Recently,  multiple foodborne outbreaks caused by shiga toxin-producing E. coli (STEC) have been epidemiologically linked to the consumption of wheat flour. This has been a hot topic of discussion with many of our flour milling clients as well as manufacturers and retailers who use flour as an ingredient or sell flour to consumers. Many in the food industry are wondering if they should be changing their hazard analyses, environmental monitoring programs, ingredient testing or finished product testing.

To understand the problem, we first need to examine the group of organisms of concern. STEC is defined as any E. coli that produces shiga toxin. There are many, many serotypes of E. coli that fall into this group, including the most famous, E. coli O157:H7. However, besides the shiga toxin, at least a dozen other virulence factors play a role in determining whether or not a particular strain of E. coli will cause illness, what the infectious dose might be and how serious the illness will be. Many combinations of virulence factors will lead to only minor, rapidly-resolved GI symptoms, such as a case of diarrhea with the person quickly feeling better. Rarely, a more serious disease such as Hemolytic Uremic Syndrome (HUS) can occur with a high mortality rate. If the patient survives, it sometimes causes a lifetime of continuing health problems such as the need for kidney dialysis.

There are seven serotypes that cause the majority of illnesses, which are often referred to as “Top 7”: O26, O45, O103, O111, O121, O145 and O157:H7. Most rapid methods screen first for the shiga toxin genes and another important virulence factor, eae or intimin. If these targets are present, the rapid methods next drill down to test for the common disease-causing serotypes. This testing protocol is not ideal, as most current methods would miss some pathogenic strains, such as the cause of the largest STEC outbreak ever. In Germany in 2011, 3842 cases were reported from fenugreek sprout consumption and person-to-person spread, all caused by a non-top 7 serotype, O104:H4. Current rapid methods will also detect members of the “Top 7” that are not pathogenic. Most rapid methods were developed to detect STEC in raw beef or produce. These methods need to be validated and sometimes modified to assure that they will work for dry ingredients such as wheat flour.

Second, we need to examine the properties of the product of concern: wheat flour. Most notably, typical wheat flour does not undergo a microbiological kill step during production. Antimicrobials such as chlorine are often added to the tempered water, but this has a minimal impact on pathogens. The grain is also cleaned before milling, but the finished flour is mainly composed of what comes in from the wheat fields. Interestingly, the demand for “whole grain” means more bran and germ makes it into finished flours. This increases the amount of fiber, but likely also the amount of bacteria. Previous studies have shown that a low percentage of wheat flour samples (0.14 – 1.32 %) will test positive for Salmonella, as would be expected for a raw agricultural commodity. No major surveys have been published yet on incidence of STEC in flour. Heat-treated flour that undergoes a process validated to kill pathogens is now available, but flour functionality is extremely sensitive. This heat-treated flour may not work for every baking need.

Finally, let’s take a closer look at details about the recent outbreaks. In May, 2016, FDA linked an E. coli O121 strain to both an outbreak of illnesses and a specific manufacturer of flour. During this investigation, Whole Genome Sequencing (WGS) also found E. coli O26. There were a total of 63 illnesses in 24 states, and many of those who got sick had eaten or handled raw dough. Flour that was produced at one mill in Kansas City from Nov. 2015 to Feb. 2016 was recalled. The recall was then expanded when WGS of isolates from additional product were also linked to the outbreak strains. In total, over 45 million lbs of flour were recalled.

Most recently, an outbreak in Canada was made public on March 28, 2017 when the Public Health Agency of Canada and CFIA confirmed that there were illnesses linked to flour. The serotype was also O121, but WGS indicated that this strain was not the same as the U.S. flour strain from the previous year.  Recalled flour was manufactured Oct. 14 through Nov. 3, 2016 at a Saskatoon flour mill. There were 28 confirmed illnesses. This recall was also expanded on May 31, 2017 to include 3 brands of flour distributed in the U.S., although no illnesses were associated with the recall in the United States.

These outbreaks and recalls due to STEC in flour carry many implications for the food industry. First, it raises a debate about the definition of ready-to-eat foods. If a consumer is reasonably likely to eat a product without a pathogen kill step, then it probably needs to be considered ready-to-eat. More heat-treated flour is being used in these types of products, and many companies are conducting challenge studies to validate pathogen kill steps. Manufacturers who use flour as an ingredient are considering the need for better air control and hygienic zoning to prevent raw flour dust from cross-contaminating the post-cook environment.

Second, the food industry also needs to educate consumers not to “lick the spoon” when working with raw dough and batter. Most consumers focus on the eggs as the main ingredient that will cause sickness from raw dough and do not realize that flour may also contain pathogens.

Third, more research is needed to understand the extent of STEC contamination of flour and how to prevent it. In both of the recalls discussed above, no real “smoking gun” was found. Some flour mills are beginning to use environmental monitoring programs to test for STEC and other indicators. More finished flour is being tested, but there is a need for further method validations and better methods for sampling this type of product. There is also a need for better STEC methods to find truly pathogenic strains. Until we find the answers to some of these questions, flour will continue to be a risk for causing foodborne illness.

Could your products contain STEC? In addition to raw flour, food manufacturers using produce and beef as ingredients run a risk of finding STEC in their products. Merieux NutriSciences’ microbiological testing experts enable you to guarantee the quality of your food products by ensuring they are free of pathogens. Contact us today to find out more about our extensive STEC testing capabilities!


Meet the Author

Tim Freier, Ph.D
Division Vice President, Scientific Affairs and Microbiology, America, Mérieux NutriSciences

Tim Freier is the Division Vice President, Scientific Affairs and Microbiology, America at Mérieux NutriSciences. He has over 25 years of experience working in various food safety and quality related positions in food facilities and in the microbiology lab. Tim has worked extensively in the area of pathogen environmental monitoring and exploring and implementing new food safety technologies.

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