Flour-Associated Outbreaks
Outbreaks have occurred because consumers often taste mixes and batters before cooking. In 2009, ready-to-bake cookie dough was identified as the culprit in the first reported E. coli outbreak associated with wheat flour in the United States. In the past, only foods such as ground beef, raw dairy, leafy green vegetables, or contact with farm animals were associated with the outbreaks. [1] This outbreak caused 77 illnesses, with 35 requiring hospitalization and ten reported hemolytic uremic syndrome cases [2].
Other E. coli serogroups were implicated in a 2016 multistate outbreak of raw flour consumption involving 56 people across 24 states. Infections were linked by whole-genome sequencing to a large domestic flour producer. Patients had all tasted unbaked homemade dough or batter. Two additional outbreaks in Canada were caused by different brands of wheat flour, resulting in the recall of pie and tart shells.
In May 2019, testing revealed E. coli contamination in an intact bag of all-purpose flour, resulting in twenty-one cases in nine states. The typical patient was young and female, many of whom reported eating or licking raw homemade dough or batter in the week previous to the infection. Cake mixes were identified in 2018 and suspected in 2021 as the source of new outbreaks.
What is Water Activity (Aw)?
Moisture describes how much water is in a product. Aw, water activity, on the other hand, measures the amount of unbound water, water that is free for use by microorganisms for survival. Water can be bound by many food components, such as fats, proteins, and sugars. Typical foods with low Aw are butter, peanut butter, and jams. Due to their high fat or sugar content, they can be stored at room temperature with a lower risk of microbial growth. Because of their low water activities, wheat and other cereal flours were considered safe from the growth and survival of pathogens such as Salmonella and enterohemorrhagic Escherichia coli (EHEC) before these outbreaks.
Pathogen Survival in Wheat
Because EHEC and Salmonella can cause illness at very low oral doses, growth to higher numbers in flour is not needed for infection. When consumed, survival of just a few bacterial cells can cause havoc in the gastrointestinal tract. This can be especially dire for the Young, Old, Pregnant, and Immunocompromised (YOPI).
Food scientists have discovered that these pathogens can survive for extended periods in wheat flour. Some EHEC have survived for up to two years in storage. This is dangerous since recalls may be overlooked when the product has been on the pantry shelf for many months. The good news is that pathogens in flour are easy to kill. It is well-known that heat can inactivate pathogens, and cooking or baking does the job.
A recent study measuring the reduction of Salmonella and pathogenic E. coli in pasta found that common cooking methods were effective in significantly reducing the risk from these microbes. In a challenge study, raw batter used to make muffins was inoculated with high concentrations of pathogenic E. coli. The pathogens were completely eradicated after 21 min of baking at 375⁰F (190.6⁰C) and cooling at room temperature.
Consumer Education & Awareness
Flours are not considered ready-to-eat (RTE) foods. After the 2005 cookie dough outbreak, the CDC suggested that manufacturers reformulate or further process prepackaged cookie dough to be as safe as RTE food. They also recommended that the risks of eating unbaked cookie dough be more effectively communicated to consumers.
An online consumer survey was conducted with 1,045 participants to assess their flour handling practices and knowledge about food safety related to this dry ingredient. Despite educational materials provided by the CDC and the flour industry, most consumers were unaware that raw dough or batter might cause foodborne illnesses. Most were also ignorant of flour or flour product recalls, with more than half of the survey participants admitting that they consumed raw dough or batter.
In 2019, the FDA collected data from adults over 18 via the Food Safety and Nutrition Survey. While responses differed significantly based on demographics, they concluded that, in general, US consumers are unaware of the risks of consuming raw flour. Sixty-six percent of their survey participants reported having eaten an uncooked flour product within the last 12 months.
Reducing the Risk
No raw product is ever completely risk-free. But scientists are trying to lower the potential for infection from raw flours. They are testing multiple measures that combine chemical, thermal, and storage conditions to prevent pathogen survival without harming flour's quality and baking properties.
The CDC and USDA are getting the message out: raw doughs and batters cannot be safely eaten. Flour safety includes not only its consumption but also how we handle flour - washing hands and surfaces to prevent cross-contamination as we do for meat. Consumers should also segregate raw flour from RTE foods and cook flour-containing products thoroughly according to package instructions.
Microwave foods are especially vulnerable to pathogen survival due to uneven heating, lack of accurate temperature control, and differences in amperage (power) between units. Follow microwave cooking instructions and consider using a thermometer to double-check the final temperature.
Social media could be a big boost. Tell your friends and family to respect raw flour as they would raw meat. “Say No to Raw Dough”
[1] The exact E. coli species or serogroup was O157:H7
[2] In hemolytic-uremic syndrome, the infection causes red blood cells to be destroyed (hemolysis), and the resulting debris causes an injury to the kidney, resulting in the kidney’s failure to clear the blood of toxins.
Portions of this article are based on material originally published in the March 2022 edition of Food Technology Magazine.
