Supplementing food antimicrobials in commercial edible coatings to enhance the safety and extend the shelf-life of stone fruits

Stone fruits are low-risk commodities but have been linked to multi-state outbreaks and recalls as recent as 2020. Stone fruits are usually waxed to preserve freshness and reduce decay, but the fungicides used in commercial coatings (“waxes”) are not active against foodborne pathogens. Conversely, the feasibility of many novel edible antimicrobial coatings has not been proved commercially. To facilitate transfer of project findings to the industry, we propose to supplement commercial, generally-recognized-as-safe food preservatives effective against common foodborne pathogens and fungi in commercial stone fruit coatings. Antimicrobial coating formulations will be established in the first objective for the antimicrobial activity and the properties needed to preserve stone fruits. In the second objective, the chosen antimicrobial coatings will be applied on yellow peaches and white nectarines inoculated with Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella enterica. Subsequently, the survival of pathogens and native fungi, and the quality and decay of stone fruits will be determined during storage at ideal and simulated retail conditions. The project directly contributes to the enhanced microbial safety and extended shelf-life of stone fruits. Project findings are also valuable to the safety and sustainability of many other produce commodities susceptible to outbreaks of foodborne illnesses.

Stone fruits are commonly considered as low-risk commodities but have been linked to multi-state outbreaks and recalls including a 2020 Salmonella outbreak. Stone fruits are usually waxed to preserve freshness and reduce decay, but the fungicides used in commercial coatings (“waxes”) are not active against foodborne pathogens and are strictly regulated by the US Environmental Protection Agency. Many novel coatings have been studied for fruits, but their feasibility has not been proved commercially. To facilitate transfer of project findings to the industry, the goal of the present project is to implement commercial, generally-recognized-as-safe food antimicrobials in commercial stone fruit coatings to enhance the microbial safety and extend the shelf-life of stone fruits. We hypothesize that implementing appropriate types and amounts of commercial food preservatives in commercial stone fruit coatings will maintain the characteristics of coatings to preserve fruit quality and effectively inhibit foodborne pathogens and spoilage microorganisms on stone fruits. The hypothesis will be tested in two objectives. In the first objective, the proximate composition of five commercial stone fruit coating dispersions and their coating properties before and after adjusting to pH 3.0-7.0 will be first characterized. Selection of antimicrobials will then be studied for the combination of polar (lauric arginate; benzoic, propionic, sorbic acids and their salts) and non-polar (parabens) antimicrobials, which may have synergistic antimicrobial activities and may maintain water vapor permeability of coatings. This will include the determination of the minimum inhibitory and minimum bactericidal concentrations of individual antimicrobials, and the fractional inhibitory concentration index to study the combination of lauric agrinate and the most effective organic acid with synergistic activity and the combination of lauric arginate/organic acid mixture and methyl- and propylparagens (2:1). Films will then be prepared with 15 antimicrobial coating dispersions to evaluate physical, mechanical, and antimicrobial properties. Five formulations leading to films with physical and mechanical properties similar to those without antimicrobials, as well as good antimicrobial activities, will be identified for coating the fruit. In the second objective, the effectiveness of antimicrobial coatings inhibiting Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella enterica cocktails inoculated at 5 or 3 log CFU/fruit on yellow peaches and white nectarines, as well as total populations of native yeasts and molds/fungi and total aerobic bacteria, will be evaluated during storage at ideal and simulated retail conditions. Antimicrobial coating treatments will be compared to controls of the uninoculated and uncoated fruits, fruits inoculated with bacteria but uncoated, and those with the corresponding commercial coating and fungicide. The quality, gloss, and decay of fruits during storage will also be evaluated. The project success will be evaluated for the absence of detectable pathogens during storage and the properties of antimicrobial coatings preserving the quality and reducing the decay of stone fruits. Findings from the project will directly enhance the microbial safety and extend the shelf-life of stone fruits and can be adopted to enhance the safety and sustainability of many other fresh produce commodities.

1. Characterize the physical, mechanical, and antimicrobial properties of films casted from commercial stone-fruit coatings supplemented with food antimicrobials. 

2. Evaluate the reduction of inoculated pathogens, native yeasts, and molds/fungi, as well as the quality of stone fruits after coating and during storage.

Commercial food preservatives of lauric arginate (LAE), sorbic acid (SA), sorbate, benzoate, benzoic acid, propionic acid, and parabens were generally more active at lower pH between 4 and 7 against Salmonella Enteritidis H4267 and Listeria monocytogenes Scott A. LAE was the most active and had synergistic activity when combined with sorbic acid or parabens at pH 6.0. The commercial stone fruit coatings had alkaline pH, negative charges, and high fat content. When adjusting to pH 6.0 and dissolving with 1% antimicrobial, only the EXC 7037 coating with original pH of 8.7 remained visually stable and was chosen. The combination of LAE and organic acids had stronger activity against Salmonella Enteritidis H4267 and L. monocytogenes Scott A in the EXC 7037 coating after adjusting pH to 6.0, and the 200 ppm LAE and 5,000 ppm SA combination was the most active, achieving 3.42 log CFU/mL reduction against Salmonella after 90 min at room temperature but only about 1 log CFU/mL or less reduction against L. monocytogenes. Parabens were active in the EXC 7037 coating without pH adjustment, reducing both pathogens by more than 8 log CFU/mL in 90 min at room temperature. With these findings, four formulations based on the EXC 7037 coating were chosen to coat fresh peaches: (A) 0.05% LAE and 0.5% SA at pH 6.0, (B) 0.1% LAE and 1.0% SA at pH 6.0, (C) 1.0% parabens at pH 8.7, and (D) 2.0% parabens at pH 8.7.

Fresh peaches were brushed, washed with deionized water, dried, and spot-inoculated with Salmonella or L. monocytogenes cocktail composed of equal populations of five strains. Separate peaches without inoculation were used to evaluate native fungi and quality. Each peach was sprayed with 1 mL of the chosen coating formulation, with controls being that sprayed with the coating without antimicrobial and that without coating. The survival of microorganisms and the quality of peaches were evaluated every 5 days during 20-day storage at 0°C and 85% relative humidity. Six peaches from each of the two different harvest seasons were used, giving 12 replicates for each data point. Peaches sprayed with formulations (A) and (B) were stored in open aluminum trays to simulate transportation in large containers, while those with formulations (C) and (D) were placed in perforated plastic pouches before placing in aluminum trays to simulate packing in boxes. When compared to the coating control without antimicrobials, formulations (A) and (B) increased Salmonella reduction by up to 1.64 log CFU/fruit (on day 20), while the impact on L. monocytogenes was insignificant; formulations (C) and (D) increased the reduction of Salmonella and L. monocytogenes by up to 1.81 and 0.44 log CFU/fruit (on day 20), respectively. Another strategy was developed by first spraying 1% or 2% parabens on peaches followed by applying the EXC 7037 coating without pH adjustment, resulting in similar pathogen populations during storage – an “instant kill” effect; the reduction by 1% and 2% parabens was increased by up to 2.19 and 2.55 log CFU/fruit for Salmonella and 0.72 and 0.87 log CFU/fruit for L. monocytogenes (on day 20), respectively. Supplementing the coating with the antimicrobials did not impact the coating to inhibit native fungi and preserve peach quality evaluated for total soluble solids content, titratable acidity, pH, weight loss, and color (L, a*, b*).

At the studied conditions, supplementing antimicrobials in the coating was more effective in inhibiting Salmonella than L. monocytogenes without altering the current coating practice used in the packing house. To further reduce pathogens on stone fruits, the produce industry may want to explore coatings feasible at acidic conditions to utilize the activity of the antimicrobials, especially those at pH 4 and below to inhibit the growth of foodborne pathogens using acidity. Alternatively, the packing house may install an additional sprayer to apply antimicrobials such as parabens to provide an instant kill of pathogens and native fungi, followed by spraying the stone fruit coating to minimize the impact of coating composition and achieve sufficient inhibition of pathogens.