Task+1_+READ

= Physics of Popping Corn = = = = Task 1: READ =

=Introduction=

Popping corn is a common experience in our daily lives but few ever think about or understand the details of what really occurs. In fact, this seemingly simple process of cooking can be understood by concepts of thermodynamics. Like all things in nature, with your new understanding of popping corn, you may then began to realise that it is really not a simple process at all!

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=Structure of Popcorn=

Popcorn differs from other cereal grains, and in fact other kinds of corn, in some important ways.

Corn is mostly starch, with smaller amounts of protein, fat and water (about 13%). There is a hard outer coating (hull, or pericarp) under which is a dense opaque starch layer and an inner starch, which consists of transulent grains that are irregular polygons about 10 micrometres across. This unique character of popcorn is caused by the fact that it has a much higher proportion of the transulent starch in its interior than other grains or other types of corn. This gives rise to its ability to pop and expand 20 to 40 times its original volume. Other grains will swell and expand upon heating, but the volume expansion is quite low, typically only two to three times as much as the volume of the kernel.



=Popping Mechanism=

In 1983, a detailed mechanism for popping corn was proposed by Hoseney, Zeleznak and Abdelrahman. In this model, the outer covering of the corn (pericarp) acts as a pressure vessel. As the popcorn is heated to the popping temperature of 175oC, the gases inside the kernel gets heated and expand. The water in the transulent starch vaporizes to a small extent, but most of it remains as a superheated liquid water because of the pressure maintained by the pericarp.



At the pressure of about 8 atm (8.1 x 10^5 Pa) or slightly greater, the pericarp undergoes catastrophic failure. At these temperatures, the starch is "jelly-like" in consistency. Most of the superheated water inside the starch granules turns to steam when the pressure inside the kernel is released by the failure of the pericarp. The steam expands to about 1500 times its original volume as water. As a result, the starch granules expand and blow up like microscopic balloons to particles that become about 40 micrometres in size, or even as large as 100 micrometres.

In the process, the vaporizing water and expanding gases produce sufficient cooling (latent heat of vaporization of water is 2.256 MJ/kg) of to solidfy the starch and form the "spongy" or fluffly popcorn kernels we used to enjoy.

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=Role of Steam and Water=

Popcorn is typically 13% water, with most of the water being located in the transulent starch granules. You may be surprised at this point that water will remain as a liquid at temperatures as high as 175oC. This unsual state is due to superheating, which is sometimes referred as boiling delay. Superheating of water in the popcorn allows the popcorn to be heated to a temperature higher than the boiling point of water, but without the water boiling. Typically, [|superheating] is caused by caused by surface tension in the liquid, which suppresses the growth of bubbles (that is boiling).

Watch superheating of water in the kitchen:

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= = =Cooking Popping Corn Using the Three Methods=

Typically, we can pop corn at home using the three different modes of heat transfer.

Put oil in the bottom of a pan. Cover the bottom of the pan with popcorn kernels. Place the pan on the stove and turn on the burner to medium heat. Cover the pan with a lid. Periodically shake the pan so the kernels move around in the oil. For a 1/4 cup (0.0544 kg) of popcorn, its takes about 4 min to pop all the corn using a 84 kW stove.
 * __Conduction__**

Obtain a popcorn popper. Place the popcorn kernels in the popper. Plug in/turn on the popper. Hot air will transfer heat to the kernels, making them expand and pop.
 * __Convection__**

Microwave a bag of microwave popcorn. For a 1/4 cup (0.0544 kg) of popcorn, its takes about 3 min to pop all the corn using a 87 kW microwave.
 * __Radiation__**