Flour explodes when its particles become suspended in the air in a dust cloud and are then ignited. The starch molecules burn relatively quickly, and it is their rapid expansion in the presence of heat that causes an explosion. Ignition is most common in the presence of a flame or heat source, though spontaneous combustion has been documented when the cloud is large enough. The risk is most pronounced in mills and processing centers where large amounts of dust are loose in closed spaces, either in silos or in transit. Explosions are generally very rare at home, and when they occur, they are typically small and easy to contain.
The Chemistry Behind the Flammability
All flours contain starch, which is a complex carbohydrate made from glucose molecules chained together. Glucose is almost always highly flammable. This property is what allows for the sugar crusts on some custard desserts, for instance, and is why marshmallows held over an open flame will quickly roast or blacken. Though most flours are not sweet to taste, they nevertheless have the highly flammable properties of sugar, which is what makes them explosive.
Air Dispersal Requirement
Flour is not prone to explode all on its own — individual grains must be separated and exposed to oxygen for there to be any risk. When stored in densely packed bags or containers, the chances of fire are quite low. Explosion becomes likely only when individual particles are suspended in the air, usually in the form of a dust cloud. Dust clouds in confined spaces both allow the starch molecules ample access to oxygen, and prevent escape — under these conditions, any heat or heat source can set the sugar molecules ablaze. In large quantities, this has a very explosive effect, and can be deadly.
Biggest Risk Areas
Mills and processing plants are usually at the highest risk for explosion, as these facilities handle massive quantities of flour at once and often have the space for the powder to separate and form dust clouds. Bags that are dropped in transit, powder that accumulates in silos, or loose dust in rail cars are often the biggest explosion risks, as any spark or errant flame can set things off. When fires start in these places, devastation is almost guaranteed and loss of life becomes likely.
Safety at Home
Flour explosions in home kitchens are very rare, and cooks should not let the fear of fire discourage them from baking or keeping flour at home. Flours that are left in store packaging or transferred to airtight containers pose virtually no risk of combustion, as they simply lack the surface area to ignite even when exposed to extreme heat. Absent a dust cloud, there will be no explosion.
Neither should cooks be worried about the small clouds that sometimes appear when adding flour to other ingredients, or that may arise as a result of a spill or accident. Even playful “flour fights” in the kitchen are unlikely to result in fire if only because of the small volumes at issue and the lack of true confinement. Any fires that do result would most likely be minor, short-lived, and very easy to extinguish.
Homemade “Flour Bombs” and Experiments
Some science classes create small flour explosions in controlled settings to illustrate the principles at work. A simple and relatively safe way to do this is for a person to light a candle inside a can with a lid. She can then poke a straw through a small hole drilled in the side of the can at approximately candle level, and slowly puff flour through the straw. A small fire should result, blowing the lid off of the can.
Suspending flour in a balloon can also be an effective, if messier, way to demonstrate the principle. When the balloon is placed near heat — often a light bulb — the flour inside can combust, bursting the plastic and showering throughout the nearby area. Though experiments like this are generally considered safe, they should only be performed under close supervision — and preferably with a fire extinguisher nearby.