Show me the data: Academy of Nutrition and Dietetics says reusable bags ‘can easily contaminate your foods.’

Today I spent a couple of hours with some new family and consumer science extension agents talking about the history of food safety, how risk is calculated and how messages should be based on data – not conjecture. We talked about why the FDA model food code provides guidance on a specific water temperature for handwashing (100F/38C). It’s mainly because folks might be more likely to wash hands when water is warm (except no one can point to that in the literature), that fat is more soluble and soap lathers better. But some research has shown that temperature isn’t a factor in pathogen removal at all (which is the desired outcome of the action).

One of the agents asked me how something like that gets into and stays in a regulatory document and I responded by saying "It probably seemed like a good idea to someone, and it stuck."

I feel the same way about the discussion about the safety of reusable bags.

The Academy of Nutrition and Dietetics has put out a press release saying that reusable bags need to be washed regularly by users as pathogens grow well and cross-contamination is likely.

From the release:

Reusable grocery totes are a popular, eco-friendly choice to transport groceries, but only 15 percent of Americans regularly wash their bags, creating a breeding zone for harmful bacteria, according to a survey by the Home Food Safety program, a collaboration between the Academy of Nutrition and Dietetics (formerly the American Dietetic Association) and ConAgra Foods.

“Cross-contamination occurs when juices from raw meats or germs from unclean objects come in contact with cooked or ready-to-eat foods like breads or produce,” says registered dietitian and Academy spokesperson Ruth Frechman. “Unwashed grocery bags are lingering with bacteria which can easily contaminate your foods.”

Sort of.

Williams and colleagues (2011) have published the only peer-reviewed study on the microbial safety of reusable bags and tested growth of Salmonella in 2 batches. They spiked the bags with 10^6 cfu and let them sit in the trunk of a car for 2 hours. One of the batches, where the temperature reached 47C/117F, showed a one-log increase in the Salmonella. The other batch, where the temperature reached 53C/124F, there was a one-log reduction. That data doesn’t show just a breeding zone – it shows they can be a killing zone too (and I’m not sure how realistic a 10^6 contamination really is).

The part of the press releases that is the least rooted in science is that pathogen-containing bags "easily contaminate your foods." The same Williams study showed generic E. coli is floating around in bags, recoverable in 12 % (n=58) of those tested but can it be (or is it likely) to be transferred to any ready-to-eat foods, or somehow to food contact surfaces in the home?

Just because the bacteria might be there, doesn’t mean it can contaminate a ready-to-eat food. No one has presented data to support that. Maybe the Academy of Nutrition and Dietetics – if so, they should share it.

In a cross-contamination event there is a dilution effect when it comes to transfer. 1000 cfus of Campylobacter on the outside of the package of raw chicken might become 100 cfus when transferred to the bag, and then only 10 cfus when transferred to ready-to-eat apples.

Washing bags frequently (as the Academy of Nutrition and Dietetics suggests) is probably a good idea (like washing hands in warm water) and probably won’t increase risk, but I wonder how much it decreases the probability of cross-contamination when compared to doing nothing.

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About Ben Chapman

Dr. Ben Chapman is a professor and food safety extension specialist at North Carolina State University. As a teenager, a Saturday afternoon viewing of the classic cable movie, Outbreak, sparked his interest in pathogens and public health. With the goal of less foodborne illness, his group designs, implements, and evaluates food safety strategies, messages, and media from farm-to-fork. Through reality-based research, Chapman investigates behaviors and creates interventions aimed at amateur and professional food handlers, managers, and organizational decision-makers; the gate keepers of safe food. Ben co-hosts a biweekly podcast called Food Safety Talk and tries to further engage folks online through Instagram, Twitter, Facebook, YouTube and, maybe not surprisingly, Pinterest. Follow on Twitter @benjaminchapman.