Popping The Corn (The Rise Of Popcorn)

When you leave the cinema, I always notice the large number of popcorn cartons strewn between the aisles. It makes me think how this snack is now synonymous with viewing pleasure and a night out in particular at the cinema. Somewhere in the auditorium, during the quiet moment of a film, there is a faint munching. In fairness, I’m also dipping into a carton looking for the largest piece and thinking when do I arrive at the fragments and uncooked grits. So to cut a long story short, we talk here about popping the corn.

Even if this is not a snack to your liking it is for many others and continues to be a growing trend in the snack market. Allied Market Research (www.alliedmarketresearch.com) valued the popcorn market globally at a neat $9,060 million in 2016. Verified Market Research in 2024 valued the market at US$13.39 billion in 2023. It could be that the compound annual growth rate will be over 14% by 2030. Another market research firm, Technavio reckons that 59% of the growth in the market will be borne in the Americas alone with the USA being behind that driving force.

Popcorn appears to be a little more sophisticated lately. It used to be about flavour – caramel, butterscotch or saltiness. In a more health conscious society, we have started returning to a more natural taste whilst others feel the need to raise the flavour tempo of what is a rather bland tasting product. A trip round food exhibitions reinforces the view that there are many more exotic flavours now available and the format for popcorn is also changing slightly. What had happened to crisps in terms of flavour is now happening with popcorn.

The health element is also improving and drawing on the experiences of marketing/ R&D in the snack food area. Almost half of all the new introductions have some form of health message attached to them.

Microwave cooking of popcorn has also extended the convenience element but it’s good to turn back to more traditional processing methods using iron pots, cooking with a small amount of oil and waiting for the pop as the corn heats up. It’s also a vehicle for trying home-based flavour combinations and I suspect has led the more innovative of us in product development kitchens to test out their taste buds.

If you are looking for social knowledge that is up to date then why not consult the USDA Agricultural Marketing Service site which contains the Popcorn Board. All the interesting data is held at Popcorn Central.

What do I look For In Popcorn Kernels?

Popcorn is generally assessed in terms of its size once it has been popped. For freshly harvested popcorn to achieve optimal expansion and thus volume when popped, several quality parameters must be carefully monitored and maintained. The most important measure for the processor is the popcorn expansion volume (PEV) which is a measure of popping volume. It is the primary criterion of quality which was noted a number of years ago. The main reason is wholly commercial; popcorn kernels are bought by weight but sold by volume. That PEV value is critical.

[As Lyerly said back in 1942 when he was doing his Ph.D. on the subject, the ‘importance of this measure to the operators of commercial establishments is obvious since their product is sold on the basis of volume rather than weight. A high expansion is also associated with palatability because the lighter and fluffier kernels are usually the more tender”.]

These are the parameters then and include the following key features which I look for in freshly harvested kernels. We’ll outline the main features and then explore some of these characteristics in greater detail.

1. Moisture Content (MC)

• Optimal Range: The ideal moisture content for popcorn kernels is typically between 13.5% and 14.5%. Some argue that a minimum of 11.0% is even reasonable. In most cases the ideal MC is closer to 14.5% implying a skew to that higher value. It is the determining factor in virtually all cases whether popping is conducted by air or by frying and whether it is in a microwave.  The range ensures sufficient internal steam pressure for optimal popping. Too much moisture can lead to unpopped kernels, whilst too little can cause the kernels to burn before popping properly. The type of popping process also dictates the optimum moisture content needed for best expansion. A value of 14% is best for air popping whilst 13.5% is required for oil popping. Whilst the measure can be conducted in the field, the processor will wait for the kernels to dry enough whilst still in the field or harvest and dry once the kernels.

2. Kernel Size and Shape

• Uniformity: Uniform kernel size and shape are crucial for evenness in heating and popping. Popcorn kernels are generally categorized by size (small, medium, large) and shape (round, oblong, etc.).
• Preferred Types: For commercial purposes, there are two primary shapes of popped corn: “butterfly” (or snowflake) and “mushroom”. Butterfly popcorn has irregular shapes and more surface area, which is ideal for coatings. It has the unfortunate habit of breaking easily during processing  and storage. As we will see later it also has a lower popping volume and dull appearance. Mushroom popcorn is more compact and less likely to breakage and can be picked more easily. It is suitable for caramel coated popcorn and other coated varieties.
• A kernel size between 5 mm and 6 mm is the optimum for popcorn quality generally.

3. Kernel Hardness and Hull Thickness

• Kernel Hardness: The hardness of the kernel contributes to the buildup of steam pressure within the kernel. A harder pericarp (outer shell) is desirable. When it comes to expansion volume, kernels that are too hard could resist popping and those that are too soft just don’t have the structural integrity to trap enough steam for expansion (Richardson, 1960; Tracy & Galinat, 1987).
• Hull Thickness: A thicker hull can increase the likelihood of the kernel popping, as it can withstand higher internal pressures. However, the hull should not be so thick that it becomes difficult to chew.

4. Crude Fiber Content

• Balance: While some fiber is necessary for the structure of the hull, too much fiber can result in a tough, less enjoyable popcorn. The crude fiber content should be balanced to ensure the popcorn pops well and is pleasant to eat.

5. Purity and Cleanliness

• Foreign Matter: Kernels should be free from dirt, debris, and other foreign matter that can affect popping quality and food safety.
• Broken Kernels: Broken or damaged kernels should be kept to a minimum, as they usually lead to uneven popping and poor quality.

6. Colour and Appearance

• Uniform Colour: The colour of the kernels can indicate their freshness and quality. Uniform, bright coloration is often preferred, as it can be indicative of proper drying and handling. Any popcorn variety or hybrid that has a flinty, vitreous, glassy (Quinn et al., 2005) or translucent look to its endosperm is most likely to be the best for obtaining a high expansion volume (Pordesimo et al., 1990, 1991). The appearance is due to tightly packed starch granules which produce rapid expansion on heating (van de Sman and Bows, 2017). Colour also has a direct impact on expansion volume and is thought to be a better predictor of expansion volume than using more classical methods. 

7. Density and Weight

• Kernel Density: A higher kernel density can correlate with a better popping volume, as it indicates a higher proportion of starch to air.

8. Starch Quality

• Amylose and Amylopectin Ratio: The starch composition within the kernel can affect popping performance. A certain ratio of amylose to amylopectin is ideal for achieving good expansion.

9. Genetic Factors

• Variety: Different popcorn varieties are bred for specific popping characteristics, such as expansion volume, flake size, and texture. Selecting the right variety for the desired end product is essential. The optimum moisture content for maximum expansion volume and percent kernels popped differs from variety to variety. Lyerly in the early 1940s assessed expansion volume of at least 29 different types of inbred corn to understand how variable expansion and this popcorn quality depended on the endosperm and pericarp of corn. This has been taken further by a number of other researchers including Sweley et al., (2013).

10. Maturity and Harvesting

• Popcorn quality is influenced by maturity at harvest and harvest condition (Haugh et al., 1976). Lien and Haugh (1975) assessed field operating conditions and evaluated kernel damage and various other parameters in relation to expansion volume. The quality of popcorn is dependent on how it is grown and when it is harvested. The husks will turn from green to yellow in the autumn and then to a paper dry and brown colour. As the ears dry, so the entire stalk starts to dry as well. If possible, wait until the entire stalk is dead and brown before picking the ears. The kernels on the cob should also look glossy and feel hard but not shrivelled. The corn needs to be picked as soon as they are dry though, to prevent pests like rodents and birds from eating them. We also note that ears dry from the outside in. They can  look like they are yellow and dry on the outside but the inner leaves will still be green for sometime. The grower usually has considerable experience in deciding on harvesting based on a number of visual factors but as we will discuss later there are analytical tests available to confirm the grower’s requirements
• Maize/corn needs to dry further before it is used because the moisture content is still to high at harvesting and there is extreme variability here too because of the different centres of harvesting and weather conditions. Mould is always an issue and so picking is judged on how soon the weather turns to either cold or rain. Corn cobs tolerate light frost but if any heavy frost is due then picking should occur.
• Industrial corn shellers are available in corn picking machines but it is still best to shell when the corn has reached the correct dryness. For the home user, test popping some kernels in a hot dry pan works. If there is no popping the popcorn is simply not suitable for popcorn and would have to be used for the production of corn flour. It can be the case that the kernels are too dry and so adding a small amount of water to soften the kernels actually works effectively. The corn does still need to be test popped however

By ensuring these quality parameters are met, producers can optimize the popping performance, resulting in a high-expansion, high-volume product with desirable taste and texture.

Analytical Methods Used To Assess Kernel Quality

Assessing the quality of corn kernels, including popcorn, involves a variety of analytical methods (Pordesimo et al., 1990). These methods evaluate different physical, chemical, and functional properties of the kernels to ensure they meet quality standards. Below are some commonly used analytical methods which I’ve employed at various times and are especially useful with fresh popcorn kernels.

1. Moisture Content Analysis

• Gravimetric Method: Using an oven to dry the kernels and calculate moisture content by weight loss. It is a common, almost universal approach and standard methods are available. For single kernel measurement, a modified ASAE Standard S352.2: Moisture measurement – unground grains and seeds (ASABE Standards, 2017) procedure can be tested, i.e., 103ºC for 72 hours (Wu et al., 2022).
• Infrared Moisture Analyzer: A rapid, non-destructive technique using infrared radiation to determine moisture content. Light reflected from or transmitted through a sample will absorb selective wavelengths of light at 1640 and 1910 nm for water.  Field based technology has become available for this purpose.  An NIR calibration model was developed by Lamonthe (2009) to predict expansion volume of microwaveable popcorn at the equilibrium moisture content of 13.5% to 14%. It had an R2 of 0.56 and RMSECV of 2.16% which means further improvement to the model was needed. Wu et al (2022) found an optimum MC of 12.5 to 13.0% using the SKNIR approach (SK – single kernel). Such improvements are being made based on proprietary and trade secret information that FoodWrite is aware of.

The most common moisture analysers use the dielectric principle. The dielectric principle is based on water which is a strongly dipolar molecule. In the presence of an electric field between two parallel plates, water molecules line up with the plus dipole towards the negative voltage side whilst the negative part of the dipole lines up towards the positive side. The dielectric constant is said to be a measure of the ability of a material to store energy when exposed to an electric field. Water has a dielectric constant of 80 which is much higher than any other constituent in maize (corn).

2. Physical Properties Analysis

• Kernel Size and Shape: Sieving, image analysis, or caliper measurement to determine size distribution and shape. Large, medium and small kernel sizes were separated by passing the sample through a set of U.S.A. standard testing sieves (Fisher Scientific Company, Chicago, IL), No. 3 % (5.6 mm opening) and No. 4 (4.75 mm opening). The kernels retained on the No. 3 % sieve were classified as large, the kernels which passed through the No. 4 sieve were classified as small, and the rest of the sample was classified as medium kernels. The Winchester bushel meter is also used as a weight measuring system with the added benefit of checking grain hardness and condition.
• Kernel Weight: Weighing individual kernels or a sample batch to determine average kernel weight. The 1000 kernel weight is a seed counter that relates the average size of the kernel to expansion volume. Lower sized kernels have less expansion volume than larger grains. The range of 1,000 kernel weight for the large, medium and small kernels are 149-152, 133-147 and 95 – 131 kernels per gram.

3. Density and Specific Gravity

• Bulk Density: A large sample is used to fill a defined volume and the weight measured.
• True Density: Using air pycnometry or a gas displacement method to measure the density excluding the intergranular spaces. It is the only method that truly allows an estimate of how much volume is needed for a box of popcorn

Kernel density will range between 1.375 and 1.389 mL/g. The single kernel volume ranges between 0.093 to 0.130 mL and the thousand kernel weight is between 126 and 178 g

Density values are higher than those reported by others. More commonly the gas displacement method uses helium. This gas penetrates the interior of the kernel which decreases their effective volume and results in higher density values.  Haugh (1976) used a density-gradient column method. Any differences in kernel density are attributed to gas penetration (Chang, 1988). 

4. Hardness and Texture Analysis

• Hardness Testing: Using instruments like the hardness tester or a texture analyzer to measure the force required to break the kernel. One can use the Stein breakage tester (Model CK-2M. Atchinson, KS). The dried kernels are subjected to impact for different lengths of time. The breakage susceptibility is determined as the ratio of the amount of kernel fragments sieved thorough a Strand Sizer Shaker (Model P, Seedburo Equip. Company, Chicago, IL) for 30 cycles to the original sample weight and then expressed as a percentage (AACC-55-20, 1983). Some experts still rely on visual inspection otherwise tests using density differences are said to be reliable. One other test is the Stenvert hardness test – a Stenvert hammer-mill grinder is used to grind about 20 grams of corn and the time taken to fill a collection tube to a particular volume – 17ml is noted. A hard endosperm means it takes longer to grind. It also has higher percentages of coarse particles than soft endosperm maize. Typical hardness values remain between 25 and 35 seconds.

One other factor that has an effect on popping expansion is weight per bushel or cob. The heavier the weight per bushel, the greater the level of expansion. It is probable the starch is more dense and there are more starch grains per kernel. Kernel damage during any form of processing will lower popping expansion.

• Instrumental Texture Profile Analysis (TPA): Measures properties like hardness, cohesiveness, and chewiness.

5. Chemical Composition Analysis

• Proximate Analysis: Determination of moisture, ash, protein, fat, and fiber content. Can be conducted using classical chemistry based methods such as Biuret and Soxhlet.
• Starch Composition: Analyzing the amylose and amylopectin content using methods like iodine binding or high-performance liquid chromatography (HPLC).

6. Popping Quality Assessment

• Expansion Volume: Measuring the volume increase of kernels after popping, typically using a graduated cylinder or volumetric container. It is quoted as total popped volume (mL) divided by original kernel weight (g). A high expansion volume is between 25 and 31 volumes whilst a low expansion is from 15 to 23.5.  The Cretors test (popcorn expansion test; C. Cretor Wood Dale, Il USA) is now the recognised method for evaluation. We also know it as the MWVT Popping Expansion or Metric Weight Volume test that quotes volume as CCs per gram. The current value of approx. 30ml/g is the ideal standard. The measure is so important that it dictates the sales price per box of popcorn.
• Unpopped Kernel Count: Counting the number of unpopped kernels to assess popping efficiency. Usually quoted as a percentage figure.
• Flake Size (FS) Distribution: Assessing the size distribution of popped flakes using sieving or image analysis. Measured as total popped volume versus number of popped kernels. There is also a total flake volume measure (TFV) which is the absolute volume of 100 popped kernels.

7. Colour and Optical Properties

• Colorimetry: Using a colorimeter or spectrophotometer to measure the colour of kernels or popped flakes.
• Visual Inspection: Subjective assessment by trained personnel for appearance and uniformity. A light box is the only method available with a microscope for assessing stress cracks and fissures. One of the main reasons for low popping and poor flake volume.

8. Chemical Analysis

• Near-Infrared Spectroscopy (NIR): Non-destructive analysis for measuring the content of components like moisture, protein, oil, and starch. A potential method too for assessing the phenotype of kernels for their PEV. It is regularly used now to assess all sorts of parameters associated with legumes for example (Pasquini et al., 2003). The USDA-ARS, Center for Grain and Animal Health Research has developed SK NIR instruments that have proven feasible to detect different quality parameters for various commodities. The SK part of the acronym refers to single kernel. Partial least square models (PLS) using NIR are currently too inaccurate but a PCA-QDA model can discriminate between high and low PEV kernels (de Oliveira et al., 2020). Wu et al., (2022) have developed the modelling element further which we alluded to when making claims about using NIR for moisture content measurement. They have calibration models using SKNIR which have R² values of 0.94 and an SEP of 0.25%.
• NIR spectroscopy can be used to identify the glassy or translucent endosperm that has densely packed starch granules that indicate a higher expansion volume. The hard kernel type is found in popcorn whilst soft kernels contain floury endosperm which is useless for popcorn. 
• Gas Chromatography (GC): Used for determining the presence of volatile compounds and fatty acid composition.

9. Microbial Analysis

• Standard Plate Count: Assessing the microbial load by culturing on specific media.
• Mycotoxin Testing: Detecting contaminants like aflatoxins using enzyme-linked immunosorbent assay (ELISA) or chromatography techniques. Has to be the most important safety test conducted. Less affected by fungi than dent corn!

10. Purity and Cleanliness

• Foreign Material Inspection: Visual or automated inspection to detect and quantify foreign materials, including stones, dust, or other seeds. The Dockage test meter is employed here to determine whether clean-up is needed.

11. Differential Scanning Calorimetry

• The method assesses the thermal properties of popcorn kernels by analyzing how they absorb and release heat. This method provides insight into the gelatinization of starch and the vaporization of water inside the kernel during popping. Kernels with optimal heat absorption profiles (e.g., proper starch gelatinization and water vapourization temperatures) tend to have higher expansion volumes. DSC can help identify these thermal properties.

12. Nuclear Magnetic Resonance (NMR) Spectroscopy

• NMR spectroscopy can be used to measure the distribution and mobility of water within the kernel. This technique provides detailed information about the moisture content and water activity within different regions of the kernel. The distribution of moisture within the kernel impacts how well it pops. NMR can help identify kernels that have the ideal water distribution for optimal expansion.
• 13C CPMAS shows that unpopped corn is a mixture of two types of starch – A type and amorphous starch. Starch granules are alternating concentric rings of amorphous and crystalline regions. A -type starch is a type of starch granule which is larger than 10 micrometres in diameter and has a high amylose content. Amorphous starch is a disordered part of the starch granule composed of both amylose chains with amylopectin branching points. Spectra of popped corn compared to cooked and unpopped corn are virtually the same because heating and then dehydration of A type starch is converted into amorphous starch. The C-Type starch contains a mix of A- and B-type polymorphs. The B-type starch granule is only found in wheat flour and is smaller than the A-type so is of little relevance when discussing popcorn.

13. X-Ray Micro-CT (Computed Tomography) Scanning

• X-ray micro-CT scanning is a non-destructive imaging technique that creates 3D representations of popcorn kernels. It can assess the internal structure, air pockets, and density distribution. This method helps to visualize the internal structure of kernels, such as the pericarp (outer shell) integrity and the arrangement of starch granules. Kernels with more uniform structures often produce better-expanded popcorn. However, at the moment it is still a technology in early infancy and may be better placed in the assessment of fungal infection for example rather than determining PEV (Orina, 2018).

14. Genomic or Biochemical Profiling

• Genomic studies or biochemical profiling of popcorn kernels can identify specific genetic markers or biochemical compounds that are associated with higher expansion volumes. Techniques like DNA sequencing or profiling could be employed. Certain genetic markers are linked to traits like starch composition and water retention, which directly influence expansion volume.

15. Machine Vision

• Machine vision systems coupled to neural networks have been used to assess  ‘hard-to-pop’ kernels (Yang et al., 2005). Visible features which reflect on popcorn quality were measured. The accuracy of the method was 75%. Better accuracy was achieved by improving the number of neurons and gathering better data on popcorn kernels. It is feasible that with further development, the method could address other quality parameters based on physical dimensions and colour. The technology is now routinely used for seed quality inspection and grading (Gong et al., 2015).
• Neural networks linked to machine vision can also be linked effectively to near infra-red inspection systems.

These analytical methods are employed to ensure that corn kernels, including popcorn, meet the necessary quality standards for safety, nutritional value, and consumer satisfaction. The choice of methods depends on the specific quality attributes being assessed and the intended use of the corn.

The Cretors Test (MWVT)

Here we discuss a really important analytical method. The Cretors Test is a standardized method for evaluating the expansion volume of popcorn, which is a key quality attribute. This test, named after the C. Cretors and Company (a leading manufacturer of popcorn machines), assesses the volume increase of popcorn kernels after they are popped. Here’s how the test works:

Key Steps in the Cretors Test

1. Kernel Measurement: A specific weight of popcorn kernels is measured before popping. Typically, a certain weight (e.g., 100 grams) of unpopped kernels is used.
2. Popping Process: The kernels are popped under standardized conditions using a controlled popcorn popper. This step ensures consistency in temperature and time across different tests. The unit is a batch-type oil popper with a cylinder into which popped corn falls.
3. Volume Measurement: Once the popping is complete, the expanded popcorn is poured into a volumetric container, like a calibrated beaker or cylinder. The volume of the popped corn is then measured.
4. Calculation of Expansion Volume (EV): The expansion volume is calculated as the ratio of the volume of popped corn to the weight of unpopped kernels. It is expressed in cubic centimeters per gram (cm³/g) or in quarts per ounce, depending on the system of measurement used.
5. The equipment is designed so that temperature and energy consumption can be measured as this helps establish the economics for specific batches of corn It is also easily duplicated with similar machines.

Types of Corn Used In Popcorn

Popcorn is actually a type of flint corn. It differs from dent and other soft corns used in other culinary products in two ways. Firstly, it contains almost entirely ‘hard’ starch. Secondly, flint corn has a very hard pericarp and ‘hard’ outer layers of endosperm, which permit the internal pressure due to steam and temperature to rise high enough for popping.

There are two distinct shapes of popcorn kernels. These are rice and pearl type and are hybrid forms of sweetcorn. Rice types have long kernels with a sharp point at the top and are usually associated with white popcorn. They are the most commonly used commercially. The pearl types are round with no sharp point at the top and are associated commercially with yellow popcorn. The pearl type has now become the dominant commercial type for popcorn (Ziegler & Ashman, 1994).

As we mentioned earlier, classification is also based simply on size – small, medium and large. Consumers have their preferences – the small yellow types  i.e. small pearl is preferred by home consumers because they produce nicer, tender flakes and have fewer hulls. The commercial producers of popcorn go for larger flakes which are tougher and more visually appealing. Sizes in between are used by both consumer and commercial manufacturers (Ziegler et al., 1984).

Other buyers of popcorn will describe 4 major types quite specifically based on similar criteria referenced  above:

(1) white hull-less – for home popping

(2) yellow hull-less, sold in kernel form for home popping but in much larger quantities where the climate is humid and hot. It retains god texture at higher moisture contents compared to other hybrids.

(3) large-kernel yellow – used for commercial popping and in the theatre trade. It forms large flakes, good resistance to handling but toughens up with humidity.

(4) medium yellow – good appearance when popped with good texture.

The large-kernel yellow hybrids produce large popcorn in size and have a creamy yellow appearance. When popped they produce a mix of the two basic shapes – 25% mushroom or ball kernels to 75% butterfly kernels. Such hybrids are popular in confectionary circles. These require more oil and need humidity-resistant packaging. They suffer from becoming chewy, elastic and tough even when they absorb a small amount of moisture.

The medium yellow kernel was developed by growers as a compromise between the yellow hull-less and large yellow hybrids. It forms a reasonably high percentage of mushrooms but not as many as a quality large-kernel hybrid. Used to make a good caramel corn or cheese corn.

The small yellow hull-less popcorn is used by home consumers because it pops at a relatively low temperature with excellent eating quality. It is preferred for low-volume operations such as cinemas and other vending locations.

Karabaha (2006) in his study examined the physical characteristics of popcorn kernels and came up with some fascinating insights. He noted the following:

• The surface area of the kernel increases with moisture content. There is a 40.16% increase in surface area from a moisture content of 8.95–17.12%.
• Bulk density of the kernel decreases and volume increases both linearly as moisture content rises

We also mentioned how genetics has such a bearing on expansion volume – it is one of the key quality attributes. Selective breeding is now conducted to obtain popcorn genotypes. Measuring kernel length was seen as a simple means of assessing a positive attribute ((Guimarães et al., 2000; Saito et al., 2021). In recent years, targeted breeding and selection has led to a significant improvement in the PEV. In fact, the expansion volume has doubled to approx. 30ml/g  when modern day cultivars were compared to some older American varieties quoted back then at 25ml/g and Brazilian varieties at 15ml/g (Zinsly and Machado, 1987; Amaral Junior et al., 2012; Galvão et al., 2015;  Oliveira et al., 2019). It is reckoned that the best popcorn varieties are sourced from the USA because these varieties have been selected for their superior PEV values.

In recent years, much more attention has been paid to popcorn germplasm than to either flint or dent ‘field corn’ cultivars. Popcorn generally should not have a lower agronomic performance but it is the case when compared to the level of attention paid to other corn (maize) cultivars. Lower agronomic performance is attributed to fewer cycles and smaller levels of inbreed-hybrid breeding. All the effort has been paid to improving traits associated with expansion volume rather than selecting for better agronomic traits and achieving a narrower genetic base (Ziegler, 2001).

The inbreds that are derived from landraces capture a sample of the vast genetic diversity, which is fundamental to the breeding program in maize (McLean-Rodriguez et al., 2021). Gopinath et al., (2024) assessed 48 inbreds for popping quality and agronomy traits. They identified inbreds for use in an Indian breeding program. They emphasized that good quality maize based on a diverse germplasm would offer a wide selection of genotypes with favourable alleles that could be targeted in their breeding programs. The same principles have and are being applied to other nation’s maize breeding programmes.

One of the main centres for maize cultivar improvement is the International Maize and Wheat Improvement Center, Mexico (CIMMYT). A great deal of study into popcorn varieties has come out of this centre. There are many other notable centres though in the USA.

In summary for this section, we know that a variety of factors such as kernel sphericity, moisture content, density and pericarp thickness are said to have significant effects on popping volume (Sweley et al., 2013). If there was an optimum based on all this research it would be that the maximum expansion occurs in popcorn genotypes with the following kernel characteristics: >70% kernel sphericity, small to medium kernel size and minimal opaque endosperm core with 13.0-14.5% kernel moisture.

At the biochemical level, the quantitative amount of linoleic acid and protein such as α-zeins positively affects popping expansion volume (PEV) and flake volume (FV) (Borras et al., 2006). [More on proteins later]. At the genetic level, popping quality is a complex quantitative trait affected by just a few major genes (Ziegler, 2001). Coan et al (2019) found the main quality attributes including expansion volume were based on genetic effects associated with a few dominant genes. Dominance genetic variation and additive variation have been suggested as the way forward to improving PEV for example. This improvement occurs by the transferring of favourable alleles from good quality normal maize to popcorn using  a method of two generation-based backcross breeding. This is enough to recover the PEV from “recurrent” popcorn parents (Ren et al., 2018). Popping quality is said to be selected using molecular markers linked to PEV.

 One of the most difficult jobs in developing a popcorn breeding programme is checking the phenotype of a large number of genotypes when assessing their PEV. It takes time and resource, it means destroying the kernel to enable a phenotype to be analysed because heating the kernel means using microwave energy to rupture the pericarp. Non-destructive methods are now being developed.

Non-destructive methods such as NIR have been used to predict the composition of kernels which has led to rapid selection of individual seeds with desirable traits. It has also meant assessment of starch types, protein and oil contents. The main NIR spectra absorption bands are believed to be associated with the presence of lignin (1116 nm and 2200 nm), hemicellulose (1300 nm), cellulose (1854 nm and 2026 nm), and starch (2404 nm). Their presence reflects the glassy state. Maize kernels which are graded as hard, intermediate, and soft kernel types contain both glassy and floury endosperm in different ratios (Williams et al., 2009). The hard kernel means the endosperm is primarily glassy in nature whereas in soft kernels the starch is more like flour.

Starch content and type has been mentioned a few times in this post. There is still plenty of debate over the importance of the chemistry of starch such as its overall composition, structural organization, the arrangement with proteins, intercellular spacing and compaction of starch granules (Lee et al., 2014).

Higher amylose contents correlate well with high popping expansion (PEV). The amylose content is between 24 and 26% in these types of popcorn and the PEV was between 30 and 35 ml/g. Where the popping expansion was lower, between 20 and 23 ml/g, the amylose percentage was lower and this correlation was the case too with increasingly lower expansion volumes (Freire et al., 2020).

It is possible that amylose content also determines endosperm hardness (Dombrink-Kurtzman et al., 1997). Where the endosperm is hard, the peripheral amorphous region of starch is larger. Compression of this type of endosperm produces a higher amylose content (Bemiller et al., 2011). Where the endosperm is floury, starch granules contain more amylopectin and appear to be less subject to compression. Amylopectin appears to correlate negatively with PEV (Sweley et al., 2012; Lee et al., 2014). 

Protein content (PC) also has a bearing on PEV. A very recent study highlighted earlier using the USDA-ARS tube SKNIR instrument can sort out individual kernels according to their PC level (Wu et al., 2022). A higher protein content also produces a higher expansion volume. Borras et al. (2006) thought that the influence of key proteins such as alpha-zein and glutelins were best correlated with optimum PEV and to endosperm hardness. The protein content (PC) of popcorn was relatively higher compared to normal dent corn. This was attributed to the presence of tightly packed starch granules (hard starch) throughout the endosperm implying that the associated protein was significant. {I assume they had not confused starch, a carbohydrate with protein} (Wu et al., 2022). Borras et al., (2006) also noted that the protein elution could be a marker tool for discriminating between flint, dent and popcorn types.

The number of unpopped or half popped kernels decreases as kernel PCs increase. In the Wu study (2022), the degree of unpopped kernels was more directly correlated with PC rather than MC.  Not everyone accepts these findings though. They point back to work by Soylu and Tekkanat (2007) who found protein content and PEV were uncorrelated. It is worth noting that USDA-ARS, Manhattan is developing a sorting model using moisture content and protein content to accurately select individual kernels.

The fat content of kernels also has a part to play in quality. Borras et al., (2006) not only looked at zein content (i.e. protein) but also particular fatty acid values. They studied seven Argentinean popcorn varieties which may or  may not have a bearing. For example, there was a negative correlation between oleic acid content and popping volume but a positive one linoleic acid. The starch in these varieties were different from those in ‘normal’ corn but it wasn’t clear how. Why the type of fatty acid would affect expansion volume was not clear either.  .

A number of studies based on QTL have mapped PEV, FV and PR in maize (Babu et al., 2006). In the case of popcorn, the mapping did not lead to any conclusive results. Meta-QTL analysis is the main approach. A number of meta-QTLs for popping quality have been identified on chromosome 1 in maize (Yongbin et al., 2012; Kaur et al., 2021). The dent x popcorn crosses have a thick pericarp but when they explode the popcorn shatters into small pieces producing a poor sensory experience. Breeders have started selecting genotypes with a slightly thinner pericarp (Ziegler, 2021) to optimise pericarp thickness.

• Butterfly/Snowflake Popcorn

If you want butterfly or snowflake popcorn then choose the following. These are the varieties that  pop into irregular, light, and fluffy shapes, commonly used for movie theater popcorn and snacks.

1. White Cloud
2. Lady Finger (also known as Tom Thumb)
3. South American Yellow
4. White Cat
5. Dakota Black
6. Japanese Hulless
7. Baby White
8. Tender White
9. Yellow Pearl
10. Yellow Butter

A good commercial yellow popcorn which is widely available under its brand name of Bonnie Lee is available from Weaver Popcorn Co., Inc.. I notice it is available in Sweden for example. White Cloud is a very early variety and is best adapted for northern regions. It produces white popcorn kernels and is available from WH Perron. Sunfresh Foods Ltd ., Toronto, Canada produced a commercial variety called Sunspun which is probably not available anymore.

• The Mushroom Popcorn Cultivars

These varieties pop into round, ball-shaped kernels, often used for confections like caramel corn or kettle corn because they are less prone to breaking.

1. Mushroom Pearl
2. Red Ruby
3. Robust 128YH
4. Robust 147
5. Super Mushroom
6. Caramel Crisp (more a brand type but included because it is sometimes offered as a varietal).
7. Big Ball
8. Butterfly Popper
9. Hybrid Mushroom
10. Monster Mushroom

Robust varieties are available from most seed suppliers and these are grown successfully in the UK as well as North America.

– Specialty Popcorn Cultivars

These varieties often feature unique colors or kernel traits.

1. Black Jewel (produces dark-colored kernels that pop white)
2. Blue Hopi
3. Strawberry Popcorn (small, red kernels)
4. Glass Gem (multi-colored kernels, used as ornamental corn but can also be popped)
5. Calico (multi-colored kernels)
6. Red Popcorn
7. Purple Passion

– Commercial Hybrid Popcorn Varieties

Large popcorn companies often develop their own hybrid cultivars for specific performance traits, such as high expansion volume, pest resistance, and better shelf life. Some of these commercial varieties may not be widely available for home gardeners but are essential in the commercial popcorn industry.

1. Robust 48-93
2. Midwest Hybrid
3. Red River
4. Yankee Hybrid

– Organic and Heirloom Varieties

Some growers focus on producing heirloom varieties that have been preserved for their unique flavors, colors, and popping characteristics.

• Purdue 39W
• Heirloom White
• Cherokee Long Ear
• Reventador

India has a burgeoning popcorn industry and it has highlighted various composite varieties such as VL Amber Popcorn, Pearl Popcorn, Jawahar Popcorn 11, KDPC 2 (Shalimar), and Bajaura Popcorn. There are also a few unnamed hybrids of interest.

Processing Of Popcorn

The method of popping corn that we have all grown up with is to heat dried popcorn kernels on the base of a pan with the lid and watch them jump against the lid. The sound of them popping against the lid is the sound we come to listen for and expect. Nowadays, popcorn is popped by air- and by oil-frying.

Studies in the 70s and 80s showed that the moisture content of the kernels was the dominating factor. The degree of popped volume and the number of kernels that actually popped  was dependent on this factor. It didn’t matter either how the heat was being delivered to the kernels (Haugh et al., 1976; Hoseney et al., 1983).

In recent times we have developed an interest in microwaveable popcorn. Microwaving is now highly successful and used commercially as well as domestically. The issues with microwave use are the following (Schiffmann, 1986a):

• having a larger number of unpopped kernels left
• low expansion volume
• scorching and burning of popped kernels if microwaving is too length

Using oil in the microwave brings its own issues. Oil of any sort has a relatively low dielectric constant and loss factor (Pace et al., 1968; Mohensin, 1984). It also has a low specific heat however which means there will be a very rapid heating in a microwave oven (Schiffmann, 1986b). The higher temperatures of oil surrounding the kernel normally produces a lower unpopped kernel ratio. The main issue though is that oil causes slower gelatinization of starch hence the lower expansion volume as a result.

The Biochemical And Physical Effects That Produce Popping

Popping occurs between 180 and 190ºC. It is equivalent to a steam pressure of 9.3 bar (135 psi) or 930 kPas within the kernel. The physical process of popping occurs when moisture which is trapped inside small spaces left by densely packed polygonal starch granules suddenly builds up pressure. The pressure is generated internally by water vaporizing to become superheated.  At first, this water vapour is held within by the confines of the thick pericarp when it becomes superheated but a critical vapour pressure is reached at which the pericarp breaks.

The kernel’s pericarp releases the superheated steam becoming steam at the instant of popping.  The change in phase involves an extremely rapid expansion within a microsecond. This is the part of the physical change that provides the driving force for further expansion of the endosperm. The water vapour is also forced into the starch granules under pressure (Van der Sman et al., 2017). The starch incidentally melts in the presence of superheated steam but with rupture, the starch granules also expand rapidly where they gelatinize along with crystalline melting. The result is a three-dimension network of exploded starch as flakes are formed (Hoseney et al., 1983).

With the opaque endosperm, the starch granules are never hydrated because water vapour is unable to penetrate them. It simply enters the voids amongst them. There is no gelatinization during popping and so they just move apart from each other. In the translucent or vitreous endosperm, the starch granules are highly expanded which produces the flake formation.

The interest in the pericarp and any other outer layers of the kernel is because they are integral to the popping action. They are a pressure vessel enclosing the starch of the endosperm. The ideal endosperm for popping comes from having a kernel which is rich in type A amylose, with tightly packed granules of a polygon shape with few if any voids that give a vitreous appearance. The polygon shaping also have an angular shape. This vitrous endosperm also has a dense protein matrix and the protein bodies are ‘well-structured’. The endosperm walls are also firm if not hard.  What is not desired is a floury endosperm!

The interest in moisture content comes from how it influences expansion rate and volume. We know from early empirical measures that the ideal is between 13.5 and 14% w/w. The most in depth study was by Gokeman in 2004 who investigated popping methods and moisture content in 5 different genotypes of popcorn. The popping methods were cooking pan, microwave oven, hot-air popper, with and without salt and oil.  The optimum moisture content was 14%. Microwaving produced the best results with highest flake size and lowest number of unpopped kernels. The cooking pan method with salt and oil produced the worst expansion volume and flake size. A medium kernel size was best for for PEV and flake size.

Whilst 14% moisture content (MC) is optimum for one study, an earlier study showed that an 11% MC was ideal (Allred-Coyle et al., (2000) and another in 2009 (Ertas et al., 2009) found the range of 12 to 14% was suitable. The reality is that everyone has slightly different test methods and ways of adjudicating on popcorn performance. This may then come down to whether a dry or wet-popping method is used (Serna-Saldivar, 2008)!

Dry-Popping

Popcorn kernels are placed in a rotary wire drum or steel auger equipment and heated to between 230 and 250ºC for between 65 and 105 seconds. The moisture is reduced to 1.5%w/w. The dry-popped kernels are sifted in a rotary drum or sifting table. The sifted flakes are then coated in salt and oil, flavourings, caramel etc. in either a batch or continuous system. They are dried slightly before packing.

Wet-Popping

In a wet-popping process, the kernels are agitated at a slightly lower popping temperature of 170 to 185ºC for 120 to 150 seconds. The moisture level is also reduced to 1.5%. The same process as for dry-popping is followed but this popcorn is usually just salted.

Wet-popping  is preferred for popping at the point of sale and relatively inexpensive. 

Popcorn Suppliers

Weaver Popcorn is still the largest bulk popcorn producer in the world. It sells under the Pop Weaver brand. It produces over a third of all popcorn and distributes to 90 countries worldwide.

Product Development and Packaging

Popcorn has become a symbol of healthier eating in the Western world and especially the USA. To meet demand and maintain sales volumes, new flavours have been regularly sought which take consumers away from classical flavours like butter, dairy and cheese. We will see a number of products and brands which have moved into very different quirky flavour systems.

In terms of standard ingredients, ready-prepared popcorn will probably be coated with palm oil and salt and then natural or synthetic flavours are added as part of that coating. Cheese and milk based flavours are popular because of the association with dairy. To generate sweetness, popcorn is coated in sugar and other flavours. I’ve discussed caramel coatings for popcorn elsewhere but this is a really popular combination.

Antioxidants for added stability comes in the form of BHT and vitamin E but rosemary extract also works well. Colours come in the form of paprika and annatto extracts which provide pale yellows and oranges.

Butter flavours are incredibly popular still!

 In December 2020, Safe + Fair (USA) produced a range of whole grain, allergen free or friendly snacks which were also non-GMO and gluten-free. What really caught the attention was the flavour –Dill Pickle Seasoning! This followed a more conventional offer of Sea Salt . In keeping with the health trend, their dill-flavoured popcorn has 43 calories per cup whilst the sea salt version has 47 calories!

A recent trend in flavour and experience is to recreate the type of product that is associated with that great occasion, the movie theatre. You can now buy popcorn that has ‘movie theatre butter’ flavour. You can purchase in the USA, Pop-Secret which ‘bring the movie theatre home’. That is a clear reference if ever there was one to an occasion of great significance to the American public.

• Purchase Pop Secret Microwave Popcorn, Movie Theater Butter Flavor, 3 Oz Sharing Bags, 30 Ct online

When it comes to claims then popcorn is positioned nowadays as very healthy and to remove the ultra-processed food stigma that has been built up lately. Pop-Secret (USA) have made the following claims for their brand:-

• 0g Trans fat per serving
• 100% whole grain
• Made with non-GMO corn
• No high fructose glucose syrup

Popped popcorn is packaged usually in cardboard boxes but also in packaging that is microwaveable. If you want it ready prepared then it is possible to find popcorn in plastic packaging too. 

Popcorn Brands

I’ve taken a look at some of the brands currently available in the market place which are now offered by localised producers as well as some of the bigger players in the market place. In the USA there are a large number of exceptional brands. If you are interested in the brands then look at those from Conagra Brands, Snyder’s-Lance (as Diamond Foods) and PepsiCo under the Frito-Lay brand name. A few other global players are Amplify Snack Brands and the Weaver Popcorn Company.

Amazon produce their own label popcorn which is available through the Whole Foods Market.

One famous brand called Orville Redenbacher’s was founded in 1952 who asked a select group of farmers to grow particular popcorn varieties. That tradition continues to this day. It is the only leading brand that professes to use real butter in its formulation. It is also the leading brand of microwave popcorn and has no artificial colour, flavour or preservatives in its formulation.

UK Brands

In the United Kingdom we have the major brands but some of our own unique players such as Joe & Seph’s.  So Joe and Seph’s have become mainstream in the UK certainly with a range of flavours which is constantly being updated.

One of my favourites – Lord Poppington is a branded offering from Savoury & Sweet (acquired in 2018 by Burts Potato Chips Ltd). Grabbed as many bags as I could reasonably hide at an IFE  exhibition ba, simply to try the range of flavours they offered. Lightly sea-salted, Sweet and Salty, Chilli and Lime, and Four Cheeses are available. The web-site is informative and well-arranged, drawing on aristocratic British themes with a slight quirkiness. The flavours don’t appear too overpowering, especially in sweetness. The Chilli and Lime flavour variant probably provides the strongest flavour combination and most interesting.

Another great UK brand is Popcorn Shed which contains bits of nuts, chocolate and dried fruit so we have a taste of texture as well as flavour. They have Berry-licious, Salted Caramel, Butterly Nuts, Pop’n’choc, Say Cheese! and finally Sweet Cheesus.

More localised producers include Yorkshire Crisps who also offer their particular brand called ‘Yorkshire Popcorn’ that comes in a salted and a luxury toffee flavour.

 

 

Incidentally, if you are keen on growing your own popcorn then I would recommend a number of sweetcorn varieties which will do the job for you.

 

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