Microwave Oven (Encyclopedia of Food & Culture)
MICROWAVE OVEN. While experimenting with radar during World War II, Percy Spencer of Raytheon Corporation in Waltham, Massachusetts, discovered the heating properties of microwaves. With a candy bar in his pocket, he leaned in front of the microwave tube and the candy bar promptly melted. This event led to the birth of microwave ovens.
In 1945 Spencer submitted his first patent application for heating food with microwaves. The patent described two parallel magnetrons that heat food that passes by on a conveyor belt. Two years later, William M. Hall and Fritz A. Gross, Spencer's co-workers, applied for a patent for a microwave-heating device enclosed in an oven. This device consisted of two microwave-generating magnetron tubes packed in a metallic box. The oven included a timer and a means of controlling power.
Raytheon's president, Laurence Marshall, was interested in Spencer's patent. A prototype microwave oven was constructed in 1946 costing an estimated $100,000. Marshall was also enthusiastic about the prototype and ordered engineers to develop an oven in which cold sandwiches could be heated. A contest was held to name the new ovenhe winner was "Radarange."
The first commercial Radarange model was a freestanding white-enamel unit operating at 220 volts of electricity and with an internal water-cooling system. The first Raytheon microwave oven was sold to a restaurant in Cleveland, Ohio, in 1947. Subsequent Radaranges incorporated sliding vertical doors. With a price tag of $3,000, sales were mainly limited to restaurants, railroads, cruise ships, and vending-machine companies.
Development of the microwave oven continued during the 1950s. Raytheon dominated the field of commercial microwave ovens and heating applications: It was the only manufacturer of ovens for restaurants and was the principal magnetron manufacturer. Raytheon licensed other companies, such as Hotpoint, Westinghouse, Kelvinator, Whirlpool, and Tappan, to manufacture the ovens. Raytheon furnished power supplies, magnetrons, and basic-oven design data to each company. The Tappan Company began experimenting with a Radarange installed in their lab. Tappan engineers, who were experts in cooking, teamed up with the Raytheon microwave engineers. In January 1952 the Tappan Company developed the first domestic commercial Radarange. It was powered by a 1,400-to 1,700-watt magnetron that was water cooled and required plumbing connections. The unit was five and a half feet high and weighed 750 pounds.
The experimental unit developed by Tappan was impractical for domestic use. What was needed was a magnetron requiring less power and a heat dislocation system that could replace the water cooling mechanism. Tappan engineers designed a cabinet with an air-cooled system. Eventually, the magnetron and related components, which had fed microwaves directly into the cavity, were relocated behind the oven. In October 1955, Tappan introduced the first domestic microwave oven for the consumer market. Designed to fit a standard forty-inch range or for built-in use, the unit had a stainless-steel exterior and aluminum oven cavity with a glass shelf. The oven featured two cooking speeds (500 or 800 watts), a browning element, timer, and a recipe-card file drawer. It retailed for $1,295. The unit was marketed as an "electric range." Its advertised advantages were cooking speed, a cool oven, and a unique reheating capability.
General Electric's Hotpoint division, which also had been researching microwave cooking, unveiled its electronic oven the following year. Both the Tappan and Hotpoint oven generated unprecedented enthusiasm and interest in 1956, but sales were dismal. The price was high for the average consumer, and food-processing techniques for the microwave were not well understood. Few food processors took the technology seriously, thus few microwaveable foods were produced.
Tappan continued to improve its product. By 1965 Tappan had introduced the first "microwave cooking center," which consisted of a microwave oven mounted above a conventional range. This unit still retailed for well over $1,000. Despite these advances, only ten thousand households in the United States owned microwaves by 1966.
Two events revolutionized the microwave industry. The first was the invention by Keisha Ogura of the New Japan Radio Company0 percent of which was owned by Raytheonf a compact, low-cost magnetron. The second was Raytheon's acquisition of Amana Refrigeration, Inc. George Forestner, Amana's president, was a microwave visionary. Amana appliance engineers teamed up with Raytheon experts to develop and design a household Radarange. In August 1967, Amana released its first microwave oven, the Amana RR-1. It operated at 115 volts and sold for $495. The unit was well received. The Amana RR-1 set off a revolution in microwave oven technology, and Amana's success encouraged other appliance manufacturers to produce microwave ovens.
Another important microwave oven manufacturer was Litton, which acquired a small microwave manufacturer called Heat & Eat in 1964. Previously, Litton had manufactured commercial microwave ovens for restaurants. Its newly named Microwave Cooking Products Division in Minneapolis targeted the home market. Litton's Model 500 used 115 volts and was compact. These ovens were installed on TWA planes in 1965, and Litton dominated the restaurant business by 1970.
Despite the initial successes, there were still problems to overcome before the microwave oven would be generally accepted. Manufacturers needed to convince the public that microwave ovens were safe. This fear began with the U.S. Congress's passage of the Radiation Control for Health and Safety Act in 1968. On 4 January 1970, the U.S. Department of Health, Education, and Welfare published the results of microwave oven radiation tests. The tests showed that microwave ovens leaked microwaves. Thus the federal government developed new standards and required changes in the construction of ovens beginning on 6 October 1971. These new regulations required design changes that would result in safer microwave ovens. Public apprehension slowly abated.
Another crucial challenge was convincing food processors to repackage their products. Foods packed in foil blocked microwaves and damaged ovens. Also, frozen foods contained too much water for microwave use. At first, food processors were not interested in working with microwave manufacturers. By the 1970s, however, more than 10 percent of all U.S. homes possessed microwaves, many microwave ovens were in use in vending businesses, and numbers were steadily increasing. Major food processors quickly reversed their direction and invested in microwaveable food products, and specialized microwave cookware was introduced. By 1975 microwave ovens out-sold gas ranges, with sales of over one million units. In the early twenty-first century, the primary use of microwave ovens in the United States was to reheat food.
See also Fast Food; Frozen Food; Kitchen Gadgets; Kitchens, Restaurant; Popcorn; Preparation of Food; Storage of Food.
Behrens, Charles W. "The Development of the Microwave Oven." Appliance Manufacturer 24 (November 1976): 72.
Buderi, Robert. The Invention That Changed the World: How a Small Group of Radar Pioneers Won the Second World War and Launched a Technological Revolution. New York: Simon & Schuster, 1997.
Osepchuk, John. "A History of Microwave Applications." IEEE Transactions on Microwave Theory and Technique 32 (September 1984): 1211.
Smith, Andrew F. Popped Culture: A Social History of Popcorn in America. Columbia: University of South Carolina Press, 1999.
Andrew F. Smith
Microwave Oven (How Products are Made)
Microwaves are actually a segment of the electromagnetic wave spectrum, which comprises forms of energy that move through space, generated by the interaction of electric and magnetic fields. The spectrum is commonly broken into subgroups determined by the different wavelengths (or frequencies) and emission, transmission, and absorption behaviors of various types of waves. From longest to shortest wavelengths, the spectrum includes electric and radio waves, microwaves, infrared (heat) radiation, visible light, ultraviolet radiation, X-rays, gamma rays, and electromagnetic cosmic rays. Microwaves have frequencies between approximately .11 and 1.2 inches (0.3 and 30 centimeters).
Microwaves themselves are used in many different applications such as telecommunication products, radar detectors, wood curing and drying, and medical treatment of certain diseases. However, certain of their properties render them ideal for cooking, by far the most common use of microwave energy. Microwaves can pass through plastic, glass, and paper materials; metal surfaces reflect them, and foods (especially liquids) absorb them. A meal placed in a conventional oven is heated from the outside in, as it slowly absorbs the surrounding air that the oven has warmed. Microwaves, on the other hand, heat food much more quickly because they penetrate all layers simultaneously. Inside a piece of food or a container filled with liquid, the microwaves agitate molecules, thereby heating the substance.
The ability of microwave energy to cook food was discovered in the 1940s by Dr. Percy Spencer, who had conducted research on radar vacuum tubes for the military during World War II. Spencer's experiments revealed that, when confined to a metal enclosure, high-frequency radio waves penetrate and excite certain type of molecules, such as those found in food. Just powerful enough to cook the food, the microwaves are not strong enough to alter its molecular or genetic structure or to make it radioactive.
Raytheon, the company for which Dr. Spencer was conducting this research, patented the technology and soon developed microwave ovens capable of cooking large quantities of food. Because manufacturing costs rendered them too expensive for most consumers, these early ovens were used primarily by hospitals and hotels that could more easily afford the $3,000 investment they represented. By the late 1970s, however, many companies had developed microwave ovens for home use, and the cost had begun to come down. Today, microwaves are a standard household appliance, available in a broad range of designs and with a host of convenient features: rotating plates for more consistent cooking; digital timers; autoprogramming capabilities; and adjustable levels of cooking power that enable defrosting, browning, and warming, among other functions.
The basic design of a microwave oven is simple, and most operate in essentially the same manner. The oven's various electronic motors, relays, and control circuits are located on the exterior casing, to which the oven cavity is bolted. A front panel allows the user to program the microwave, and the
Image Pop-UpThe oven cavity and door are made using metal-forming techniques and then painted using electro-deposition, in which electric current is used to apply the paint. The magnetron tube subassembly includes several important parts. A powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated, while a thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating. An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. Also, a blower motor used to cool the metal fins of the magnetron is attached directly to the tube.
Near the top of the steel oven cavity is a magnetronn electronic tube that produces high-frequency microwave oscillationshich generates the microwaves. The microwaves are funneled through a metal waveguide and into a stirrer fan, also positioned near the top of the cavity. The fan distributes the microwaves evenly within the oven. Manufacturers vary the means by which they disburse microwaves to achieve uniform cooking patterns: some use dual stirrer fans located on opposite walls to direct microwaves to the cavity, while others use entry ports at the bottom of the cavity, allowing microwaves to enter from both the top and bottom. In addition, many ovens rotate food on a turntable.
The cover or outer case of the microwave oven is usually a one-piece, wrap-around metal enclosure. The oven's inside panels and doors are made of galvanized or stainless steel and are given a coating of acrylic enamel, usually light in color to offer good visibility. The cooking surface is generally made of ceramic or glass. Inside the oven, electromechanical components and controls consist of timer motors, switches, and relays. Also inside the oven are the magnetron tube, the waveguide, and the stirrer fan, all made of metal. The hardware that links the various components consists of a variety of metal and plastic parts such as gears, pulleys, belts, nuts, screws, washers, and cables.
The Manufacturing Process
Oven cavity and door manufacture
- 1 The process of manufacturing a microwave oven starts with the cavity and the door. First, the frame is formed using automatic metal-forming presses that make about 12 to 15 parts per minute. The frame is then rinsed in alkaline cleaner to get rid of any dirt or oil and further rinsed with water to get rid of the alkaline solution.
- 2 Next, each part is treated with zinc phosphate, which prepares it for electro-deposition. Electro-deposition consists of immersing the parts in a paint tank at 200 volts for 2.5 minutes. The resulting coating is about 1.5 mils thick. The parts are then moved through a paint bake operation where the paint is cured at 300 degrees Fahrenheit (149 degrees Celsius) for 20 minutes.
- 3 After the door has been painted, a perforated metal plate is attached to its window aperture. The plate reflects microwaves but allows light to enter the cavity (the door will not be attached to the cavity until later, when the chassis is assembled).
The magnetron tube subassembly
- 4 The magnetron tube assembly consists of a cathode cylinder, a filament heater, a metal anode, and an antenna. The filament is attached to the cathode, and the cathode is enclosed in the anode cylinder; this cell will provide the electricity that will help to generate the microwaves. Metal cooling fins are welded to the anode cylinder, and a powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated. A metal strap holds the complete assembly together. A thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating.
- 5 An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. The waveguide is connected to the magnetron on top of the protruding antenna, while a blower motor used to cool the metal fins of the magnetron is attached directly to the tube. Finally, a plastic fan is attached to the motor, where it will draw air from outside the oven and direct it towards the vanes. This completes the magnetron subassembly.
Main chassis assembly
- 6 The chassis assembly work is performed on a pallet work-holding device used in conjunction with other toolsocated at the station. First, the main chassis is placed on the pallet, and the cavity is screwed on to the chassis. Next, the door is attached to the cavity and chassis by means of hinges. The magnetron tube is then bolted to the side of the cavity and the main chassis.
- 7 The circuit that produces the voltage required to operate the magnetron tube consists of a large transformer, an oil-based capacitor, and a high voltage rectifier. All of these components are mounted directly on the chassis, close to the magnetron tube.
- 8 The stirrer fan used to circulate the microwaves is mounted on top of the cavity. Some manufacturers use a pulley to drive the fan from the magnetron blower motor; others use a separate stirrer motor attached directly to the fan. Once the stirrer fan is attached, a stirrer shield is screwed on top of the fan assembly. The shield prevents dirt and grease from entering the waveguide, where they could produce arcing and damage the magnetron.
Control switches, relays, and motors
- 9 The cook switch provides power to the transformer by energizing a relay and a timer. The relay is mounted close to the power transformer, while the timer is mounted on the control board. The defrost switch works like the cook switch, activating a motor and timer to operate the defrost cycle. Also mounted on the control board are a timer bell that rings when the cooking cycle is complete and a light switch that allows viewing of the cavity. A number of interlocking switches are mounted near the top and bottom of the door area. The interlocking switches are sometimes grouped together with a safety switch that monitors the other switches and provides protection if the door accidently opens during oven operation.
- 10 A front panel that allows the operator to select the various settings and features available for cooking is attached to the chassis. Behind the front panel, the control circuit board is attached. The board, which controls the various programmed operations in their proper sequence when the switches are pushed on the front panel, is connected to the various components and the front panel by means of plug-in sockets and cables.
Making and assembling the case
- 11 The outer case of the microwave is made of metal and is assembled on a roll former. The case is slipped onto the preassembled microwave oven and bolted to the main chassis.
Testing and packaging the oven
- 12 The power cords and dial knobs are now attached to the oven, and it is sent for automatic testing. Most manufacturers run the oven from 50-100 hours continuously as part of the testing process. After testing is complete, a palletizer robot records the model and serial data of the oven for inventory purposes, and the oven is sent for packaging. This completes the manufacturing process.
Extensive quality control during the manufacture of microwave ovens is essential, because microwave ovens emit radiation that can burn anyone exposed at high levels for prolonged periods. Federal regulations, applied to all ovens made after October 1971, limit the amount of radiation that can leak from an oven to 5 milliwatts of radiation per square centimeter at approximately 2 inches from the oven surface. The regulations also require all ovens to have two independent, interlocking switches to stop the production of microwaves the moment the latch is released or the door is opened.
In addition, a computer controlled scanner is used to measure emission leaks around the door, window, and back of the oven. Other scanners check the seating of the magnetron tube and antenna radiation. Each scanner operation relays data to the next-on-line operation so that any problems can be corrected.
Because of their speed and convenience, microwave ovens have become an indispensable part of modern kitchens. Many developments in the microwave market and allied industries are taking place fairly rapidly. For example, foods and utensils designed specially for microwave cooking have become a huge business. New features will also be introduced in microwaves themselves, including computerized storage of recipes that the consumer will be able to recall at the touch of a button. The display and programmability of the ovens will also be improved, and combination ovens capable of cooking with microwaves as well as by conventional methods will become a standard household product.
Where To Learn More
Davidson, Homer L. Microwave Oven Repair, 2nd edition. Tab Books Inc., 1991.
Gallawa, J. Carlton. The Complete Microwave Oven Service Handbook: Operation, Maintenance. Prentice Hall, 1989.
Microwave Oven Radiation. U.S. department of Health and Human Services, 1986.
Pickett, Amold and John Ketterer. Household Equipment in Residential Design. John Wiley and Sons, 1986.
Raytheon Company. Appliance Manufacturer. Cahners Publishing, 1985.
Klenck, Thomas. "How It Works: Microwave Oven." Popular Mechanics. September, 1989, p. 78.
Roman, Mark. "The Little Waves That Could." Discover. November, 1989, p. 54.