Reducing populations of Salmonella Typhimurium and Listeria innocua on tomatoes, apples, cantaloupe and romaine lettuce with cold plasma-activated hydrogen peroxide aerosol

Authors: Fan, Xuetong; SONG, YUANYUAN - Jiangnan University

Submitted to: United States Japan Joint Protein Research Panels
Publication Type: Proceedings
Publication Acceptance Date: 10/16/2019
Publication Date: 11/16/2019
Citation: Fan, X., Song, Y. 2019. Reducing populations of Salmonella Typhimurium and Listeria innocua on tomatoes, apples, cantaloupe and romaine lettuce with cold plasma-activated hydrogen peroxide aerosol. United States Japan Joint Protein Research Panels. pg. 25-26.

Interpretive Summary: Introduction Fresh fruits and vegetables such as leafy greens, tomatoes and melons have been implicated in outbreaks of foodborne illnesses in recent years in the U.S. Among the human pathogens of concern are Salmonella spp. Escherichia coli O157:H7 and Listeria monocytogenes. Controlling and destroying these pathogenic microorganisms in foods remains a formidable challenge. Washing with chemical sanitizers, such as chlorine, is the most commonly postharvest method for minimizing cross-contamination and risk of pathogens on fresh produce. However, washing with sanitizers has limited effectiveness in alleviating the problem of pathogen contamination in fresh produce partially due to low reactivity and potency of sanitizers, and pathogens residing in protected sites such as crevices, stomata, or cracks that aqueous sanitizers cannot reach. Therefore, more effective treatment methods are needed to meet consumer demand for fresh produce free from microbial contaminants. Our earlier study demonstrated that combination of cold plasma and aerosolized H2O2 reduced population of bacteria by up to 5 logs depending on characteristics of produce surface without causing significant damage to produce quality (Jiang et al., 2017). The present study was conducted to optimize treatment conditions of the cold plasma-activated aerosolized H2O2 process for tomatoes, Granny Smith apples, Romaine lettuce and cantaloupes, and to evaluate whether cold plasma activation affected the efficacy of aerosolized H2O2 against S. Typhimurium and L. innocua on the produce items. Materials and Methods Stem scars and smooth surfaces of grape tomatoes, surfaces of Granny Smith apples and Romaine lettuce (both midrib and upper leaves) and cantaloupe rinds were inoculated with two-strain cocktails of S. Typhimurium and 3-strain cocktails of L. innocua. The inoculated samples were treated with 7.8% aerosolized H2O2 with and without cold plasma activation (Fig. 1). The aerosolized H2O2 was activated by cold plasma generated between two pin electrodes. Parameters such as treatment time, dwell time, and repeated treatment cycles were evaluated. In addition, cold plasma-activated water aerosol was applied as a control. Results and Discussion In preliminary experiments, we evaluated the effects of treatment time and dwell time, and found that for bacteria on smooth surfaces of apples and tomatoes, a very short treatment time (a few seconds) was enough to inactivate bacteria to an undetectable level (detection limit, 0.70 log CFU/piece). However, for bacteria on rough surface, the short treatment times had very limited effectiveness on bacteria populations. Therefore, we increased the treatment time and the number of cycles to improve the effectiveness of the treatments. For S. Typhimurium on stem scars of tomato, ionized hydrogen peroxide was repeatedly applied in bursts for a total of 60 s treatment time to optimize efficacy. Three cycles of 20s spray time plus 20 min dwell time proved to be the most effective application to reduce S. Typhimurium populations on the stem scar, achieving a 2.73 log CFU/piece reduction. A 30s treatment reduced the bacterial populations by > 2.8 log CFU/piece on Romaine lettuce and by 2.48 log CFU/piece on the rind of the cantaloupe. After establishing the optimum conditions for inactivating Salmonella, and demonstrating that cold plasma-activated H2O2 reduced populations of Salmonella by 2.5 to >5 log CFU/piece (depending on type and nature of produce), we studied whether cold plasma played any role in the H2O2 inactivation of both Salmonella and Listeria. Results showed that, for all fresh produce items and surfaces, cold plasma significantly (P<0.05) improved the efficacy of aerosolized H2O2 against Salmonella and L. innocua (Fig. 2). Without cold plasma activation, H2O2 aerosols only reduced populations of Salmonella by 1.54

Technical Abstract: A study was conducted to develop and evaluate the cold plasma-activated aerosolized hydrogen peroxide process in inactivating Salmonella and Listeria on tomatoes, Granny Smith apples, Romaine lettuce and cantaloupes. Results showed that cold plasma and hydrogen peroxide aerosol had synergistic effects in reducing the population of Salmonella and Listeria inoculated onto different types and surfaces of fresh produce items, i.e. cold plasma enhanced the antimicrobial activity of hydrogen peroxide aerosol. Listeria was more sensitive to the treatment than Salmonella. Possible involvement of hydroxyl radicals, as a result of advanced oxidation process, will be discussed, and other relevant research will be presented.