Survival and persistence of Salmonella enterica in dry bulb onion production practices; a risk assessment approach

The first chapter of this research thesis focuses on onion production practices and the potential risks associated with contamination of dry bulb (DB) onions along the cropping cycle with human enteric pathogens. Dry bulb onions are a widely consumed vegetable globally that has been previously thought to be safe from human pathogen contamination. This literature review summarizes the history of outbreak-related information, the biology of etiologies of interest, the body of literature associated with microbial risks and onions, and pre-and post-harvest production practices with respect to the risk of contamination with human pathogens. The information discussed in this review is useful for portraying the complex interactions of the microorganisms of interest and for industry professionals, producers, and consumers with respect to management and applicable risk mitigation efforts in the future for DB onions. The second chapter of this research thesis focuses on determining the microbial risk factors associated with pre-and post-harvest commercial DB onion production practices. In the past 30 years DB onions have not been involved in foodborne illness outbreaks in the United States. However, two major multi-state foodborne outbreaks linked to Salmonella spp. (Sal) in 2020 and 2021 have altered the perception of producers and consumers about the microbial safety of this crop. Despite significant efforts to identify the source and route of contamination, little knowledge exists regarding the risk factors associated with enteric pathogen contamination along the DB onion cropping cycle. Thus, the goal of this research was to develop risk assessment profiles of DB onion production practices capable of identifying and reducing Sal contamination in pre-and post-harvest activities. DB onion cultivars grown in the state of Colorado were used to determine the ability of these onions to potentially inhibit Sal. This was achieved by testing the minimum inhibitory concentration (MIC) with a cocktail of attenuated and pathogenic Sal at 3,600 mg/L of onion slurry. All evaluations indicated that there was no significant inhibition of Sal irrespective of the type of strain or DB onion cultivar. Pre- and post-harvest risk quantification was determined based on field inoculations of a 2-strain attenuated Salmonella (attSal) cocktail. Survival and persistence of attSal was assessed at multiple production stages including at the 3 leaf stage, lifting, topping, curing, harvest, transport, and packing house environments. From these evaluations, results indicate that attSal is capable of surviving in both DB onions and soil for over a period of 64 days across the entire cropping cycle including harvest. Cultivar, agronomic practice, and UV index had no significant impact in our ability to recover attSal and in the survival of these strains along the cropping cycle in soil or DB onion. At harvest, the population of attSal on DB onion was 3.4 MPN/g at the three-leaf stage development (3LS), log 2.07 cfu/g at topping and log 1.87 cfu/g at lifting irrespective of DB onion cultivar. During interstate transport, the population of attSal further decreased to undetectable levels (< 3.0 MPN/g of DB onion). This scenario was considered a low-risk event for packinghouse purposes. Commercially grown DB onions were also included in all packinghouse evaluations. These onions were free from naturally occurring Sal and were inoculated with chalk containing attSal to mimic soil dust contamination. This chalk had an initial attSal population of log 5.5 cfu/g DB onion and for packinghouse purposes, it was considered a high-risk contamination event. A total of 14 locations within the packing line were selected to test the potential transfer of attSal from inoculated DB onions to control treatments and food contact surfaces. Additionally, DB onions from both high and low-risk contamination events were collected during sorting and packing. In both high and low -risk packing line contamination events, attSal was not recovered from any food contact surface or DB onions (Totl N= 897) over the course of 4 days of processing. A dry sanitation event was implemented in the packing line to assess whether such approaches could reduce contamination from attSal or any other residues left by the crop or by previous activities at the packing line. Our dry sanitation cleaning protocol involved the cleaning of the crop contact surface with a dry brush-single use paper towel, followed by sanitation with a food grade alcohol wipe, followed by a spray of an ethanol alcohol solution (food grade) at 75%, followed by a final removal of alcohol residues with a dry single-use paper towel. This approach proved to be effective in reducing packing line residues Adenosine triphosphate (ATP) measurements and the population of two indicator organisms Enterobacteriaceae (EB) and Escherichia coli (EC). However, the effectiveness varied with the type of surface. Plastic and camel hair bristles were not cleanable. There was no correlation between the population of EB and EC and the presence of attSal from high and low-risk contamination events. Dry sanitation events clearly indicated that it is a viable and useful practice that could be implemented on DB onion packing lines. The absence of cleaning and sanitation will be conducive to significant accumulation of DB onion debris and for the potential proliferation of indicator and pathogenic organisms. These findings are important to industry professionals, producers, and consumers regarding developing risk profiles and application of risk mitigation strategies to improve the microbial safety of DB onions.
2023 Summer.
Includes bibliographical references.
food safety, Salmonella, fresh produce, food borne illness