08/06/19: Researchers confirm pathogens in raw wheat, urge cautious use of raw flour

Only about 10 percent of the wheat flour milled in the United States makes it to consumers via retail shelves, but that’s been enough to cause at least four E. coli outbreaks in Canada and the United States since 2015. One in the United States is ongoing, with 17 people in three states having been confirmed infected as a May 24 update from the Centers for Disease Control and Prevention. The outbreak has been traced to Aldi’s Baker’s Corner flour, which has been recalled.

Research data published this month in the Journal of Food Protection by a team of researchers from ConAgra Inc., WhiteWave Foods Co., and the third-party IEH Laboratories & Consulting Group documents dozens of kinds of bacteria in raw wheat, including many strains of E. coli and Salmonella. One of the researchers said the contamination problem has been known to industry and others for many years but will take time to solve without a mandate fro the Food and Drug Administration.

“This is a complicated issue. In the absence of FDA dictating a remedy, it takes industry years to respond. The first outbreak of E. coli O157 for the beef industry was in 1982. It took 20 years, until December of 2002, and regulatory action by FSIS for the industry to effectively address the issue,” Mansour Samadpour told Food Safety News.

“While we fully expect the baking process to eliminate the pathogens such as Salmonella, (enterohemorrhagic E. coli) and Listeria, cross-contamination, and consumption of raw dough can and will result in a public health impact.”

Samadpour is president and CEO of IEH Laboratories & Consulting Group. He has led numerous government, industry and academic research and development projects during his more than 30 years in food safety. Other scientists on the research team were Samuel P. Myoda and Seana K. Davidson of IEH Laboratories & Consulting Group; Stefanie Gilbreth of WhiteWave Foods Co.; and Deann Akins-Leventhal of Conagra Foods Inc.

In the project reported this month, the research team tested more than 5,000 wheat samples for enterohemorrhagic E. coli (EHEC), Salmonella spp., Listeria spp., and Listeria monocytogenes. The researchers found all except Listeria monocytogenes.

The scientists used pulsed-field gel electrophoresis (PFGE) to fingerprint pathogens found in the wheat samples. They found 22 types of E. coli and 47 kinds of Salmonella.

“All (E. coli) PFGE patterns were unique, and 39 of 47 Salmonella patterns were unique,” according to the research abstract. “These results indicate a diverse background of naturally occurring organisms. These findings suggest that microbial contamination is coming from diverse sources and provide no evidence in support of a specific pathogen load. 

“Altogether, our surveillance study shows that contamination of wheat with pathogens is clearly evident and poses a foodborne illness risk.”

Salmonella was found in more samples than E. coli and Listeria, with 1.23 percent of samples tested being positive for at least one of the 47 kinds of Salmonella detected. The researchers found E. coli in 0.44 percent of the samples. Listeria spp. was found in 0.08 percent of the wheat samples.

Little information is available regarding microbial pathogens in wheat and wheat flour, according to the research report. The scientists said more information about the prevalence of pathogens is needed to develop effective methods to prevent foodborne illnesses caused by wheat products.

Samadpour said there are several active research projects in the works to develop food safety procedures for raw flour. There has been at least one validated microbiological heat inactivation process in use commercially for several years, he said. 

The Nestle Co. began using the heat-treatment process for its raw cookie dough after an outbreak in 2009 sickened 80 people across 31 states. Of those patients, 35 had to be hospitalized and 11 developed a kind of kidney failure known as a hemolytic uremic syndrome (HUS). However, the flour milling industry has not embraced heat treatment or irradiation, which is also known to kill pathogens in flour. Industry representatives say there is a concern that heat treatment or irradiation will have a negative impact on the rising properties of flour. 

“In general all raw agricultural commodities are reasonably likely to harbor pathogens,” Samadpour told Food Safety News. “In the absence of a kill step in the flour milling process, the contamination will carry over from wheat to flour.

“At the present time, the only viable option to address the issue is to subject lots of wheat flour intended for retail use to high-resolution sampling and testing for the pathogens of concern.”

Key points highlighted by the research team included:

  • Prevalence of Salmonella and E. coli in raw wheat emphasizes the need to cook wheat products;
  • Salmonella diversity was high, indicating various animal sources that are difficult to prevent; and
  • Cooking wheat products is the best preventative measure against foodborne illness from wheat.

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