The explosive potential of custard powder

Custard powder makes a delicious dessert but also has a rather less savoury potential – in certain conditions, it can be a powerful explosive.

Instant custard powder is a staple of many kitchens. Just add water and heat, and the powdery mixture of cornstarch and flavourings will transform into an unctuous treat. It's hard to imagine anything more inoffensive. But on 18 November 1981, at the Bird's Custard factory in Oxfordshire, the substance showed its dark side. A hopper of powder overflowed, and the resulting dust cloud ignited, exploding into flame.

Nine people were injured in the explosion. They were lucky – powder explosions can be lethal. Fourteen people were killed in Minnesota in 1871 when a flour mill went up. Forty-four people, including a child, lost their lives to a cornstarch-based explosion in 1919 in Cedar Rapids, Iowa, which levelled part of the town. "The main explosion seemed to lift great buildings and hold them in tension for a moment, letting them drop with their own weight," one observer wrote later.

Dust explosions, of various substances, have continued to take lives – in 2014, 97 people died when dust in a factory in Kunshan, China, ignited. In 2022, one industry report counted 50 such explosions globally, although not all of them involved foodstuffs.

How can the makings for a simple dessert cause carnage, though? In all dust explosions, there are a few common factors. The powder must be made of a flammable substance – that means you're safe from sand, which is made of minerals like silica. But flour, cornstarch, sugar, coal dust, powdered plastic, and aluminium powder can all burn, meaning that if they get airborne, the risk for a truly devastating explosion goes up.

That's because, suspended in a cloud, all those particles have an enormous surface area exposed to oxygen. That makes them swift to combust. If a few of them heat up to ignition – and in each of these cases, there was a source of heat like friction or static electricity involved – then the fire can spread almost instantaneously to the rest of the cloud. Like a pile of confetti ignited in a fireplace, suspended dust burns fast.

That evening at the Bird's Custard factory, there were 20 workers on duty. Several noticed that cornstarch was pouring out of the top of one of the bins, according to the accident report. "At this point several witnesses saw a flash near the top of the bin and a wall of flame spreading outwards and downwards from the bin top," the report continues. "Descriptions were of a gale-force wind with a flame front behind, which flashed through the area." Later inspection revealed that the machinery meant to pour cornstarch into various bins had malfunctioned, so cornstarch continued to pour into the container until it overflowed.  

Keeping the risk of such explosions down means coming at the problem from many angles – grounding all the machines in a factory to reduce static electricity, building in filtration systems that remove dust from the air, and diligent patrolling for any dust build-up are all recommended by health and safety agencies.

Look at a spoonful of cornstarch or custard powder, and it is nearly impossible to imagine it as an engine of destruction. Very few of us have any direct experience of these events, so it is hard to see these substances the way you might see a sheet of paper held near a candle, or a cigarette smouldering by a pile of leaves, as an accident waiting to happen. 

There are situations in which you might encounter this dangerous chemistry in real life, however, outside of a custard factory or flour silo.

Parties in which coloured powder, often cornstarch, is shot into crowds, inspired by the Hindu festival of Holi, are popular spring-time events. In 2015, at a water park in Taiwan, coloured cornstarch sprayed over a concert audience ignited, provoking a maelstrom in which more than 500 people were injured. Twenty people died as a result of their injuries.

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The park's management stated that they had no knowledge that such explosive fires were possible as a result of cornstarch clouds. In fact, coloured powders are often sold with labels incorrectly marking them as non-flammable, a team of scientists found in 2023 when they tested the combustible qualities of various powders.

At Bird's Custard, that day in 1981, the wall of flame thankfully did not kick up any dust that could have provoked a secondary explosion. But it's not uncommon for dust explosions to go from bad to worse as lingering dust is kicked up and rises into the air. The only way to prevent these disasters is to pre-empt them, say consultants who assess the risks for companies.

Keep spaces where powders are used clean, patrol for sources of heat and, above all, have a healthy respect for the destructive potential in innocent, edible powder like custard.

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