Traditional popcorn corn stick puffed food why there is a black line

Old-fashioned popcorn has a lot of small black spots is a normal phenomenon

This is because in the process of heating, the temperature inside the pot continues to rise, and the pressure of the gas inside the pot also continues to increase. When the temperature rises to a certain level, the rice grains will gradually become softer, and most of the water within the rice grains becomes water vapor. Because of the higher temperature, the pressure of water vapor is very large, so that the rice has become soft expansion.