Wet is better for bacteria; produce cross-contamination in the kitchen

Rutgers food safety something Don Schaffner writes:

Every paper I’ve published has a history behind it. This one has its origins in Aberdeen Scotland in 2008.

Michelle Danyluk was a newly minted PhD out of Linda Harris’s lab at UC Davis and had just started as an assistant professor at the University of Florida. Like most good assistant professors Michelle was always on the lookout for grant opportunities, and she had recently seen a call for proposals from the U.S. Food and Drug Administration on schaffnerconsumer risk from fresh-cut produce. Michelle had offered to lead the grant writing efforts with Linda and I as co-principal investigators. The grant submission deadline fell during the Food Micro 2008 meeting in Aberdeen, Scotland and of course, as the deadline loomed closer, the proposal wasn’t finished.  Michelle and I were in Scotland, and Linda was back in Davis, California. Fueled by Scottish Thai food (actually not that bad), and beer, as well as a passible Internet connection and the occasional trans-Atlantic phone call, the three of us hammered out the details of who would do what in the proposal.  Somehow it all worked, and FDA decided to give us the money. And Michelle got tenure.

Three years later, the grant is mostly spent, and Michelle is sitting on a pile of data collected by super-technician Lorrie Friedrich. Michelle and Lorrie have amassed an astounding pile of cross-contamination data for four different surfaces (ceramic, stainless steel, glass, and plastic) and four types of fresh-cut produce (carrots, watermelon, celery and lettuce) under both dry and wet conditions, and considering transfer in either direction. Because Michelle was foolish enough to listen to me when I suggest we repeat each experiment 20 times to “capture inherent variability,” the entire dataset was composed of more than 600 observations. Enter graduate student Dane Jensen.

Dane is from the Jersey shore. The real Jersey shore, near Asbury Park. I pointed Dane at the pile of data and told him to come back when it was graphed and analyzed. He did a good job, and it was part of his Masters thesis, and our recent publication in the Journal of Food Protection.

What did we discover when we set off down this road in Aberdeen?  Mostly that surface moisture and direction of transfer have the greatest influence on microbial transfer rates: bacteria would rather be on a wet produce surface than a dry counter-top or cutting board.  Want to learn more? Order up some Thai food, crack open a beer and read the article.

Quantifying transfer rates of Salmonella and Escherichia coli O157:H7 between fresh-cut produce and common kitchen surfaces

Journal of Food Protection®, Number 9, January 2013, pp. 1488-1657, pp. 1530-1538(9), DOI: http://dx.doi.org/10.4315/0362-028X.JFP-13-098

Jensen, Dane A.; Friedrich, Loretta M.; Harris, Linda J.; Danyluk, Michelle D.; Schaffner, Donald W.



Cross-contamination between foods and surfaces in food processing environments and home kitchens may play a significant role in foodborne disease transmission. This study quantifies the cross-contamination rates between a variety of fresh-cut produce and common kitchen surfaces (ceramic, stainless steel, glass, and plastic) using scenarios that differ by cross-contamination direction, surface type, produce type, and drying food.safety.talktime/moisture level. A five-strain cocktail of rifampin-resistant Salmonella was used in transfer scenarios involving celery, carrot, and watermelon, and a five-strain cocktail of rifampin-resistant Escherichia coli O157:H7 was used in transfer scenarios involving lettuce. Produce or surface coupons were placed in buffer-filled filter bags and homogenized or massaged, respectively, to recover cells. The resulting solutions were serially diluted in 0.1% peptone and surface plated onto tryptic soy agar with 80 μg/ml rifampin and bismuth sulfite agar with 80 μg/ml rifampin for Salmonella or sorbitol MacConkey agar with 80 μg/ml rifampin for E. coli O157:H7. When the food contact surface was freshly inoculated, a high amount (>90%) of the inoculum was almost always transferred to the cut produce item. If the inoculated food contact surfaces were allowed to dry for 1 h, median transfer was generally >90% for carrots and watermelon but ranged from <1 to ∼70% for celery and lettuce. Freshly inoculated celery or lettuce transferred more bacteria (<2 to ∼25% of the inoculum) compared with freshly inoculated carrots or watermelon (approximately <1 to 8%). After 1 h of drying, the rate of transfer from inoculated celery, carrot, and lettuce was <0.01 to ∼5% and <1 to ∼5% for watermelon. Surface moisture and direction of transfer have the greatest influence on microbial transfer rates.