Compared with other types of fresh produce, broccoli has not often been the focus of food safety-related research, despite the fact that it is commonly served raw in salads, salad mixes, and crudités platters. So, when Cornell M. Eng. student Yuezhi Wu consulted with Produce Safety Alliance Director Betsy Bihn about possible food science-related topics for his Masters in Engineering thesis, Bihn (who had recently signed on as a collaborator with the Eastern Broccoli Project) naturally thought of broccoli. The resulting study is the first to examine transfer rates of E. coli between broccoli and two surfaces commonly encountered in small packing operations. Though preliminary, the results will inform future risk assessments of the broccoli postharvest environment and help identify measures to prevent contamination events.

Wu’s co-authors on the study are his advisor, Cornell Biological and Environmental Engineering Professor Ashim Datta; Bihn; and Extension Specialist Lindsay Springer, a recent Food Science PhD who at the time was working with Bihn. After considering the postharvest surfaces that broccoli typically encounters in a small farm operation, the authors decided that gloved hands and conveyor belts pose the most risk. In a series of experiments with a nonpathogenic strain of E. coli, the team then measured bacterial transfer rates from contaminated broccoli to previously clean glove and conveyor belt material, and from contaminated conveyor belts and gloves to broccoli crowns and stems. To mimic contact between gloved hands and broccoli, small disks cut from nitrile gloves were pressed against broccoli stems or crowns for 5 seconds; to mimic contact with the conveyor belt, broccoli was dropped onto squares of belt material and kept there for 20 seconds.

The researchers found that the transfer rate from contaminated conveyor belt material to broccoli was much higher than the rate of transfer from contaminated broccoli to the conveyor belt. Contaminated glove disks transferred more bacteria to broccoli crowns and stems than contaminated crowns and stems transferred to glove material. Broccoli crowns in particular picked up more bacteria from contaminated gloves than did broccoli stems, which was attributed to the more porous structure of the crown. The authors noted that these results are consistent with studies using other fresh produce, which have found that transfer rates from non-organic surface (like the gloves and conveyor belt) to organic surfaces (broccoli and other vegetables) is typically higher than the reverse. The results emphasize the importance of cleaning and sterilizing conveyors and of sterilizing or replacing gloves frequently. Ultimately, this type of data can be used to construct models that will help growers understand contamination risks and determine the ideal cleaning schedule for their operation.

Reference

Wu, Y., L. Springer, E. Bihn, and A. Datta.  Quantifying Escherichia coli Cross-Contamination Rates among Broccoli, Conveyer Belt and Glove. https://hdl.handle.net/1813/57072