be performed in laboratory settings and validated in challenge studies with inoculated stone fruits. The coating will prevent Listeria contamination on fruits and bacterial persistence on packing equipment. Results from this study will provide improved pathogen control in addition to the basic good agricultural practices, thereby helping fruit industry to produce safer produce for human consumption.

The number of human outbreaks associated with foodborne pathogens contaminating fresh produce has increased in the past decade. Stone fruits such as peaches and nectarines are usually consumed raw but these fruits were considered ‘safe ‘since they had not been implicated in major human outbreaks. In 2014, a stone fruit packing company issued the first recall of certain stone fruits because of concern about contamination with Listeriamonocytogenes. Although limited with only 2 illnesses, this outbreak highlighted the potential of L. monocytogenes to cause outbreaks via foods that had been considered unlikely vehicles for this pathogen. Traditional preservation techniques such as heating, packaging or reduced water activity do not apply for these commodities. In the absence of any practices that prevent L. monocytogenes survival on stone fruit, the exposure of the fresh fruit to contaminated water or postharvest contact surfaces will increase the likelihood of contamination and foodborne outbreaks. In this project, we propose to investigate L. monocytogenes biofilm formation and transfer rate from surfaces typical for the packing house (rubber peach holders, stainless steel surfaces and cleaning and waxing brushes) to stone fruits (Objective 1). In addition to identifying problematic areas, biofilms will be inactivated with commonly used sanitizers and optimal conditions will be determined regarding sanitizer concentration and contact time. We also propose to design a novel fruit coating with antilisterial properties and intended for a hygienic application such as a spray or water flume. The coating will be formulated to contain antilisterial agents that can (i) inactivate L. monocytogenes on contact with equipment and, (ii) maintain the safety of the fruits (Objective 2). Furthermore, the coatings will be tested in challenge studies, regarding fruit preservation, antilisterial properties and consumer acceptability (Objective 3). Stone fruits will be coated with the antilisterial coating and with a commercially available wax for comparison purposes. A set of samples will be inoculated with a L. monocytogenes cocktail. Samples will be stored in conditions to mimic packing house and retail environment and survival of the pathogen over time will be determined by plate counts. Coated samples (antilisterial and wax) not inoculated with L. monocytogenes will be evaluated side-­‐by-­‐side for their consumer acceptability and shelf-­‐life. We anticipate that results from research will point out practical preventive intervention strategies for safer fruit supply. This proposal will address the identified research area (Part3) “California Fresh Fruit Association”.

1. Develop a flow-through system to determine cleaning efficacy on surfaces with flat and topographical features and determine surface role and cleaning procedure in the possible pathogen contamination of stone fruits. 

2. Evaluate novel fruit coating formulations with antimicrobial properties that can be developed as brush-independent (spray) applications and can replace traditional wax treatments to maintain fruit integrity and shelf life. 

3. Determine efficacy and properties of selected coating formulations in challenge studies in controlled conditions.

We would like to point out the resilience of Listeria monocytogenes in the environment, especially when forming biofilms. Although our experiments were designed to simulate the “worst-case” scenario by growing the mature biofilms for 48 hours, we recommend that possible niches of L. monocytogenes should be identified in packinghouses and eliminated. Desiccation or exposure to low relative humidity will not kill the pathogen, and it can detach and contaminate other surfaces and food products. Also noticeable is the difference in the attachment on different brush filaments, since smoothness of the filament seems to be important in holding the pathogen. When using a mineral oil coating such as Prima Fresh 220, the brush should be cleaned thoroughly to eliminate the coating, prior to sanitizer treatment. Alternatively, use a vegetable oil coating, which will provide a less hydrophobic environment. We also determined that in the case of brushes and their filaments, the cationic surfactant benzalkonium chloride (BK), is more effective in disinfection than chlorine. However, in the case of brushes, the sanitizer treatment should be allowed to continue for a longer time (i.e., 20 minutes). Regarding coatings, the incorporation of P100 phage (Listex™ P100 phage mixture) was effective for reducing L. monocytogenes populations in solution but its effectiveness was inconclusive when tested on coated peaches. The process of mixing and pH adjustment would need more development to be industry ready, as the process was lengthy. Peaches with traditionally used coatings (mineral or vegetable oil) had less weight loss than those coated with gelatin or pectin (with and without phage) but were less firm. Based on sensory observations, all peaches were generally liked the same, indicating that the coatings did not affect the overall liking of the peaches. Overall, coatings traditionally used in industry were not negatively affected by the addition of P100 except for pH adjustment. In addition, gelatin and pectin-based coatings with and without phage had similar properties to industry standard coatings (vegetable and mineral oil based) and provided peaches with increased firmness. The project data shows there were no negative effects, thus further research is warranted.