Novel coating systems with sustained release of food antimicrobials to improve safety of cantaloupe.

The netted surfaces of cantaloupes provide harborage sites for pathogens that are protected from washing and sanitizing and can continue to proliferate during storage. Antimicrobial coatings can potentially improve microbial safety and quality of cantaloupes but have not been studied extensively. Application of antimicrobials is somewhat complicated because they can be absorbed by components of cantaloupes which could reduce their activity. Thus sustained release of antimicrobial compounds from coatings is needed to maintain sufficient quantities to inhibit microorganisms on cantaloupe rinds. In this project, we propose to study novel antimicrobial coating systems to solve several challenges of improving safety and quality of cantaloupes. We propose to use plant-based food antimicrobials, essential oils, to develop systems that can overcome barriers to diffuse into cavities on cantaloupe surfaces and continuously release antimicrobials therein. The technology also can maintain visual appearance of cantaloupes and reduce moisture loss during storage. Specific research objectives are proposed to develop such technology and characterize physical and antimicrobial properties of coatings, as well as impacts on quality of cantaloupes. Coatings have already been adopted in the produce industry, and the novel technology can be applied to other melons and fresh produce products for enhanced microbial safety and quality.

Cantaloupes have been linked to several outbreaks of foodborne illness due to contamination by pathogens such as Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella. It is becoming clear that netted rinds of cantaloupes are topographically irregular and pathogens tend to adsorb and be trapped in cavities. Conversely, the microscopic cavities induce capillary forces that keep sanitizers such as hypochlorite outside the concave areas, which has been correlated to poor sanitization performance. Therefore, strategies are needed to overcome capillary forces, e.g., by adopting an appropriate surfactant to lower interfacial tension and thus capillary forces so that sanitizers and other antimicrobials can gain access to the protected areas where pathogens are harbored. Further, because cantaloupes could be stored for two weeks or longer, intervention systems such as antimicrobial coatings are needed to insure microbial safety during storage. In addition to inhibiting pathogenic and spoilage microorganisms, antimicrobial coatings that are transparent are desirable to maintain visual appearance and be effective in preventing quality losses like dehydration and oxidation. In this project, we propose to study novel antimicrobial coating systems that can overcome the problems associated with safety and quality of cantaloupes. Based on our recent research findings, we propose to prepare transparent nanoemulsions of essential oils (EO) from clove, cinnamon, and thyme that will be incorporated in film-forming hydrocolloids to coat cantaloupes. We hypothesize a phase-inversion temperature method can be effective in preparing transparent nanoemulsions of EO that can be further used to prepare transparent coatings. We further hypothesize that the proposed novel antimicrobial coatings are effective systems in improving safety and quality of cantaloupes. Because surfactants such as Tween 80 are used in preparing nanoemulsions, these nanoemulsions are expected to overcome capillary forces and diffuse to cavities on cantaloupe surfaces to inhibit potential pathogens therein. Further, vapor pressure and sustained release of volatile EO will be controlled by inclusion of soybean oil in the oil body of emulsions. The oil-containing coatings also are expected to improve barrier properties for loss of water vapor and thus quality of cantaloupes during storage. The hypotheses will be tested in the following research objectives: (1) identify conditions of preparing nanoemulsions of EO, (2) characterize physical properties of films and release properties of EO, (3) evaluate antimicrobial effectiveness of coatings against spoilage and pathogenic microorganisms on cantaloupes, and (4) assess the cost of antimicrobial coatings and impacts on the quality of cantaloupes. The objectives will be led by a team with overall expertise in food biophysics, food microbiology, food antimicrobials, and produce safety and quality. The project directly addresses the priority area of 2.2 in the RFA. Although only cantaloupes are studied this project due to time limitation, we expect the novel antimicrobial coatings can be applied to numerous fresh produce products for improved safety and quality. Because coatings have already been used for produce products such as citrus and apples, we anticipate the successful coating formulations can be timely translated to the produce industry.

1) Identify conditions of preparing nanoemulsions of EO 

2) Characterize physical properties of films and release properties of EO 

3) Evaluate antimicrobial effectiveness of coatings against spoilage and pathogenic microorganisms on cantaloupes. 

4) Assess the cost of antimicrobial coatings and impacts on the quality of cantaloupes

Microemulsion formulations were identified to dissolve both EOs and SBO at various mass ratios using Tween 80 as a surfactant and equal mass of PG and water as the polar phase. These microemulsions have been characterized for fundamental antimicrobial activities and physical properties including viscosity, droplet dimension, and stability. When chitosan was used as the film forming polysaccharide, the mixtures of chitosan and microemulsions were used to prepare films to predict properties of coatings when applied on cantaloupes. These films were transparent, had suitable physical, mechanical, and antimicrobial properties, and retained CBO better than conventional formulations during storage. Transparent films were also prepared using the CBO and SBO blend and alginate. These films were also found to be suitable for coatings and were less costly than those of chitosan microemulsions. To reduce the amount of EOs and costs of coatings, LAE was studied as an emulsifier with excellent antimicrobial activity, and synergistic antimicrobial activity was observed using the mixture of LAE, CO, and EDTA. Three mixtures were chosen to study impacts of coatings on safety and quality of whole cantaloupes: (1) 1% chitosan and microemulsions with 2% CBO, (2) 1% alginate and 2% CBO with 0 or 0.5% SBO, and (3) 1%w/w chitosan, 0.1%w/w LAE, 0.1% w/w EDTA, and 0, 0.5, or 1% w/w CO. These coatings, especially after addition of SBO, showed the effectiveness of inhibiting common foodborne pathogens L. monocytogenes, S. enterica and E. coli O157:H7 inoculated on cantaloupes, as well as native microflora. Antimicrobial coatings also slowed the color change and improved the firmness of cantaloupes during storage, and had no impacts on other quality parameters. Costs of coatings were estimated to be less than 1 cent per cantaloupe. Due to time limitation, the third coating mixture was not studied for alginate as the film-forming polysaccharide for possible further reduction of cost. Based on antimicrobial activities and effects on storage quality, two coating formulations studied in this project are recommended. The first mixture is recommended to contain 1% w/v alginate, 0.05% w/v CaCl2, 0.3% w/v glycerol, 0.5% w/v Tween 80, 2% w/v CBO, and 0.5% w/v SBO that can be prepared by simple mixing. The material cost of this coating mixture is $0.410 for 100 cantaloupes. The second mixture is formulated with 1% w/w chitosan, 0.5% w/w acetic acid, 0.1% w/w LAE, 0.1% w/w EDTA and 1% w/w CO, and the estimated material cost for 100 cantaloupes is $0.637. Chitosan may be replaced by less expensive alginate to additionally reduce the cost. The costs of these coating mixtures can be offset by the reduced number of recalls and the extended shelf-life of cantaloupes.