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The popcorn kernel (Zea mays everta) it pops because it contains more water (between 12 and 16%) and has a more resistant husk than common corn. When the seed is exposed to heat (minimum temperature of 170°C), the encapsulated water becomes steam, which expands. The pressure is so great that the shell breaks. In the first contact with the air, the starch (gelatinous inside the grain) solidifies and turns into the white foam that we eat. If there are irregularities in the husk, such as holes or cracks, the water vapor escapes and the husk does not explode, forming piruá (corn that did not pop). Other factors that prevent the transformation of corn into popcorn are continuous exposure to the sun – which ends up drying the inside of the grain – and heating below the necessary temperature.
Source
Articles A Culture of Popcorn in Brazilby Cleso Antônio Pacheco, researcher at Embrapa, and The Cultivation of Popcorn in Brazilby Eduardo Sawazaki, researcher at the Center for Graniferous Plants – IAC