The first is the simplest, that is, heating (frying) under normal pressure, that is, western-style popcorn. This method mainly depends on the vaporization of water, which requires the skin of the ingredients to be dense and airtight, otherwise the water vapor inside will be lost directly and the food cannot be expanded. If rice does this, it can only be fried rice instead of popcorn. Among the common ingredients, corn is the most convenient to obtain, which is one of the reasons why western-style popcorn mainly chooses corn. In fact, rice (rice with skin), peas, broad beans, etc. can all be made in this way, but few people in the city see it.
The second is called airflow expansion. This method is to put the granular food into the pressure-resistant container and heat it. After the internal temperature and pressure are high enough, the pressure is suddenly reduced, and the food which has been partially gelatinized at high temperature is expanded rapidly because of the internal water vapor and the infiltrated high-pressure air. Friends born in the 1980s should be very impressed with this kind of thing. It is called a manual air expander, which was invented by the Japanese and looks like a bomb. The master of popcorn opens his bow left and right, shaking the bellows and shells, and looking at the barometer on the shells from time to time. At a certain pressure, the master picked up the shell and stuffed it into a pocket, only revealing a steel bar. He knocked it out/kicked it out/pulled it directly with his hand if he was not afraid of the heat. With a bang, white smoke rose, and then the children around him covered their ears and grinned at the other end of the pocket. This is a unique childhood joy in the 1980 s, and it is becoming more and more rare now.
Because there are already high-pressure containers, all solid ingredients with gaps can explode, not limited to crops with hard skins. Rice, wheat, soybeans, corn, even rice cakes and gluten can be used.
In addition to the manual air blower, there is also an industrial air expander, which looks like a cannon. Granular commercial puffed food, such as popcorn, basically comes out of the air expander.
The third type is called extrusion expansion, and the specific implementation method is also called screw expander. That is the protagonist of Zhihu's classic question "Why can a diesel engine make popcorn?".
It cuts up the raw materials and squeezes them through the intermeshing screws, or screws and inner screw sleeves, so as to realize the high pressure and high temperature of the starch paste, and at the same time, it is transported outward with the rotation of the threads. When the raw material paste emerges from the molding mouth, the pressure suddenly decreases, so the water evaporates and expands. Compared with the airflow expander, the extrusion expander has one more step of chopping raw materials, so it can not only use raw grain, but also use scraps such as powder, and the food it produces can be in various shapes.
Our common puffed biscuits, such as snow cakes and scallops, are all made by large extrusion expanders.