Novel Methods to Sanitize Fruits and Vegetables Using Chlorine Dioxide Gas

There are increasing concerns about minimally processed and refrigerated fruits and vegetables because of outbreaks caused by pathogens, including Escherichia coli O157:H7, Listeria monocytogenes, Salmonella, and Shigella. Current antimicrobial technologies are unable to effectively achieve a significant reduction (5 log or higher) of microbial hazards in fruit and vegetable products. Results from the proposed research will help improve the safety of fresh and MPR fruits and vegetables. Furthermore, it will enhance our understanding of the potential benefits and feasibility of using ClO2 gas as a new disinfectant for food industry.
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Apples, strawberries, cantaloupe, lettuce, or mushrooms will be selected as five model systems for fruits and vegetables because of their different surface properties, high safety concerns, and limited shelf life. In this study, whole apple used for raw material for processing of apple cider and cut or shredded lettuce will be used. Strawberries, cantaloupe, or mushrooms will be studied in whole or cut samples. Targeted pathogenic and spoilage microorganisms with different levels (102, 103, 104, 105, or 106 cells/cm2 or g fresh weight) will be inoculated on the surface of whole or cut apples, strawberries, cantaloupe, lettuce, or mushrooms. The inoculated samples will be treated with different concentrations (0.1 - 3 mg/l) of ClO2 gas in a model system in a treatment chamber that we will design and develop. The populations of target microorganisms before and after ClO2 gas will be examined using colony enumeration methods. Efficiency of ClO2 gas on inactivation of these target microorganisms will be determined based on microbial log reduction data after treatments. Two kinds of technologies applying ClO2 gas treatments will be studied using the five selected commodities: Method 1 is ClO2 gas treatment followed with water spray prior to processing and packaging of uncut selected commodities; Method 2 is active MAP with ClO2, O2, CO2 and N2 gases for uncut and cut selected commodities. Parameters in each method, including concentration of ClO2 gas, exposure time and initial population of the target microorganisms on the surface of each commodity, will be derived from previous results in this study. Log reduction of the target microorganisms will be determined after each treatment and will be used to evaluate efficiency of each treatment. The effects of ClO2 gas treatments on safety (ClO2 residue), physiological quality (texture and respiration rate), sensory quality (color), nutritional value (vitamin C and A), and shelf-life of selected commodities will be investigated in this part of the study. Two optimized technologies from the studies using the different treatment methods will be applied for whole or cut apples, strawberries, cantaloupe, mushrooms, and lettuce without inoculation of target microorganisms. Variables will include concentration of ClO2 gas and storage time. Control samples made without ClO2 gas will also be included. The optimized treatment methods for two technologies selected from the studies on previous research objectives will be validated and tested on a pilot scale. The quality and safety of these products will be evaluated during refrigerated storage and their shelf-life will be determined. The cost and operation safety of applying these technologies will also be evaluated.
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A pilot scale ClO2 gas treatment system has been developed. This system features automated ClO2 gas generation, concentration, exposure time, and relative humidity control and continuous monitoring, and 30,000 square feet capacity. More than 5 log reductions of selected pathogens (E. coli O157:H7, L. monocytogenes, and Salmonella spp.) were achieved on green peppers, strawberries, mushroom, and oranges after continuous ClO2 gas treatments. Treated products showed insignificant changes in surface color, minimal and acceptable residues (<1mg/kg) of oxidative species, and extended shelf-life. The results strongly demonstrate that ClO2 gas sanitizing treatment is a promising alternative to improve the safety and quality of most fruits and vegetables. The kinetics of aqueous and gaseous ClO2 treatments in inactivating E. coli O157:H7, L. monocytogenes, Bacillus spores, and recombinant bioluminescent E. coli O157:H7 has been studied. The results suggested that moisture content in the environment surrounding cells plays an important role in microbial inactivation by ClO2 gas. Bioluminescence from the recombinant bacteria could be correlated to microbial survival and may have a potential to on-line monitor the efficacy of ClO2 gas treatment. Mechanism studies indicated that visible damage was not observed on cell membrane instead of interior cell structure after the treatment.
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Chlorine dioxide gas provides a much greater antimicrobial effect compared to currently used sanitizers and antimicrobial processing systems. The use of chlorine dioxide gas, when conditions are optimized, should provide a far better alternative for treating fruit and vegetable products. In turn, it is expected that produce will be safer and have a longer shelf-life. This provides benefits to the public health sector and economically to the produce industry.