Baking bread might be a relaxing weekend activity, but making the flour that goes into that bread is a dangerous business. Ever since the Washburn flour mill explosion near Minneapolis in 1878 killed 18 people, the milling industry has tried to reduce the risk of flour particles igniting into flour bombs. Unfortunately, that’s tougher than it sounds. In fact, General Mills’ Gold Medal Flour factory, uses "explosion-proof" motors for added protection.

“Flour dust that is suspended in air is more explosive than coal dust," Paul Steinlage, milling manager, told Food Engineering Magazine this month. "With the number of motors we need to run the mill, we absolutely have to have the best explosion-proof motors so we feel protected.”

Throughout agriculture and food processing, there were 115 reported dust explosions [pdf] between 1994 and 2003, the most recent numbers available, most of them in grain elevators. These stats left me asking: why is dust explosive?

I called up the Bill Stevenson of CV Technology, which bills itself as a dust explosion consulting company. He explained that there are five key components to a dust explosion: 1) dust 2) suspended in a cloud 3) within a confined space 4) with enough air 5) and some type of ignition. He said this was called the "explosion pentagon". Without all of those factors, you won’t get a dust explosion.

"If you had a layer of (flour) on your kitchen counter and you put a benzine torch to it, you could get it to ignite but it wouldn’t blow up," he said.

Flour sitting on your counter just doesn’t have enough oxygen to burn, he said. But airborne flour particles (or corn starch, as in the video above) have enough oxygen available for combustion AND yet remain close enough to each other to create the chain reaction that results in an explosion when one flour particle is ignited.

If you find yourself in a romantic comedy style flour fight in the kitchen, however, don’t worry too much. Stevenson noted that the amounts of grain used in home cooking aren’t really enough to do serious damage.

"The amount of corn starch or flour that you use at home is a 1 cup here or 2 cups there. That’s not what we’re worried about," he said. "What we’re worried about is the railroad car load."

Video: A quick tutorial on "safe" fire-breathing using corn starch to create blow fire… Notice that the corn starch used in the video isn’t confined, so it doesn’t cause an explosion, just a nice fireball.

Context:

German Dust Explosion Database

CV Technology

The Washburn Mill Explosion (Minnesota Historical Society)

Combustible Dust in Industry: Preventing and Mitigating the Risks of Fire and Explosions (OSHA)