This tasty ingredient might be the most dangerous part of making sushi.

Multiple restaurant fires in Wisconsin have been attributed to tempura flakes, the crispy deep-fried bits of batter used to give sushi a crunch. Specifically, the way the flakes are prepared can cause the flakes to spontaneously combust.

Security footage from a restaurant in Madison, Wis., shows the flakes catching fire six hours after they had been left out to cool, Fox 43 reports. According to an expert, the way the flakes are processed, and the density of the flakes, do not allow the heat to dissipate and can result in piles of the flakes igniting.

“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,” fire investigator Kara Nelson told Wisconsin Public Radio. “So the heat builds and builds, and once it hits ignition temperature, that’s when a fire occurs.”

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Nelson described this process as “spontaneous combustion.”

Security camera footage at Sumo Steakhouse and Sushi Bar in Madison, Wis., reportedly shows this process occurring. It was shared online by the City of Madison Fire Department with the caption, “Investigation into two separate restaurant fires this spring turned up a surprising conclusion: The fires were caused by a food preparation technique wherein the oil used to make a tempura-like ‘crunch' self-heats and spontaneously combusts.”

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The ATF Saint Paul Twitter page also shared the results of the investigation, with a message that read, “As ATF certified fire investigators assist state and local agencies, they sometimes come across fires with accidental causes. Here, Madison, Wis., Fire and ATF found a cooking technique was to blame for two Madison, Wis., restaurant fires.”

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The Madison Fire Department issued a fire hazard in July for the cooking technique.