This project will focus on the integration of effective intervention technologies and treatments to enhance microbial safety of fresh fruits and vegetables with a holistic approach addressing major elements (safety, quality, and shelf-life), necessary for the implementation of technologies. The ultimate goal is to reduce the risk of foodborne illnesses associated with consumption of fresh produce, while maintaining acceptable food quality and shelf-life. Specific objectives of the research program are: Objective 1 - Develop and optimize single intervention technologies to reduce pathogen populations, maintain quality, and extend shelf-life of foods. Sub-objective 1.1. Develop and optimize aerosolizing technology and pulsed light to reduce pathogen populations, maintain sensorial and nutritional quality, and extend shelf-life of fresh produce. Sub-objective 1.2. Develop new antimicrobial packaging/coating-based technologies by incorporating natural, modified and novel materials to inactivate foodborne pathogens on fresh produce. Objective 2 - Determine the synergistic/additive effectiveness of combining non-thermal processing, antimicrobial packaging and effective chemical interventions utilizing information generated from the first objective.
An integrated approach to enhance microbial safety while maintaining product quality and extending shelf life of fresh produce will be adopted by combining aerosolizing antimicrobials, pulsed light antimicrobial packaging and other interventions. The types of fresh and fresh-cut produce evaluated in the project will be those frequently involved in outbreaks of foodborne illnesses, those that are hard to sanitize due to surface characteristics, and those that cannot be washed. During the first part of the proposed project plan, we will develop and optimize new aerosolization systems, pulsed light technology, novel antimicrobial coating with incorporation of nature and bio-based substances, and antimicrobial packaging materials with controlled-release mechanisms triggered with either acids or pulsed light. The optimized/developed interventions and antimicrobial packaging will then be combined with each other, and with other effective antimicrobial treatments to study the synergistic or additive effects on pathogen inactivation while maintaining quality and shelf-life of fresh produce. When selecting combinations, technologies with different pathogen-inactivation mechanisms or synergistic interactions will be desirable. We will utilize the advanced oxidation, photochemical, and photothermal, and triggered-release mechanisms and other hurdle technologies to increase the efficacy of combined antimicrobial treatments. By combining effective intervention technologies and treatments, synergistic effects with a targeted 5-log reduction of common pathogens may be achieved. Pathogens to be included in the proposed project plan are Salmonella spp., E. coli O157:H7, L. monocytogenes and other emerging pathogens (such as non-O157 STECs). We will use a representative cocktail of 3-5 strains from each genera of bacteria that are associated with outbreaks of relevant fresh produce. Scientifically well-established inoculation, recovery, and enumeration procedures will be used. Appropriate controls will be included in each experiment, and experiments will be replicated independently at least three times. Inoculation of fresh produce will be achieved either by surface ‘spot inoculation’ where specific locations on the produce surface will be inoculated or by a ‘dip inoculation’ technique where the whole produce item will be submerged in the experimental inocula. The inoculated fresh produce will be drained and air dried in a laminar flow hood before being subjected to various treatments. After treatment with various chemicals and physical interventions, the total number of viable and injured bacteria will be determined using amended media. The effects of the individual and combined treatments on the physiochemical and sensorial quality and shelf-life will be evaluated during storage. Shelf-life will be determined based on the deterioration in product quality and increasing populations of microorganisms that render the product unacceptable to consumers.
Experiments have been undertaken to evaluate the feasibility of aerosolized antimicrobials for the inactivation of Salmonella, E. coli and Listeria and quality maintenance of tomato, cantaloupe and spinach leaves, to optimize pulsed light technology for the reductions of Salmonella on tomato fruit, and to evaluate potential antimicrobials and polymers to be incorporated into packaging systems. Detailed progress to achieve the overall objectives is listed below. Spinach leaves, cantaloupe rinds, and stem scar and smooth surfaces of tomatoes were inoculated with Escherichia coli O157:H7, Salmonella Typhimurium and Listeria innocua. The inoculated samples were then treated with aerosolized hydrogen peroxide for 45 sec in a treatment chamber. Non-inoculated samples were used to study effects on quality and native microflora populations. Results showed that the effects of aerosolized hydrogen peroxide depended on types of inoculated bacteria and produce items. The treatments also significantly reduced populations of native bacteria, and yeast and mold. Furthermore, firmness, color or odor of tomato, spinach and cantaloupe was not significantly affected by the aerosolized hydrogen peroxide treatments. The aerosolized sanitizers could potentially be used for sanitizing fresh fruits and vegetables. Further studies are planned to optimize the technology. Whole and sliced apples have been found to be contaminated with Listeria monocytogenes. New formulations consisting of antioxidants and organic acids were validated for their effectiveness in preventing browning of cut apple surface and inactivation of L. monocytogenes. Solutions prepared from most promising formulations developed earlier by our group were used to dip apple slices. Color of cut surface and skin discoloration of fruit pieces were measured during simulated 21-days shelf-life. In addition, a cocktail of L. monocytogenes was added in the solutions of anti-browning/antimicrobial formulation to study inactivation of the pathogen. Results showed that the formulations maintained the freshness of cut apples while minimizing Listeria contamination. Several new antimicrobials and polymers, including olive leaf extract, doped titanium dioxide, acrylic polymer with citrus terpenes and ternenes hydrocarbons, corn fiber gum, and glycerol have been screened and evaluated for their antimicrobial activities against E. coli and Listeria and for film forming capacity. Results show that these compounds have a potential to be used as antimicrobial packaging materials and will be applied on food to improve safety and shelf life. As a physical preservation method, pulsed light has a positive consumer image and is efficient in inactivating microorganisms in a relatively short period of time compared to other technologies. Research is now on going to optimize the pulsed light system to achieve a minimum 99.9% reduction of foodborne pathogens such as Salmonella and E. coli on fresh produce. Preliminary results showed that pulse light was very promising inactivating greater than 99.9% Salmonella enterica in the stem scars of cherry tomatoes after about 30 sec treatment. No visual alteration of color and texture was observed. Research on reduction of background spoilage microflora and on retention of quality is being carrying out.