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Laws of Thermodynamics
Zeroth Law states that if two thermodynamic systems are each in thermal equilibrium with a third, then they are in equilibrium with each other. Putting it in algebraic terms, if A = C and B = C, then A = B. The First Law of Thermodynamics is that energy can neither be created nor destroyed. It can only change forms. In other words, the net heat given to a system equals the net work done by the system. The Second Law of Thermodynamics states that heat energy moves from less structure to more structure. In other words, heat will always be transferred from a higher source which is hotter and more disorganized to a lower one which is colder and more organized, until the two are in equilibrium. The Third Law of Thermodynamics is that as temperature approaches absolute zero, the disorder and randomness of a system approaches a constant minimum. In other words, at absolute zero, molecular movement is almost fixed in time and space.
Types of Heat Energy
Conduction is heat energy transferred from one thing to another through direct contact. Convection is heat energy transferred through indirect contact, for example through the flow of air or water. Radiation is heat energy transferred from one object to another through space by electromagnetic radiation.
Phases of Matter
There are five main phases of matter: solid, liquid, gas, plasma, and Bose-Einstein Condensates. For the purpose of this unit, I will focus on the first three. The main difference in the structures of each state is in the densities of the particles. In the solid phase, matter has a definite shape and volume. The particles that make it up, either atoms or molecules, are joined together by strong bonds which give it a fairly tight structure. Sometimes the bond arrangements create a repeating pattern known as a crystal lattice. Quartz, granulated sugar, table salt, and ice all display a crystal lattice pattern. From the outside, solids look static but inside, the molecules continue to vibrate but stay in place relative to each other. In some cases, like water, volume may be larger in the solid state than the liquid state due to the arrangement of the molecules. Matter in a liquid phase has a definite volume but no definite shape. The bonds are weaker than in the solid phase and the particles are able to move further apart. While volume remains constant, liquids take on the shape of their container. In the gas phase, matter has neither shape nor volume. The forces attracting the particles are so weak that the atoms and molecules can spread out to fill the available space.
Elements and compounds can move from one state to another when specific physical conditions change. A sufficient change in energy and/or pressure can cause a phase change. Most elementary students are familiar with “melting” and “boiling” to describe the state changes from a solid to a liquid and a liquid to a gas, respectively, as they are things they experience in their everyday lives; ice melts and water boils. They may also know “freezing” and “condensing,” liquid to solid and gas to liquid, from learning about the weather or the water cycle. Most, however, do not know that “sublimation” describes going from a solid phase to a gas phase, as in dry ice. When demonstrating sublimation with dry ice, I also discuss condensation because carbon dioxide gas is colorless and odorless. The smoke that students see is the water vapor in the air changing state from a gas to a liquid as the heat is transferred to the colder carbon dioxide gas.
The easiest way to add energy is to add heat. The more energy the atoms and molecules have, the more they move. When a substance transfers its heat, it loses energy and the particles move closer together. A second way to cause a phase change is to increase pressure. Examining the ideal gas law will help us to understand the relationship between pressure, volume, and temperature. The kinetic theory states that all gases are made up of very small particles with no forces of attraction or repulsion between them. These particles move randomly, independently, in straight lines until colliding with the walls of their container or each other, which causes them to mix uniformly. The temperature of the gas reflects the average kinetic energy of the gas particles. When looking at the mechanism of popping corn, the two most important variables are pressure and temperature. Since the water is contained within the pericarp, the popcorn hull, volume is constant. Gay-Lussac’s Law states that the pressure of a given amount of gas held at constant volume is directly proportional to the Temperature in Kelvin units. As temperature rises, the pressure of the gas, water vapor, rises.1
The Kelvin scale is the logical extension of the relationship between temperature and volume. At absolute zero, theoretically there is no molecular movement. Each unit on the Kelvin scale equals one degree on the Celsius scale. Although my students will use the Celsius scale for collecting and analyzing data, I will give them the conversion formulas: F = C x 9/5 + 32 and C = (F – 32) x 5/9
Research
What makes popcorn pop? The short answer is water found inside the kernel. Yet science is as much, if not more, about the questions rather than just the answers. Lots of foods have water inside them, but they don’t pop in the microwave. Or do they? Consider a watermelon; it has so much water that it is even part of the fruit’s name, yet we don’t put it in the microwave. Maybe it’s a question of seeds. We don’t expect the fruit to pop; however, neither do the watermelon seeds. In fact, there are very few seeds that meet the structural conditions needed to pop; amaranth is one, but none of them do it as well as popcorn.
History of Popcorn
Who invented this culinary marvel? Archeological evidence suggests that popcorn is a naturally-occurring variety of corn. In the 1940s, the Rockefeller Foundation, in conjunction with the Mexican Ministry of Agriculture, collected more than 2000 varieties of corn, maize, from Mexico. Among those listed were four “ancient indigenous” races of corn: Palomero Toloqueno, Arrocillo Amarillo, Chapalote, and Nal-Tel based on ear and kernel characteristics. Like modern popcorn varieties, all four have pine cone shaped ears and small dense kernels. Archeological findings support their claim of antiquity. Brown Kernels about 4000 years old related to Chapalote were discovered in the Bat Cave of New Mexico. In La Perra Cave, in northeastern Mexico, 2000 year old cobs related to Nal-Tel were found.2
There are five main types of corn: flint, sweet, dent, flour, and popcorn. Flint corn (Zea mays indurata) has kernels with hard outer shells and can range in color from white to red. This is the type associated with ornamental corn. It can also be used as animal feed. Central and South America are the largest growers of flint corn today. Sweet corn (Zea saccharata or Zea rugosa) is the type most eaten by people, either fresh on the cob or processed as a frozen or canned food. It gets its name because it has more natural sugars than other types of corn at the same developmental stages, 10% for sweet corn compared to 4% for field corn. However, once picked, the sugars begin converting to starch, losing up to 50% of its sweetness after one day. Dent corn (Zea mays indenata) is also known as field corn. It gets its name from the indentations on the kernels at maturity. It can be white or yellow, contains both hard and soft starches and is usually used as animal feed, in processed foods and in various industrial products including ethanol. The fourth type’s name flour (Zea mays amylacea) explains its use. It has a soft, starch filled kernel, that easily grinds down to produce the starch flour that is used in many different food preparations. In addition to supplying nutrition, starch is what gives cooked food its shape. It is usually white but can also be other colors as in blue corn tortilla chips. Flour corn is one of the oldest cultivated types and the one mostly grown by Native Americans. Last, but certainly not least, is popcorn (Zea mays everta). Popcorn is related to flint corn in that it has a hard exterior shell, known as the pericarp. Inside is a soft endosperm and water.3
Agronomists classify the popcorn endosperm as everta because it turns inside out, or everts, when heated. The water inside the kernel is changed to steam. The increased pressure causes the pericarp to split and the starchy endosperm flakes producing the treat we call popcorn. In other types of corn, either the steam escapes gradually or the kernel splits without exploding.
That the everta class has remained distinct over so many centuries is due to a quirk in its genetics. Although its pollen can fertilize other types of corn, most but not all evertas have a gene, Gametophyte Factor I, that inhibits it from being pollinated by other types of corn. The gene comes in 2 forms, Gal-M gene slows down the pollination by other species, allowing time for everta pollen to sneak in and do the job. Gal-S gene is even more effective by completely blocking other varietal pollen.
Breeders use this one-way flow of genetic information by breeding their everta with purple dent corn. If the resulting ears have the purplish tinge then the everta lack the desired gene. If, however, the pigment is missing, then the trait is there. This test allows organic seed corn producers to exclude genetically modified pollen from dent and sweet corn types. As of this writing, although many other varieties of corn have been genetically modified to be more drought and disease resistant, the popcorn variety of corn has not.4
Physical Description
A popcorn kernel has three important parts: the pericarp, also known as the seed coat or hull, the endosperm, and the embryo. The pericarp provides protection for the endosperm and embryo. In the case of popcorn, damage to the pericarp can mean a loss of water and less popping. The endosperm fills most of the kernel and is the main energy source for the developing seedling. The embryo contains the miniature plant. It also contains a cotyledon, which provides energy for germination.5 Popping corn kernels have a dense, hard translucent endosperm with a tiny bit of soft endosperm next to the embryo. Of all the grain corns, popcorn types have the smallest kernels, appearing as either pearl type or rice type. Pearl have a rounded top whereas rice come to a point. Some varieties may produce ears with both types but a single ear is always homogenous. There are anywhere from 14 to 28 rows of kernels on a cob and they appear in a variety of colors. The color of the kernels has no effect on the color of the popcorn as it is the white endosperm that explodes. So, while the gourmet colored popcorn will look prettier than its plain golden yellow cousin when displayed in the glass jar on the counter, there will be no color difference in appearance when popped. When fully dry, the kernels and pedicel, the portion of the pericarp attached to the cob, easily detach but the brittle nature of the kernels make gentle handling a must. Kernels with damaged pericarps will not pop as well because the steam inside will escape through the cracks before building up enough pressure to cause the endosperm to explode.6
Modern Cultivation
Commercial popcorn cultivation has existed in the United States for more than 150 years. In 1865, Fearing Burr (a noted seeds man) listed 5 “parching corn” varieties available: two with white kernels, two with yellow and a red rice type. As of 2011, the Seed Savers Exchange inventory listed 31 varieties of popcorn. Commercial popcorn today is bred for their flake types and popping expansion. The more the starchy endosperm expands, the softer the piece. There are two types of flakes: “Butterfly” and “Mushroom.” Butterfly popped kernels are more angular, softer and are generally the shape most associated with the popcorn made at home and eaten in theaters. Mushroom type is compact and more rounded. Since it has less breakage, it is better suited for being kettle flavored i.e. caramel corn. Mass producers tend to favor hybrids that produce a bland neutral flavor so as not to interfere with the added flavorings. 7
Recently, there seems to be a resurgence in interest in heirloom varieties and a backlash against the hybrids. In the New York Times article “Heirloom Popcorn Helps a Snack Reinvent Itself,”8 Kim Severson suggests that this may be the latest example of popcorn’s renaissance. In the 1880s, steam popping machines brought the treat from the farm kitchen to the cities. It was an integral part of movie escapism in the Great Depression. Jiffy Pop, with its ease of preparation, brought new life to the snack in the late 1960s after television replaced going to the movies. The nature of the popping mechanism and the way that a microwave works were a perfect match. Most microwave ovens still have a popcorn setting. A walk through the supermarket aisle reveals at least a dozen different brands and types of microwave popcorn. Yet even as the popularity seemed to be increasing, there were a few people wanting something different: a popcorn whose taste comes from the seed without any added oils, butters, flavorings, or salts. Gene Mealhow, farmer and owner of “Tiny but Mighty” heirloom popcorn, blames Orville Redenbacher for the change in emphasis from taste to size. Redenbacher, a 1928 graduate of Purdue University, used genetic material developed through his university’s alumni seed improvement association to create a hybrid seed that produces bigger kernels with a higher popping rate—all at the cost of taste. According to Mealhow, “Orville produced a giant popcorn to be a delivery vehicle for butter and salt.”9
As part of developing this unit, I obtained several different types of popcorn kernels to see for myself what, if any, differences existed. I won’t influence the reader by sharing my opinions, but the experience did affect my planned teaching of this unit.
Globally, the United States is the largest producer of popcorn, but we consume 90% of it almost exclusively as snack food. This leaves only 10% for export. Argentina ranks second in production, though their yield is approximately half of the U.S. total. Their domestic use is much lower, being largely limited to public places like movie theaters and bars but little home consumption. As a result, they export 95% of their yield, making them the largest exporter of un-popped corn in the world.10
Ways to “Heat” Popcorn: Historical to Present
If we had a time machine, we’d probably discover that the first instances of “popping” corn were accidental. Tomie DePaoli’s Popcorn Book, an almost mandatory elementary school read around Thanksgiving, states that Native Americans used to throw kernels directly into a fire and then try to catch the morsels as they popped and flew back out. While it seems fanciful, scientifically it is possible. However, De Paoli also depicts popcorn as being served at the First Thanksgiving.11 According to Stephanie Butler, writer for the History Channel, there are no contemporary accounts that reference popcorn. What she does verify is that in 1612, French explorers saw some Iroquois women popping corn in clay pots. They would fill the pots with hot sand, then add the kernels before stirring it with a stick. When the corn popped, it came to the top of the sand.12 Holding the dried ear of corn over a fire is another method presented by de Paola and verified by National Geographic.13 However, I wasn’t able to verify tossing the kernels directly into the flames as a cooking method.
Until the invention of the microwave, all methods of popping corn were basically the same. Heat was applied to the outside of the popcorn kernel, either directly or through oil. That heat energy was then conducted through the pericarp, the hull, and transferred to the starchy endosperm and water found inside the seed. Two physical changes occur. The endosperm becomes a gelatinous starch that solidifies when released. The water molecules go through a change of state from a liquid to a gas. When the pressure of the water vapor exceeds the internal strength of the pericarp, the kernel breaks open, turning itself inside out in the process with the starchy endosperm now the fluffy outside and the brittle pericarp reduced to almost nothing.
The first widely successful commercial poppers appeared in the United States in the late 1800s. Just as Edison is associated with the light bulb, Charles Cretors was the man for roasting and popping machines. He wasn’t an engineer but a shopkeeper who sold candy. Disappointed with a peanut roaster he’d purchased, he decided to make a few adjustments. He was so successful that one of his providers decided to sell his improved version, thus starting C. Cretors and Company, which continues to make popcorn poppers today largely for commercial venues. The history of popcorn machines is fascinating and there are even two museums devoted to them: J.H. Fentress Antique Popcorn Machine Museum in Holland, Ohio and the Wyandot Popcorn Museum in Marion, Ohio.14 A common element of most of the early machines was the use of oil to enhance the flavor and the aroma and it is still the method used most often today in non-microwave popping.
Contemporary non-commercial popcorn poppers fall into two main types: those that use air and those that use an “arm” to move the kernels along and away from the heated surfaces. Rather than discuss both, I want to focus on the air popper as it generates the most student misconceptions. The name is misleading as it gives the impression that the popping comes from heating the air rather than the kernel. Like other stand-alone electric poppers, the kernels are heated by coming in contact with a metal plate, either directly or through the heated oil. Under the plate is a heating element which is connected to both a thermostat and a thermal fuse. The thermostat consists of two different metal arms that react differently to changing temperatures. As the thermostat gets heated, one arm will expand and break the circuit as it bends away from the other. As it cools, it returns to position and current can flow again. It’s what helps regulate the temperature for optimum popping. The thermal fuse is a safety feature. In case of dangerous overheating, it melts and the circuit is permanently broken until a replacement piece can be installed. The purpose of the air is to move the popped kernels away from the heat source. Since they have greater surface area, they are lifted out of the way, making room for the un-popped kernels. This is done with a small fan powered by a direct current motor. Four diodes, placed on the circuit board, keep the current moving in only one direction, in effect, turning the alternating current from the wall to the direct current needed to power the motor. As the motor spins, it turns a small plastic disc with fan blades molded into it. The air then escapes from slots in the heating element portion.15 In a rotating “arm” model, the oil, if used, heats up first and then the heat energy is transferred, conducted, to the popcorn seed. The arm helps to insure more even heat by turning the kernel over and moving the lighter popped pieces out of the way of the heavier un-popped kernels.
Microwave energy works completely different. Instead of the heat being applied externally, the rays directly excite the water molecules inside the pericarp. Popcorn played an important role in the development of the microwave oven as a cooking device. The first patent for microwave popcorn actually belongs to a company that provided magnetron tubes, an essential part of the radar systems used in World War II. The story goes that one day, Percy Spencer, a scientist at Raytheon, was experimenting with magnetrons in his lab, looking for new uses. He noticed that the candy bar in his pocket had melted and wondered if it was due to the device. He sent out for popcorn kernels, put them in a paper bag and held them next to a magnetron. Voila! Popcorn. Spencer then built a rudimentary metal box with a magnetron in it, creating the first microwave oven. After researching further and conducting more experiments, Percy Spencer filed for and successfully received a patent in 1945. For his invention, Spencer received no royalties, but he was paid a one-time, two-dollar gratuity from Raytheon—the same token payment the company made to all inventors on its payroll at that time for company patents.16 Before you think bad of Raytheon, it was and still is the way things work in industry. As a research scientist with Hercules, my father developed many innovative processes and was paid the same two dollars for each time he had his name on the patent.
In 1954, Raytheon produced the first commercially built microwave oven which was a little under six feet tall and weighed around seven hundred and fifty pounds. It cost between two and three thousand dollars and was initially used in restaurants, railways and ships as they were too bulky and expensive for home use. It also had some shortcomings; for instance, meat, while it cooked, wouldn’t brown, and not all things heated evenly. After further research and modification in design, the first microwave oven for home use was put on the market by Amana, a Raytheon subsidiary, under the name “RadarRange” in 1967. It could be had for around five hundred dollars and fit on a kitchen counter.17
As I researched the origin of the microwave oven, I was taken back in time to Christmas of 1971. There was a large wrapped box sitting in the dining room addressed to my mother from my father. For weeks, she had tried to guess its contents: sheets and towels, clothes, a whole lot of smaller boxes inside one big box. Each time, my dad only smiled and said no. He was definitely more excited than she was when the gift was finally opened: a brand new microwave oven! I can still remember the first thing we made in it, cups of water for either tea or hot chocolate. Yes, we could have used a tea kettle, and usually did, both before and after, but this was a new way of cooking.
I’ll confess that up until designing this unit, the microwave oven had remained a magic box to me. So how exactly does a microwave oven work? It all starts with electricity, a transformer and a magnetron. The alternating current from the wall needs to be transformed to a high voltage direct current of 2000-3000 volts which is the job of the transformer. This current is then sent to the Magnetron, which is a kind of vacuum tube. Before there were transistors, many electronic devices had tubes. They are made of glass with a filament that allows for a controlled flow of electrons. Inside the magnetron, voltage is applied to a filament sending a stream of electrons down the inside of the metal tube. As the electrons move, circular magnets cause them to spiral. The spiraling beam passes by holes in the side of the tube which causes high frequency radiation to be emitted from the end of the tube. This radiation has a wavelength of about 12 cm, 5 inches, and can pass through glass and transparent plastic, which is why glass lids aren’t a problem. The radiation, however, doesn’t pass through food but is absorbed by it, causing the food molecules to vibrate. They vibrate because they are not symmetric which produces a positive and a negative end, which aligns with the electric field. Since the electric field created by the microwaves is constantly changing, so are the food molecules. Think of it as watching a magnet flip over and over, as a stronger magnet’s pole orientations keep shifting. The back and forth motion creates the heat that cooks the food. To help things cook more evenly, many microwave ovens have a fan that interrupts the flow of radiation to spread it more throughout the cooking chamber.18
That explanation helped a little in identifying the hidden parts of the microwave oven, but I still struggled with the science part of it. I readily tell my students that I don’t have all, or even most, of the answers, but my own observations of how different foods heated told me that I was missing something important in the way microwave ovens work. I also needed to clarify what electromagnetic radiation was and how microwaves were different than other forms of radiation. Electromagnetic radiation are waves of pure energy that travel through space at the speed of light. The properties of electromagnetic radiation waves are determined by their specific wavelength and energy. It is an inverse relationship; the shorter the wavelength, the greater the energy. When comparing the potential cooking power of two microwave ovens, one needs to consider both the power output, which is usually between six hundred and nine hundred watts and the size of the cooking space. Magnetron power is a measure of watts produced not the amount of electricity used. To compare efficiencies, you need to divide power by cubic feet.19
So why doesn’t all food heat up the same? The answer is water content, or something else that is hydrophilic, attracted to water and also polarized. The molecular structure of water is a dipole, which means that it lines up with an electric field, like a compass. The microwaves produce an electric field that shifts polarity almost five billion times per second. Likewise, the individual water molecules shift their orientations. This causes them to bang against each other and gain speed. Faster moving molecules mean hotter molecules. Although they have a lot of energy, microwaves don’t penetrate very far which is why there are two other heating mechanisms occurring. Think back to popping corn. Even though there are recommended times, we use our ears to decide when we should stop the cooking.
The radiation causes the water molecules in the food to heat up. Even before they pop, kernels get warm. This warmth is then conducted to the adjacent less hot molecules; The Second law of Thermodynamics in action. At the same time, the water is being turned into steam. In most cases, the water vapor goes through the rest of the food, transferring its heat. In the case of popcorn, the steam builds up inside the pericarp until the internal pressure exceeds the hull capacity, approximately 135 psi, pounds per square inch. At that point, the hull ruptures, releasing both the steam and the starchy endosperm. The steam now acts as convection heat so that the remaining kernels are being heated from within and without. After the initial few pop, we hear most of the rest pop shortly after. When popping slows down, it is because the bulk of the steam energy has been used. A way to demonstrate this is to pop popcorn in a microwave safe container with a lid and without. Since we know that the waves can penetrate glass, any difference in efficiency will be due to the steam and not the microwaves.
As Spencer had demonstrated back in 1945, microwave energy could be used to cook popcorn but it would require a change in the food packaging to increase its popularity. The first patent for a microwave popcorn bag was issued to General Mills in 1981, and home popcorn consumption increased by tens of thousands of pounds in the years following.20 The modern microwave popcorn bag was actually the center of a very big lawsuit between Hunt Wesson, Orville Redenbacher brand, and General Mills, Act 11 brand. While it is possible to use a plain folded paper bag to hold the kernels, companies soon saw the added value of creating a better vessel. People loved the convenience of Jiffy Pop as it contained the oil, popcorn, and seasonings all in one. What if the same could be done with a coated bag that could be used in a microwave oven? Act 1 popcorn went so far as to use real butter, oil and salt but that required refrigeration. Act 11 solves the problem by using synthetics. Adding flavoring agents also meant coating the inside of the bags. Flavor was better but there were still too many un-popped kernels. What was needed was a way to distribute the heat more evenly. Although microwave ovens operate at higher efficiency ratings and can cook food quicker, the nature of the heat means that the waves are not always evenly distributed. To get that effect, you need a material that absorbs a small portion of the radiation and converts it to heat.21 Metal, used correctly, can do that job. If it is too thin, it overheats and melts. If it has sharp points, it can cause sparks. The metal threads now found in most microwave popcorn bags, have been designed to avoid both issues while still providing a conduction heat source. As a result, the modern microwave popcorn bag utilizes all three times of heat: radiation from the magnetron, conduction from the bottom of the bag and convection from the steam released by the bursting pericarps. Evidence of the latter two can be found in heat outside the “floor” of the bag, the inflation of the bag during cooking and the release of steam when opened carefully.
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