Popcorn Kernels And The Law: What's The Legal Crunch?

Popcorn is a unique variety of corn, and the only one that pops. The reason for this is the specific amount of moisture inside the kernels. Popcorn kernels contain around 13%-14% water, and when heated, this water turns to steam. The steam builds up pressure against the hard, nonporous outer shell, and when the pressure gets high enough, the kernel bursts, turning inside out. This process is governed by the ideal gas law, which states that as temperature changes, the volume of a gas changes. Thus, the ideal gas law can be used to determine the pressure inside a popcorn kernel when it pops.

Ideal gas law

The ideal gas law, also known as the general gas law or the perfect gas law, relates the pressure, volume, and temperature of a gas in a confined space. The law is expressed as PV = nRT, where P is the pressure, V is the volume, n is the number of moles of gas, T is the temperature, and R is the universal gas constant. This law applies when gases are at low pressures and high temperatures, causing the molecules to move almost independently of each other.

The ideal gas law is a combination of simpler gas laws, such as Boyle's Law, Charles's Law, Avogadro's Law, and Amontons's Law. It can be derived from the kinetic theory of gases, which assumes that gas molecules are in random motion, obey Newton's laws of motion, and have negligible volume compared to the volume occupied by the gas.

The ideal gas law is often used to understand the behaviour of gases in various conditions. For example, let's consider the case of bursting popcorn kernels. Popcorn kernels contain about 13% water. When heated, this water vapourises, increasing the pressure inside the kernel. The ideal gas law can be used to calculate the pressure inside the kernel as the water vapour expands. This pressure can reach about nine times the atmospheric pressure, causing the kernel to rupture and pop.

Here's how the ideal gas law can be applied to this scenario:

PV = nRT

P = Pressure inside the kernel

V = Volume of the water vapour inside the kernel

N = Number of moles of water vapour

R = Universal gas constant

T = Temperature of the water vapour

By measuring the volume of the kernel, the mass of water inside, and the temperature, you can calculate the pressure required to rupture the kernel. This calculation provides insight into the physics behind the simple act of popping popcorn.

Moisture content

The moisture content of a popcorn kernel is critical to its transformation into the fluffy white popcorn we enjoy. Popcorn kernels must contain about 13-14% moisture for optimal popping. This precise amount of water inside each kernel determines whether the kernel will pop or remain unpopped.

The process of popping involves heating the corn until the pressure inside the kernel is great enough to cause it to burst, turning the kernel inside out and releasing the trapped moisture. The hard outer shell, or pericarp, and the specific moisture content set popcorn apart from other types of corn. The pericarp's strength and the starch type of the endosperm (the fluffy white part) are crucial for the popping process.

If the moisture content is too low, the kernels won't generate enough steam to pop, resulting in many unpopped kernels. On the other hand, if the moisture content is too high, the kernels may burst prematurely, leading to smaller, less fluffy popcorn. The ideal moisture content of 14% ensures that the steam builds up enough pressure to rupture the pericarp and create the characteristic "pop" of the kernel bursting.

The moisture content also affects the popping volume, flake size, and percentage of unpopped kernels. Studies have shown that the maximum popping volume is produced at moisture levels ranging from 11.0% to 15.5%. The expansion volume and flake size increase as the moisture content increases up to 14%, and then decrease with any additional moisture. Therefore, maintaining the optimal moisture content is crucial for achieving the desired popping results.

Heat and pressure

Popcorn kernels pop due to the heat and pressure that builds up inside them. The kernel consists of a very hard, mostly nonporous outer shell called the pericarp. Inside the kernel, there is not only the seed for a new corn plant but also water and soft starch granules that serve as a food source for the seed during germination.

When the kernel is heated, the water inside becomes vapour, and due to the hard and mostly nonporous shell, the steam has nowhere to go, resulting in a buildup of pressure inside the kernel. The pressure continues to increase until the pericarp ruptures and the insides, now melted, spew out. The ideal popcorn kernel has an optimal moisture content of about 13-14% and is popped at a temperature of about 180 degrees Celsius.

Popcorn pops at roughly nine times atmospheric pressure. The steam needs to reach a much higher pressure to burst the kernel open, and thus a higher temperature is required to create steam at this pressure. The pressure needs to reach roughly 135 PSI (~9 bar), or 9-40 times the pressure of a consumer pressure cooker. The temperature at which this pressure is reached is roughly 180-200°C or 350-390°F.

The characteristic popping sound does not originate from the cracking of the hull, but instead, results from the vapour release after the kernel has cracked. The popping sound comes from the release of vapour after the kernel has cracked. The starch inside the kernel is also responsible for the characteristic popcorn consistency and white-yellowish foamy appearance. With high temperatures, the starch gelatinizes and then expands with the rapid burst of the kernel. Once it cools down, the solidified flake we know as popcorn is formed.

Starch gelatinization

Firstly, during granule swelling, water is absorbed into the amorphous spaces within the starch granules, causing them to swell. This is possible due to the presence of heat, which allows water to penetrate these tightly bound areas. The crystalline regions within the starch granules become more diffuse, and the number and size of these regions decrease.

Secondly, the melting of double-helical structures occurs as water enters the amylopectin through the amorphous regions. The heat causes the amylose chains to dissolve and separate into an amorphous form, further contributing to the breakdown of the starch granules.

Finally, amylose leaching takes place as the penetration of water increases randomness in the starch granule structure, leading to swelling. Eventually, the amylose molecules leach into the surrounding water, causing the granule structure to disintegrate. This process is irreversible, as the starch granule dissolves in water.

The gelatinization temperature of starch depends on various factors, including the plant type, the amount of water present, pH, and the types and concentrations of salt, sugar, fat, and protein in the recipe. Unmodified native starches may start swelling at temperatures as low as 55°C, while others require temperatures of 85°C or higher. The gelatinization temperature also depends on the degree of starch modification, such as cross-linking, acid treatment, or acetylation.

Gelatinization plays a vital role in cooking, as it makes starch digestible and helps thicken or bind water in sauces, soups, and roux. When cooled for an extended period, gelatinized starch will thicken and rearrange into a more crystalline structure through a process called retrogradation. This process results in the formation of various molecular associations, such as amylose-amylose, amylose-amylopectin, and amylopectin-amylopectin.

Popping yield

The popping yield can be influenced by several factors, including the moisture content, variety, and quality of the kernels, as well as the temperature and method of heating. For example, popcorn kernels typically require a temperature of around 350°F / 177°C to pop effectively. If the temperature is too low, the kernels may not pop, resulting in a lower yield. Similarly, if the temperature is too high, the kernels may burn, producing an unpleasant taste and reducing the yield.

To optimise the popping yield, it is important to start with high-quality kernels. Fresh kernels with a hard pericarp (outer covering) are ideal, as they can better contain the pressure build-up during heating. The moisture content of the kernels is also crucial. Popcorn kernels contain approximately 13% water, and this water needs to be heated and vaporised for the kernels to pop. If the kernels are too dry, they may not have sufficient moisture to build up enough pressure to rupture the pericarp.

Additionally, the heating method can impact the popping yield. Even heating is essential to ensure that all kernels are exposed to the same temperature and pressure conditions. Stirring or shaking the kernels during heating can help distribute heat evenly and prevent burning. Using a small amount of cooking oil during the process can also improve the popping yield by facilitating heat transfer and providing a medium for the kernels to pop in.

In summary, achieving a high popping yield depends on a combination of factors, including the quality and moisture content of the kernels, the heating temperature, and the method of heating. By optimising these factors, a higher proportion of kernels will successfully pop, resulting in a desirable popping yield.

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