Antimicrobial efficacy of commercial produce sanitizers against artificially inoculated foodborne pathogens and natural fungal contaminants on the surface of whole melons
Foodborne disease outbreaks linked to enteric pathogens on cantaloupe and watermelon over the past few years has raised concern in the melon industry. As a result, new formulations of antimicrobial treatments have been developed, creating more choices for melon producers to utilize in attempts to improve microbial safety of their product. Previous research has indicated that the use of these strong post-harvest sanitizers may have detrimental quality effects on produce. The overall objective of this research was to evaluate the broad spectrum antimicrobial activity of commercially available produce sanitizers marketed towards the melon industry.
Sanitizers selected represented various chemical categories of sodium hypochlorite (chlorine), hydrogen peroxide, liquid chlorine dioxide, organic acids, quaternary ammonium, and hydrogen peroxide/acid combinations. Bacterial reduction of E. coli O157:H7, non-O157 STEC, Listeria monocytogenes, and Salmonella spp. following in- cell suspension treatment (5 minutes, 4°C) with the ten different commercial sanitizers was evaluated at varying concentrations. Subsequently, selected sanitizers and concentrations were evaluated for their effectiveness in reducing the pathogen populations on the surface of artificially inoculated whole cantaloupe and watermelon rinds at representative packing house conditions of 24°C for 2 minute application times. Sanitizers displaying the greatest antimicrobial activity against pathogens and on both melon types were selected for evaluation of antifungal (yeast, mold) activities as well as quality effects over the shelf life of whole cantaloupe and watermelon following similar treatment conditions. Additional work using UV spectrometry and viable pathogen reduction was completed to investigate the antibacterial mechanism of action these produce sanitizers may utilize.
Results of the cell suspension study revealed ability of all tested sanitizers to reduce pathogens by 0.6-9.6 log CFU/ml. One organic acid (citric/lactic acid combination) treatment resulted in minimal reduction (3 log or less) of all pathogens and was eliminated for further analysis. The application of sanitizers on melon rinds resulted in significant differences in pathogen reduction observed between sanitizers, but not between melon types. The most effective sanitizers against pathogens on melon surfaces were quaternary ammonium and hydrogen peroxide/acid combinations with 1.0-2.2 log CFU/gm and 1.3-3.5 log CFU/gm reductions, respectively, for all pathogens tested. The other tested sanitizers were less consistent with microbial kill, with reductions ranging from 0-3 log CFU/gm depending on pathogen and sanitizer. Results of the quality analysis following treatment with quaternary ammonium and hydrogen peroxide/acid combinations indicated no reduction of natural yeast and mold contaminants, and no changes in firmness or color of melon rinds (P>0.05). Minimum bacterial concentration (MBC) of sanitizers against E. coli O157:H7 and L. innocua were determined, and varied by active ingredient. Mechanism of action analysis indicated that cell viability of E. coli O157:H7, non-O157 STEC, L. innocua, and Salmonella spp. decreased with time following treatment with all chemical sanitizers, with the exception of sodium hypochlorite. An increase in cellular leakage of A260 material was found to result in cells treated with quaternary ammonium, but not with any other tested sanitizer.
Based on the results of the present work it can be concluded that 1) the use of broad spectrum produce sanitizers at recommended concentrations and treatment applications on the rind surfaces of whole melons has the potential to reduce bacterial foodborne pathogens of concern in the melon industry, 2) the use of commercial produce sanitizers may not reduce the presence of natural fungal contaminants on melon rinds but will not negatively affect the sensory quality of melons, and 3) death of bacterial cells exposed to quaternary ammonium products involves damage to cytoplasmic membrane but the mechanism of action utilized by additional tested produce sanitizers may involve more complex cellular interactions in addition to membrane damage. Overall, results of this research will be utilized to provide guidance to the melon industry on best produce sanitizers for a broad spectrum pathogen intervention strategy.