You already know that in this blog, we often discuss bakery and baked goods, but from time to time, we allow ourselves to talk about other cereal-based products as well.

Popcorn is one of the most consumed snacks in the world. It’s an iconic product in cinemas, and it has also traditionally been made at home. Nowadays, there are also packaged industrial popcorn and microwave-friendly products for home preparation. In this entry, we will analyse how popcorn is made, understand the expansion mechanism, and discuss the key parameters to ensure proper preparation.

Why does corn pop?

The first thing we need to understand is why corn pops and expands when heated. The outer part of the corn (pericarp) is tough and completely covers the grain. So when the temperature rises, and the interior of the grain reaches 100°C, the water should evaporate. However, contrary to that, what happens is that the pressure increases. This occurs because the pericarp acts as a pressure chamber. As you know, the temperature at which water evaporates increases as pressure does. This gradually increases the pressure until the pericarp can no longer hold, and it bursts open. At this point, the temperature of the corn can reach approximately 180°C. When the pericarp opens, the pressure inside the grain drops suddenly to atmospheric pressure. The water inside the grain, which is at temperatures well above 100°C, evaporates rapidly, causing the expansion of the kernels and quick drying.

There are many videos that show this process, but here‘s one in super slow motion that illustrates it well and helps you understand it better.

What parameters influence popcorn quality?

The first crucial parameter for producing quality popcorn is the type of corn used. Popcorn is made from a special type of corn called “pop” or “popcorn.” It is a Flint corn, which means it has a higher percentage of vitreous endosperm and is very compact. These corn kernels are smaller in size and have a more rounded shape. These corn varieties have two characteristics highly valued for making popcorn. Firstly, they have a pericarp that covers the grain very well, with an appropriate hardness and thickness. This allows them to act as pressure chambers without breaking too soon, which would reduce expansion. However, they are not excessively hard, which would delay the opening and could make it difficult. Secondly, because the endosperm (the inner part of the grain) is more compact, it expands more than other grains. It’s essential to understand that corn kernels are sold by weight, but popcorn is often sold by volume, so greater expansion means a better business. Besides, we are accustomed to the well-expanded popcorn. Corn kernels can expand between 300% and 500% during this process.

A second factor to consider is the moisture content of the kernels. Higher moisture content leads to greater internal pressure on the pericarp. Excessive moisture can cause kernels to burst prematurely and irregularly, with some not popping while others start to “burn.” On the contrary, if the moisture is too low, internal pressure may not rise enough, preventing the kernels from popping or resulting in less expansion. The ideal moisture content for popcorn kernels is around 13-14%. In the case of frying popcorn, it’s advisable to have slightly lower moisture, as the heating and pressure increase more abruptly.

It’s important to note that the moisture content of corn at the time of harvest is higher than the optimal level for making popcorn, so it needs to be dried. There are various methods for drying corn, but high-temperature methods should be avoided as they can damage the kernels, reduce expansion, or hinder popping. It’s possible to use higher temperatures during the initial phases to remove free water and then finish drying at a lower temperature when the moisture level is around 18-20%.

Additionally, it’s advisable to store the dried kernels for some time before the popcorn-making process. These pre-storage periods are common in many cereal applications because changes occur within the first few weeks after harvest that can affect the grains’ ability to undergo certain processes. For example, the baking quality of wheat flour changes, the germination capacity of barley grains essential for malting is modified, or the culinary quality of rice grains is altered.

Another crucial factor is the integrity of the grain. If the kernels are damaged and fissures occur in the pericarp, these kernels will not act as pressure chambers and won’t “pop.” An excess of such damaged kernels can result in a high number of unpopped kernels during the heating process. As mentioned, the thickness or pressure resistance of the pericarp is more associated with the variety.

A final important factor, closely related to the variety used, is the expansion shape. In Spain, we are accustomed to rounder expansion shapes. However, in other countries, popcorn can expand into more open or flattened shapes. The latter has the advantage of capturing flavour agents, such as salt or others, more effectively.

How should the kernels be heated?

There are three main ways to heat the kernels to make popcorn, each with its advantages and disadvantages.

Frying is the method that produces the most flavourful products because the popcorn partially absorbs the frying oil or fat. It also results in a slightly different colour due to the high temperatures involved. However, frying has drawbacks like high oil consumption, the need for cleaning, and the generation of smoke. Therefore, it’s a simple and low-cost method that produces products with superior organoleptic quality but comes with certain issues or inconveniences. Nowadays, frying is primarily reserved for artisanal popcorn making and home use, as long as microwave popcorn is not used. In this case, the choice of fat will also influence the quality of the popcorn obtained. In Spain, it’s more common to use oils like sunflower oil, and to a lesser extent, olive oil, while in other countries, butter is preferred. The final product’s taste will be different, but also the nutritional quality, although the overall fat content should not be very high and should not be a major concern.

A second option is dry heating. This is the preferred method in the industry since it’s more convenient, avoids smoke and dirt problems, and reduces subsequent rancidity, increasing the product’s shelf life. The downside of this process is a lower organoleptic quality.

The third way to pop popcorn is in the microwave. However, this method has a challenge because when the kernels burst, they jump and move around the microwave’s surface. Special equipment has been designed to keep the kernels within a certain volume. Another common option today is pre-packaged microwave popcorn. In these packages, the popcorn is enclosed in special paper bags capable of withstanding certain temperatures and with a calculated capacity to accommodate the popcorn generated. During the process, these packages expand while the popcorn is being made. The widespread availability of microwaves in almost all households and the simplicity of this method have made it the most used in homes today.

Can we do more?

Once the popcorn is prepared, we can add them flavours. In Spain, the most common practice is to add salt to them, but in other countries, there are popcorn varieties available in various flavours. You can use individual products such as caramel, butter, or cheese. However, the most common approach is to acquire flavouring blends based on natural ingredients like spices or extracts, although artificial products can also be used. Although popcorn is predominantly a savoury snack, it’s also possible to prepare them with sugar, making them slightly sweet.

These additions are more commonly found in commercial popcorn, both in pre-packaged varieties and in some microwave preparations. It’s worth noting that plain popcorn typically doesn’t adhere well to these added substances, so they often settle at the bottom of the container if left untreated. A good “adhesive” is oil. Therefore, when making popcorn by frying, the incorporation of these types of substances is usually more effective. However, it’s also possible to lightly coat the popcorn with oil before adding flavourings to improve their adhesion and distribution.

One challenge with popcorn is its limited shelf life. Popcorn has very low moisture content, less than 3%. This causes them to absorb moisture very quickly, losing their crispiness and becoming soft. Therefore, popcorn should be consumed shortly after preparation. However, they can also be stored in airtight containers to prevent moisture absorption.

Another common question is why there are no popcorn varieties made from other grains. The truth is that most other grains do not have a pericarp that covers the endosperm as effectively and acts as a pressure chamber, like certain maize varieties. While there are grains that can expand in a similar way, their expansion is much smaller, with some irregularity among grains. The most well-known of these is sorghum, but sorghum’s expansion is usually limited to countries where it is grown, and it is not easy to obtain the right type of sorghum for making popcorn.