Flour is an increasingly recognized source of Shiga toxin-producing E. coli (STEC) outbreaks. In late 2018 and early 2019, an outbreak of STEC infections linked to contaminated flour led to 21 illnesses across nine states.
“This outbreak illustrates the importance of always baking or cooking raw dough or batter before eating it,” says Michael Vasser, MPH, lead author of the report. “Raw flour is not ready-to-eat and can contain bacteria, such as STEC. You get can sick from eating or tasting unbaked products that are intended to be baked, such as dough or batter. Even tasting a small amount of raw dough or batter could make you sick.”
He explains that harmful bacteria can contaminate grain while it’s still in the field or at other steps during production, and processing steps such as grinding grain and bleaching flour do not kill the pathogens. “Bacteria are killed only when food made with flour is cooked,” Vasser says. “Because flour doesn’t look like other raw foods that people are familiar with, like raw chicken or raw eggs, people may not realize that it is raw and should be cooked or baked before eating.”
CDC investigated multistate outbreaks linked to the consumption of raw doughs or batters in both 2016 and 2018-2019. The investigations show that people may not be aware of the risks of eating raw flour in doughs or batters. “CDC does have messaging to explain the risks of eating raw dough,” Vasser says. “A recent study conducted by the Department of Food Science at Purdue University showed that, of those surveyed, many consumers were not aware of flour recalls or outbreaks and few thought they would be affected by them.”
The symptoms of STEC infections vary, but can include diarrhea, stomach cramps, and vomiting. “Some people can have more severe illness, including a potentially life-threatening complication known as hemolytic uremic syndrome,” Vasser says. “Very young children and the elderly are more likely to develop severe illness than others, but even healthy older children and young adults can become seriously ill.”
CDC and other federal agencies are working to make food safer by building state and local capacity to improve surveillance and investigation of foodborne illnesses and using data to evaluate and revise foodborne disease prevention strategies. “As new challenges in food safety arise, such as antibiotic resistance and emerging strains of bacteria, we must continue to focus efforts on ensuring a safe food supply for everyone,” Vasser added.