Salmonella risk is enhanced by onion condition or defect

Three recalls of bulb onions due to Salmonella have occurred since 2020. In each of these cases, the associated product was either a short-day or intermediate-day onion variety. Short-day and intermediate-day onion varieties are sweeter and spoil more quickly than do long-day storage onion varieties. This suggests that onion attributes (eg., moisture, sugar, acid content) that increase susceptibility to microbial spoilage may also increase risk from Salmonella. This risk may be further enhanced in head-and-tailed onions which are distributed peeled and pre-cut, removing the top and bottom of the onion. Head-and-tailed onions are subsequently processed to yield ready-to-eat sliced or diced products. The goal of this proposal is to characterize the impact of these variables (variety, physiology, mechanical damage, and handling practices) on Salmonella. The results from this work can be used to shape Salmonella risk management practices in bulb onions by better pinpointing where the greatest risk lies. In addition, we will assess the role of artificial, forced-air curing on Salmonella reduction in short-day onions. Artificial, forced-air curing is used in some short- and intermediate-day onion producing regions and may accelerate Salmonella die-off.

Recent onion recalls due to Salmonella, including those onions destined for processing, have increased the focus on production and handling practices to mitigate this risk. There is a gap in knowledge regarding the identification of onion conditions or defects which enhance the risk from Salmonella. For example, the biochemical composition (eg., moisture, sugar, acid content) and cultural practices (eg., limited curing and storage) of short-day onion varieties may enhance Salmonella persistence and survival. The use of head-and-tailed onions in processing to yield ready-to-eat (RTE) sliced or diced products may further increase risk, as head-and-tailed onions are received peeled and pre-cut, removing the top and bottom of the onion. These factors have an impact on onion spoilage from secondary plant pathogens and, therefore, may promote Salmonella internalization and growth as well. Taken together, this suggests that attributes of onions that promote post-harvest quality loss due to bacterial soft rot also enhance Salmonella risk. These attributes broadly include variety, physiology, mechanical damage, and handling practices. Similarly, strategies employed to minimize bacterial soft rot may also help reduce Salmonella contamination and growth in onions. In this study we will conduct Salmonella survival and growth studies on short-day, intermediate-day, long-day onion varieties to determine if bacterial persistence and replication are significantly impacted by onion variety. We will additionally assess onion biochemistry (eg., compositional differences in moisture, sugar, and acid content) that potentially explain these differences. We will investigate how differences in curing and handling intervals typical of short- and intermediate-day onions impacts Salmonella survival on onion surfaces. Finally, we will determine the impact of heading-and-tailing on Salmonella survival and growth under varying temperature scenarios important at harvest, processing, and retail for short-, intermediate-, and long-day varieties. The results of these analyses will help identify key risk factors in onion production that may enhance risk, so that resources for mitigation can be better directed. In addition, we hope to explore opportunities that can be used to reduce risk.

Objective 1: Determine the effect of onion condition and defect on Salmonella. 

Objective 2: Characterize the specific onion attributes that impact safety and quality. 

Objective 3: Evaluate different short-day varieties and handling practices for their impact on Salmonella.

This project is ongoing. A final report will be provided when the project is finished.