The same phenomenon that causes overheated compost piles to spontaneously combust has been causing fires in sushi restaurants.
The culprit is deep-fried tempura flakes, also known as agedama or tenkasu, which have been linked to five restaurant fires in Wisconsin alone, the New York Times reports.
Fire investigator Kara Nelson says the flakes, used to give sushi rolls some crunch, are made by deep-frying batter, putting it through a colander, and letting the mixture rest in a bowl for hours, often overnight. She says the dense piles often do not allow heat to dissipate properly and they end up igniting after anywhere from three to 10 hours.
Security camera footage from a Madison restaurant showed the mixture combusting six hours after it had been left to cool.
“As they deep fry it in batches, they put it into a colander or a mesh strainer in batches, so they’re basically adding heat every time,” Nelson tells Wisconsin Public Radio.
She says the batches were sometimes piled together in a metal bowl, which trapped the heat.
“So the heat builds and builds, and once it hits ignition temperature, that’s when a fire occurs,” Nelson says.
She says her department has been trying to spread the word about the fire hazard. Her department recommends spreading the batter out on a baking sheet so heat can dissipate—and says restaurants should make the mixture in the mornings so they can keep an eye on it during the day instead of leaving it unattended overnight.