Image copyright AFP

Scientists in France have worked out the critical temperature at which popcorn bursts.

They found that when the popcorn reached a threshold of 180C, the outer shell burst open regardless of the size and shape of the grain.

The research also revealed insights into the way that popcorn jumps as it breaks open, and the sound emitted as water vapour is suddenly released.

The results appear in the journal Royal Society Interface.

The whole process of bursting, jumping and popping occurs in hundredths of a second.

But the physicists Emmanuel Virot and Alexandre Ponomarenko studied each stage in detail to understand its underlying scientific basis.

Using high-speed cameras that record at 2,900 frames per second, they observed the popcorn as it was heated in an oven. The researchers cranked up the temperature at increments of 10C over a period of five minutes.

Moisture inside the corn began to turn to steam as the temperature passed 100C (212F), but when it rose to 180C (356F), the pressure inside climbed to around 10 times the atmospheric pressure at sea level.

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Unable to withstand the stress, the outer shell breaks open and the starchy innards expand, forcing their way out through the shattered hull.

At 170C, only 34% of the popcorn kernels had popped, but at 180C, 96% of the corn had burst.

"We found that the critical temperature is about 180C, regardless of the size or shape of the grain," Mr Virot, from the École Polytechnique near Paris, told the AFP news agency.

Then the scientists turned their attention to the way popcorn jumps. The first thing to emerge from the fractured hull is a limb-shaped structure dubbed a leg.

Image copyright Royal Society Interface Image caption The analysis revealed a starchy "leg" which shoots out and propels the kernel into the air

Pent-up energy is released into the starchy "leg", which pushes against the hot plate, launching the flake up to a height of a few millimetres to centimetres in the air.

"A piece of popcorn has a singular way of jumping, midway between explosive plants such as impatiens, and muscle-based animals such as human beings," the researchers write in their paper.

Finally, the team studied the characteristic popping sound made by the corn. They found that it was not related to the jumping action, because it occurred too early.

Instead, they conclude, it is most likely caused by the sudden, rapid release of water vapour from the kernel.

In addition, after the corn bursts, a pressure drop turns cavities inside the corn into acoustic resonators.