The recall of flour in Canada has now expanded beyond the Robin Hood brand and includes 16 products sold across the country, all over concerns the flour could be contaminated with E. coli.

At least 26 people have become sick with a matching and dangerous strain of E.coli O121; six of them have needed to be hospitalized. The actual number of infected Canadians could be even higher, since E. coli sometimes causes only mild illness that might never be reported to hospitals.

Most of us associate E. coli with raw ground beef or unpasteurized milk. But flour? How does that get contaminated with bacteria?

Food safety expert Scott Lougheed, a PhD candidate in the School of Environmental Studies at Queen's University, says while E. coli outbreaks caused by flour are not common, they can happen.

E. coli is a bacterium that lives in the digestive tracts of humans and animals. Lougheed says it’s possible that if an infected animal or rodent walks through a wheat field and defecates there, they could leave infected feces behind. That material could then be gathered up by a wheat harvester and spread throughout the harvested grain.

“Like many bacteria, E. coli prefer warm and moist environments,” says Lougheed, and flour mills can be ideal places for that bacteria to then grow.

The friction of the mills can create enough warmth to allow the bacteria to multiply, but not enough heat to kill it. And if any humidity enters the mill or the grain holding tanks, that can set up the ideal growth conditions.

E. coli also has the ability to survive in a dry environment, such as inside a flour bag on a store shelf. When that flour arrives home to our cupboards, it is still essentially the same wheat, Lougheed says.

“When we use it, it’s in its raw form. It’s been milled and ground, but it’s raw. From the field to our counters, it hasn’t been cooked,” he explained to

Other grains such as oats are steamed at the mill before they are rolled or cut; barley or rice are usually eaten cooked. But flour remains raw wheat.

The danger from contaminated flour isn’t that it ends up in our recipes. In fact, baking the flour will kill the E. coli. The real problem is that uncooked flour tends to get everywhere in our kitchens when we work with it, Lougheed says.

We can contaminate our counters and kitchen items when they come into contact with raw dough, and the flour itself can find its way everywhere. Since most home bakers don’t think of flour as a potential bacterial source, they don’t usually sanitize kitchen surfaces after working with it, Lougheed says.

“We tend not to treat flour as a raw food; we treat it as a ready-to-eat product even though it’s not. So we don’t tend to wash our hands afterwards the way we would after handling raw chicken, for example,” he said.

Because of that ability of flour to cross-contaminate surfaces, Lougheed says tracing flour-based E. coli outbreaks can be challenging. He says it’s often not enough to ask ill patients, “Did you eat raw cookie dough or pancake batter?’ What needs to be asked instead is, “Did you come into any contact with uncooked flour?”

Neither Robin Hood nor Ardent Mills (where the flour was produced), has given any indication how this E. coli outbreak occurred, but Lougheed says preventing flour contamination in the first place is difficult, because of the challenges controlling conditions in farmers’ fields.

It is possible to heat-treat flour to kill off bacteria, but he says that process tends to ruin the flour's gluten.

“So your bread wouldn’t puff up and be as chewy, your cakes wouldn’t be as tender. You’d be pretty disappointed about how it performs in our kitchens,” he said.

Irradiation could help, he says, but the general public doesn’t like the idea of irradiation and the equipment is also “extraordinarily expensive,” says Lougheed, so smaller mills wouldn’t be able to afford it.

As this CFIA investigation continues, Lougheed expects the number of recalled brands and products will expand further.

“I expect we’ll see more recalls as we go forward. Robin Hood is a big flour maker. They are very likely to have lots of wholesale customers and commercial customers,” he said.

“So this is probably just the beginning.”