Popcorn: critical temperature, jump and sound

To our knowledge, so far, few people are aware of the origins of unique "popular" sounds. In our scenario, this sound is thought to be caused by (i) slamming, (ii) ground bounce, or (iii) release of pressurized water vapor.

To understand the origin of "popular" sound, our experimental equipment has added the microphone Neumann KM 84 (40-16 000 Hz). The microphone is placed 30 cm from the popcorn on the hotplate. Acoustic recording is synchronized with pencil lead disconnection and high speed camera phantom milo 4 (2000 frames / sec) with less than 1 ms error

As shown in FIGS. 6 a, b, the popcorn first opens a part of the starch without any sound. Next, after 100 ms, the second rupture begins (Figure 6c), then the "burst" sound begins 6ms later (Figure 6f). Both cracks expand and the legs of the starch continue to face the hotplate. The "pop" sound starts at 106 ms and lasts about 50 ms, but there is no obvious dominant frequency, but there is a sharp burst at 110, 115, and 121 ms (Figure 6 f).

We first confirmed that the "pop" sound occurred before the jump, so it was not due to bounce. Since most of the cracks in popcorn are not related to any sound, careful observation also distinguishes between snoring (see also [31]). Then there is a reason to assume that the 'generated' sound is caused by vapor emission. More specifically, the pressure drop stimulates the cavity in the popcorn as if it were an acoustic resonator. This situation has been applied to volcano acoustics and "popularity" of champagne cork [32]. The burst observed in FIG. 4f can then be interpreted as a continuous release of a pressurized water vapor bladder to initiate continuous excitation. In addition, since the room being tested has a reflecting surface several meters from the popcorn, consecutive bursts may be irreversibly associated with repeating artifacts from the echo. A short time delay of 6 milliseconds between cracks and "burst" sounds can be interpreted as the time required to reach and release the first vapor pocket. It is still surprising that our sound record lacks the dominant frequency, but it reflects the dramatic change in the nature of popcorn during the transition.

When the temperature exceeds 100 degrees Celsius, the water in the popcorn boils and reaches thermodynamic equilibrium under vapor pressure (as in the case of pressure cooker). Exceeding critical steam pressure will damage the outer casing (or hull). At the same time, in the interior, the starch granules are swollen to form sponge-like flaky petals. Then popcorn jumps a few millimeters, you will hear familiar "pop music". Surprisingly, the origin of the physical explosion, jump, and "pop!" Are still unknown. Therefore Emmanuel Virot and Alexandre Ponomarenko of École Polytechnique of the University of Grenoble looked at thermodynamics and fracture mechanics. First of all, they tried to play microwave popcorn at various temperatures in the oven. Only 34% of people appear at 170 ° C. The critical temperature and hull fracture point were 180 ℃, 96% of which found rupture of popcorn.

Next, the French team determined the critical temperature of all popcorn bursts. They put the core of the microwave popcorn in the oven and raised the temperature at 10 degrees Celsius. Only 34% of the grain pops up at 170 ° C, 96% pops up at 180 ° C - regardless of the size of the kernel. They identified three possibilities: hull rupture, ground bouncing, and release of pressurized water vapor. To understand which is responsible, they placed a high-end microphone 30 cm from the hot plate and synchronized it with the high-speed camera. Because it does not sound when popcorn is opened for the first time, cracking of the hull is not the cause. A second rupture was observed after 100 milliseconds - after that, an explosion began in 6 milliseconds

Whether on the stove or in the cinema the popcorn kernel shows three interesting reactions when heated. According to an interesting new study published in the Royal Society Interface Magazine, the sound "pop!" Is caused by the release of water vapor. This is similar to tumbling. When the temperature exceeds 100 degrees Celsius, the water in the popcorn boils and reaches thermodynamic equilibrium under vapor pressure (as in the case of pressure cooker). Exceeding critical steam pressure will damage the outer casing (or hull). At the same time, in the interior, the starch granules are swollen to form sponge-like flaky petals. Then popcorn jumps a few millimeters, you will hear familiar "pop music". To my surprise, the physical origin of explosion, jump, and "pop!" Is still unknown.