The Physics
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Pressure Used to Manufacture Puffed Cereal

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Breakfast cereals and how they are made. R.B. Fast and E.F. Caldwell, Eds. St. Paul, Minnesota: American Association of Cereal Chemists. 1990. "Diligence must be shown by operators to be sure that pressure gauges and safety valves are in proper working condition and that guns in operation are not allowed to exceed the desired firing pressure of 200 psi." 1.4 MPa
What is a rice krispy? What is it made out of and how do they make it? How stuff works. "You cook/condition the grain to get the moisture right and pressurize it to something like 200 psi. Then your release the pressure instantly." 1.4 MPa
Emission Factor Documentation for AP-42 [pdf]. Environmental Protection Agency (EPA). August 1995. "As the gun begins to rotate on shafts mounted on each side of the gun body, gas burners heat the sides of the gun body, causing the moisture in the grain to convert to steam. When the internal pressure reaches 200 psi (approx 9-12 min), the lid is." 1.4 MPa
Hoseney R. Carl: Principals of Cereal Science and Technology. St. Paul Minnesota: American Association of Cereal Chemists. 1986. "The pellets are then loaded into guns or popping vessels. These guns are about 30 in. long and 6 in. in diameter. They are sealed, rotated and heated to 425 C. Pressure may build to 200 psi." 1.4 MPa
Hegenbart, Scott. Mastering the Morning: Creating Breakfast Cereals. Food Production and Design, 1995. "After the lid is closed and locked, the gun is heated with rotation to convert moisture to steam and to pressurize the chamber once the desired pressure of around 200 psi is reached, the heating process is stopped." 1.4 MPa

The pressure required to puff cereal is 1.4 MPa, 13.6 atmospheres, or 200 psi. That being said, there is more in the story to understand. Cereal puffing requires basically two pieces of equipment, a container that is sealable, heatable, and may be opened quickly and something to heat the container, like a torch. The technique used to puff cereal is called "gun-puffing". Step by step, this is the mechanism:

  1. Wheat pellets that are moistened are loaded into a container.
  2. The container is hit with high heat by flame.
    1. This will cause the pressure to rise in the container.
  3. The pressure inside the container is built to 1.4 MPa by the addition of heat.
  4. After this pressure has been reached, the container containing the wheat pellets, (still wheat pellets) is opened suddenly, which exposes the pellets to the outside pressure.
    1. When this happens, the pellets puff instantly. They operate according to the equation (P1V1)/T1=(P2V2)/T2. Since the temperature is kept constant, the pressure and volume are the variable factors. Everything inside the container experiences the pressure of 1.4 MPa before it is opened. After it is opened, the volume of the pellets increase in response to that change.

The puffed pellets shoot out of the machine at about the same time. This explosive step in the process marks how cereal puffing became "gun" puffing.

  1. The flying puffed pellets are caught in a container with a constant circulation of air to keep the puffed pellets from weighing down on and flattening each other.

George Payyapilli -- 2005