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Popcorn is a beloved snack worldwide, but its transformation from a hard kernel to a fluffy treat is more than just magic: it's chemistry in action. This seemingly simple process involves the interplay of water, starch, and heat, leading to a fascinating explosion of flavor and texture. Let's dive into the chemistry behind popcorn!
Let’s begin with the anatomy of a popcorn!
When we look at a popcorn, all we notice is a white cloud covering a hard brown center? It’s so much more!
A popcorn kernel is a tiny, tough package with three main components: Pericarp, Endosperm, and Germ. The Pericarp (also referred to as Hull) is the exterior, hard and impermeable shell. It is composed primarily of cellulose, a polysaccharide and acts as a pressure cooker building suspense. The Endosperm is made up of starch, a polysaccharide. Starch is composed of amylose and amylopectin that are made up of thousands of alpha glucose units. The Endosperm serves as the kernel’s energy source. The Germ is the seed's embryo, containing genetic material and lipids and contributes to the nutritional value of a popcorn. It contains enzymes for starch breakdown and further increment of pressure.
A short overview of how a popcorn pops: water is absorbed by the endosperm, and as the kernel is heated, the water molecules begin to vibrate more vigorously, increasing the internal pressure.
Water plays a key role in the popping process. The amount of water can either make or break a popcorn kernel. To get the fluffy popcorn we love, the corn has to be dried until the moisture content is about 13-15%. If not, the corn with a lower moisture content gives rise to the duds (also referred to as ‘old maids’). Zeal mays everta is commonly used to create popcorn, and
soaking dried kernels in water for three days and then frying them in oil helps produce the perfect popcorn.
In the Popping Process, the popcorn kernel goes through a series of physical and chemical changes.
When we heat the kernel, the temperature rises and the kinetic energy increases. This causes the water inside the kernel to heat up and vaporize into steam. This steam is trapped by the strong, impermeable hull, causing the pressure inside the kernel to increase rapidly. The germ also activates alpha-amylase, hydrolyzing starch, further intensifying pressure.
At around 100°C (212°F), the water reaches its boiling point, and the starch inside the endosperm begins to gelatinize due to the hot oil and moisture. The starch inside the kernel softens and becomes pliable, creating a thick gel.
As the temperature continues to rise beyond the boiling point of water, the steam's pressure reaches critical levels, typically around 9 atm (about 135 psi or 930 kPa) at temperatures of approximately 180°C (356°F). This pressure is strong enough to rupture the hard hull.
When the hull can no longer contain the pressure, it bursts open. The superheated steam cloud expands rapidly, which causes the cavity inside the kernel to act as an ‘acoustic resonator’ and amplifies the sound of the pop and makes it audible. This rapid expansion also causes the gelatinized starch and protein to inflate into a foam as they are suddenly exposed to lower pressure. The foam after cooling forms the fluffy part of the popcorn we eat. We know that popcorn jumps as it pops, and this happens as the first bit of gelatinized starch that emerges forms a sort of ‘leg’ which catapults the kernel like a gymnast as the remaining starch spills out. The corn kernel transforms in to a soft puff that is between 40 and 50 times bigger than the original seed. The entire process of popping occurs in a fraction of a second.
The distinctive flavor of popcorn is a result of the Maillard reaction, a complex series of chemical reactions between amino acids and reducing sugars that occur when the kernel reaches high temperatures. This reaction produces a range of flavor compounds, giving popcorn its signature: a toasty, nutty, and slightly sweet taste. The Maillard reaction is the same process that browns meat and gives baked goods their golden color. In popcorn, the amino acids are derived from the protein in the kernel and the reducing sugars are derived from starch. The process mainly uses the reducing sugars: glucose and maltose (a disaccharide made up of two glucose units).
When the popcorn is heated, the reducing sugar and the amino group of amino acid react to form water and an unstable compound called a Schiff base or N-substituted glycosylamine. This happens through the nucleophilic substitution reaction where the amino group attacks on the carbonyl carbon of the reducing sugar.
As the Schiff base is unstable, it goes through the Amadori rearrangement. It is the conversion of N-glycosides of aldoses to N-glycosides of the corresponding ketoses. The Schiff base converts to its corresponding ketose in presence of an acid or base as the catalyst. The final product of the Amadori rearrangement is a stable compound called the Amadori product or ketosamines.
Then, the ketosamines go through a series of dehydration, fragmentation, and cyclization reactions. These reactions can produce reductones, diacetyl, aspirin, pyruvaldehyde, polymerized polymers and numerous fission products of the ketosamines. These reactions mainly give rise to
the compounds that contribute to the popcorn’s flavor and aroma like aldehydes, ketones, furans, and pyrazines.
Compounds like Furans and furanones contribute to the sweet, caramel-like flavor. Pyrazines like 2-acetylpyrazine contribute to the nutty, roasted flavor. The buttery, roasted aroma of popcorn is mainly provided by aldehydes, ketones, 2-acetyl-1-pyrroline, (E,E)-2,4-decadienal, and 2- furfurylthiol. The reactive intermediates formed from the ketosamines can polymerize to form melanoidins, a brown pigment.
In popcorn, the Maillard reaction depends on temperature, moisture content and pH. The reaction is highly dependent on temperature, and usually takes place significantly from 140°C (284°F). Its optimum temperature range is around 110-170°C (230-340°F). Moisture is used to facilitate some reaction pathways in the Maillard reaction, however, excessive moisture inhibits the browning process. The optimum pH of the reaction is slightly alkaline, but in popcorn, a rather neutral pH is maintained so the reaction is still able to take place smoothly.
Talking about the creation of ‘duds’ or ‘old maids’, not every kernel pops the way we want it to. Some kernels have too little moisture content, this causes insufficient creation of water vapor and less pressure. The pressure of the vapor would be insufficient to pop the kernel. The pericarp or the hull should be strong, with more highly ordered crystalline arrangement of the cellulose molecules. This helps in retaining moisture and buildup of pressure. A weak hull causes duds. The popcorn kernel should be heated evenly, if not, then the kernel may not reach the critical temperature needed for popping.
This may be a little shocking, but ‘popcorn lung’ is a serious condition that causes inflammation of bronchioles in the lungs. Rather than consumers, this condition affects workers who are constantly exposed to popcorn. This condition is due to the inhalation of diacetyl which is used in butter flavorings. Nowadays, rather than diacetyl, 2,3-pentanedione is used to prevent ‘popcorn lung’ amongst workers.
Popcorn may seem trivial in our daily life, but the next time you have a popcorn, remember that behind each kernel is a series of great stories of different chemical and physical transformation.
References
- https://www.compoundchem.com/2017/01/19/popcorn/
- https://www.popcorn.org/All-About-Popcorn/What-Makes-Popcorn-Pop
- https://sciencenotes.org/how-does-popcorn-pop-the-science-of-popcorn/
- https://www.thoughtco.com/how-does-popcorn-pop-607429
- https://www.eurekalert.org/news-releases/516139
- https://www.biosyn.com/tew/The-Maillard-reaction-and-Amadorirearrangement.aspx#:~:text=Chemically%2C%20the%20Amadori%20rearrangement%2 0refers,according%20to%20the%20Hodge%20Diagram.&text=an%20unstable%20Schiff %20base
- https://www.chemistrylearner.com/chemical-reactions/maillard-reaction
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