Microwave Technology: A Half-Century of Progress
May 1997 -- Cover Story
By: Robert F. Schiffmann
All it takes sometimes is a random event to achieve major impact.
The year: 1945. The place: Waltham, MA. In the Raytheon Company radar laboratory, a vice president named Percy Spencer stumbles onto a discovery that will ultimately lead him to gain the title "father of the microwave oven" and usher in an era of unparalleled cooking convenience.
Several versions of the story exist: A chocolate bar melts in Spencer's pocket or a feeling of warmth comes over his body while working around the company's radar system; popcorn kernels placed in front of a wave guide horn start popping. Whatever it was, Spencer recognized the possibility of heating foods with microwave energy -- the same energy used in radar -- and invented the microwave oven.
His patent, filed in October 1945 and granted in 1950, (U.S. patent 2,495,429) can be paraphrased as: a method of treating food by application of microwave energy, wherein the food is exposed to microwave energy within a restricted region of space and for sufficient time to cook the food to a predetermined degree. Incidentally, that "restricted region of space" was a galvanized garbage can in the first experiments.
In 1941, Spencer invented the strapped magnetron design, still used today in most radars, microwave ovens and industrial heaters. He also received the first U.S. patent for microwavable food -- popcorn, which was popped on the cob -- in 1949 (U.S. patent 2,480,682).
This year marks the 50th anniversary of the first microwave oven -- Raytheon's Radarange(r), introduced in 1947. Most Americans (69%) consider it the most important technological breakthrough since 1969, according to a study conducted by the inventors of the Seiko kinetic watch.
"The enthusiasm for the microwave oven isn't about cooking, but about time," says William Doyle, professor of history, Pennsylvania College of Technology, Williamsport, PA. "People like the microwave because it is a time-saving device, not because it helps them cook. Time ... is the new frontier."
Is it any wonder, that 150 million microwave ovens exist in the United States -- a roughly 95% saturation level? Many homes have more than one oven and, of the projected 9 million ovens to be sold in 1997, 75% will be replacement or second ovens.
The U.S. consumer views microwave ovens as convenient, fast reheaters -- time savers -- that fit vast lifestyle changes perfectly. Manassas, VA-based International Microwave Power Institute's 1995 Microwave Industry Report: A Survey of Consumer Usage and Attitudes Regarding Microwave Products revealed that 78% of adult respondents and 54% of their spouses stated they use the microwave at least twice a day; 36% and 18%, respectively, use it four or more times daily.
More than three-quarters of work places have microwave ovens. Then there's the eat-away-from-home opportunity -- everything from convenience stores where consumers pop sandwiches, buns and soups into the in-store microwave; to fast-food chains, where employees top off sandwiches with a few seconds of microwaves.
Large and limited
The first microwave ovens were large (about the size of small refrigerators), expensive (about $4,000 each) and limited to foodservice use. The water-cooled tubes required a warm-up period prior to cooking, and carried a price tag of $200 each (two were used in each oven); tube life was restricted to 500 to 1,000 hours.
Today's consumer ovens carry an average selling price of about $150, and use air-cooled magnetrons with lives of 10,000 to 15,000 hours (10 to 15 years in typical use) that cost about $10 each.
Tappan Company, Mansfield, OH, introduced the first consumer microwave oven in 1955, incorporating it into the company's standard gas range and oven. Tappan included another important invention: the mode stirrer. This slowly rotating fan-like device overcame the problem of standing waves, thereby improving heating uniformity.
In 1967, Amana Refrigeration Company, Amana, Iowa, (purchased by Raytheon in 1965) introduced the first counter-top consumer microwave oven. It operated at 115 volts and used ordinary 15-ampere kitchen wiring instead of a special 220-volt line. This step should have launched the microwave oven market. It did, but more slowly than anyone anticipated.
In the early to mid-1970s, Minneapolis-based Litton Microwave Cooking Products introduced large-cavity models with electronic touch controls, and controls for automatic defrost and temperature. During this time, consumers began recognizing the appliance's value. By 1975, 30 years after its invention, annual sales reached 2.7 million units; During the 1978-1979 period, microwave ovens achieved 10% market penetration, the magic number at which an appliance is no longer considered just a fad.
Other important milestones included the introduction of Louisville, KY-based General Electric Company's Space Maker® oven in 1979; Japan-based Sharp Electronics Corporation's introduction of rotating carousel turntables (replacing mode stirrers) in the late 1970s; and the development in Japan of ceramic-metal cooker magnetrons circa 1980 (at an initial cost of $25 to oven manufacturers).
Today's microwave ovens are simpler to use than their more primitive ancestors. Many contain sensors that assist the user by automatically cooking certain foods, such as popcorn, and might even offer digital-display heating instructions.
"In the early days you were on your own -- you set the timer and hoped for the best," Anne Howard, general manager of the Mahwah, NJ-based Appliance Division at Sharp Electronic Corporation says. "The development of touch controls and automatic features helps people get good results. Preprogrammed time and sensor settings deliver results that are better than consumers could achieve on their own."
In the '70s, the food industry displayed a lack of interest in microwave ovens when it became apparent that nearly all conventionally heated products performed poorly in microwave ovens.
Only The Pillsbury Company, Minneapolis, and small start-up company, Edina, MN-based Golden Valley Foods, Inc., created by ex-Pillsbury employee Jim Watkins, continued investing heavily in the future of microwavable foods. Despite consumers' increasing requests, it wasn't until the mid-1980s -- when about 40% of U.S. homes contained microwave ovens -- that the food industry finally woke up to the potential market for microwavable foods.
Efforts during the late 1970s focused on making regular products dual-ovenable, sometimes only by adding some microwave reheat instructions to the label. However, by 1986, numerous microwave-only products were introduced, including popcorn, pizza, filled sandwiches and entrées.
However, what became quite clear, is that the microwave oven is an unfriendly environment for many foods. It's an effective steamer or poacher, but fails at crisping or browning. It doesn't reheat very evenly, heating too fast in some cases. Some foods overheat or erupt. It might leave some parts quite cold while others are very hot, especially frozen foods.
So, major efforts were taken to learn microwave food formulation. This was augmented by major efforts in the packaging industry, whose job became providing containers, which would support and enhance the efforts of the food technologist. This would provide foods that more closely resembled the quality consumers expected.
New microwavable food products proliferated in the late 1980s. But trouble was on the horizon, first seen by the decline in microwavable-product introductions in 1991. In 1993, an article in The Wall Street Journal described a "microwave meltdown" at the checkout counter. Of 12,000 product introductions in 1992, only 625 were microwavable. Experts cite many reasons for the decline, which continued for at least two more years:
Still, opportunities for successful microwavable foods still exist, probably more so than before. At the January conference of The Food and Drink Industry Training Advisory Council, Northern Ireland, Peter Lytle of the Wyzata, MN-based Business Development Group stated that "the total time spent preparing all meals in the average American household has declined to 25 minutes."
What better cooking appliance to meet that time requirement than the microwave oven? In addition, recent announcements by Camden, NJ-based Campbell Soup Company and others to offer home-delivered meal plans, as well as the growth of the home-meal-replacement market, tap into microwave-oven strengths, and should lead to a host of new products.
Developing microwavable foods is a formidable task. The ovens heat fast, usually in one-fourth to one-third of conventional time; avoiding overheating is a challenge. Foods often heat unevenly, especially frozen products, where the range of temperature might be more than 70°F. Color, aroma and flavor are rarely developed in short-time heating. Generally, microwaving will do little more than heat the product -- it won't improve it.
Major factors influencing the way foods heat in microwave ovens relate to their dielectric properties, specific heat, mass and shape. (For an in-depth description of microwave technology and food formulation, see "Understanding Microwave Reactions and Interactions," April 1993 Food Product Design.) Dielectric properties, such as dielectric loss factor, influence how well and fast a product will heat in a microwave oven -- the higher the loss, the faster it heats. They also affect microwave penetration depth into the food, impacting uniformity of heating. Penetration depths are generally small, a half-inch or so. Therefore, foods should not exceed two inches in depth.
Focused heating can also be a problem, especially in small cylindrical containers such as baby food jars, where the center can reach temperatures of 200°F while the surface might be 100°F. The specific heats of oils, fats and high sugar food components also can lead to localized overheating and excessive heating rates.
Water is most often a major food ingredient; it acts as a microwave moderator, providing more tolerance to the rapid heating rates and reducing uneven temperatures. Many products benefit from the use of higher-than-usual water content; for example, baked products, such as hamburger buns, would otherwise become tough or dried-out and leathery.
Ingredient suppliers have learned the specific requirements of microwavable food products and provided ingredients targeted to meet those needs. For example, modified starches and emulsifiers improve products, from entrees to pizzas. Instant cold-swelling starches not only provide freeze/thaw stability and tolerance to microwave heating, but they could reduce the in-plant preparation time required for cook-up starches.
"The evolution of the microwave oven has offered many challenges and opportunities," says Gary Zwiercan, vice president and general manager of the Food Product Division at Bridgewater, NJ-based National Starch & Chemical Company. "Food systems designed for the microwave oven demand everything from improved freeze/thaw stability and unique viscosity characteristics, to readily dispersible starches, while, at the same time, maintaining quality.
"We recognized these needs early on," Zwiercan says, "and have been working with customers, offering them either existing products or developing new products to meet their needs and market requirements."
Emulsifiers can double the range of acceptable microwave heating time. Their mechanism probably relates to complexing with starch and emulsifying water, thereby reducing the rate of water loss and preventing premature toughening of the crust. But emulsifiers might have other, still not well-understood effects -- as highly polar molecules, they might affect the dielectric properties of foods.
Complete flavor development is unusual during microwave heating, so the addition of flavors is often required for masking undesirable flavors or aromas, enhancing other existing flavors, and augmenting the profile. Encapsulation often protects these sensitive components.
Other problems remain. It is difficult to crisp breaded or coated food products. Susceptors have proven helpful with products such as pizza, but fall short of providing an answer for reheating frozen breaded foods. This results from a phenomenon referred to as "water pumping." When frozen breaded chicken is reheated in a conventional oven, the rate of moisture diffusion from the chicken into the coating is slow and the breading can brown and crisp as it dehydrates and reaches the oven temperature -- 350° to 400°F. However, in the microwave oven, vapor pressure increases inside the chicken meat and pumps water rapidly to the surface, swamping the coating with moisture. At the same time, air inside the oven remains cool. Water, which might otherwise evaporate, condenses and cools the surface, resulting in poor color development and a soggy crust.
One of the greatest frustrations in microwave cooking is determining exactly how to heat a product -- how much time to use -- and to know when it's done. This is because of the wide variations in oven-to-oven performance.
As the microwave oven market grew, a proliferation of ovens of all sizes, powers and features occurred. In 1977, Morris Katz, research associate at The Pillsbury Company, labeled it "the babble of power." U.S. ovens differ in many ways:
In the United Kingdom, a government-mandated program provides an oven-rating scheme. Since 1992, ovens are performance-rated, using a 350-gram water load. Ovens display a label indicating oven power (as measured by the Geneva, Switzerland-based International Electrotechnical Commission's IEC 705 power output test) and a "heating category" rating of A to E based upon the 350-gram test. Food-package labels show the heating time in minutes for the various categories and the oven wattage. This has been adopted by all oven manufacturers and nearly all food processors supplying products in the United Kingdom but seems unlikely here, since U.S. manufacturers can't even agree to use an identical procedure for measuring microwave-oven output power.
True Cook Plus+® (TCP+) just might bring the wide variety of microwave ovens in harmony with food products. Microwave Sciences L.L.C., an Atlanta-based entrepreneurial company, has a patent-applied-for technology employing a microprocessor-based operating system and process control language. Pushing the TCP+ button on an oven's control panel and punching in the digits on a food's label signals the oven to automatically cook the product.
Between the invention of the microwave oven and the mid-1970s, choosing a product container was easy -- only aluminum-foil containers were available. But these had problems, both genuine and perceived, including: arcing, microwave fires, magnetron destruction and blocking microwaves. In the late 1970s, paperboard containers were first introduced, followed by plastic versions.
Microwave ovens meant that consumers could move from food preparation to eating without handling food or washing a pot or dish. This meant containers had to be:
Containers could not adversely affect food flavor and appearance. They required graphics to encourage purchase, provide instructions and offer nutritional content listings. And, these containers required at least partial replacement of existing packaging machinery.
Initially, the task was especially difficult because most products needed to be "dual-ovenable" -- capable of use in conventional and microwave ovens. For plastics, high-temperature exposure was a problem, especially in conventional ovens. At first, expensive thermoset polyester trays were used in products such as Campbell's LeMenu(r) dinners. These required a foil overwrap, a plastic dome and a carton, making the total container cost extremely expensive.
Crystalline PET (polyethylene terephthalate) replaced thermoset polyester, but was still too costly. Less-expensive polypropylene can't match crystalline PET's heat stability, but works well for microwave-only products. These containers still require lidding and overwrapping.
A major breakthrough came with the all-folded paperboard container and lid for Budget Gourmet®. Complete with colorful graphics, it kept packaging costs down and withstood the economic recession that forced many high-packaging-cost products off supermarket shelves. Today, about 70% of all microwavable products are packaged in folded paperboard trays. Richmond, VA-based Westvaco Corporation's Ovenware Plus™ line utilizes a patented blow-molding technology to produce unique shapes, sizes and package styles, including compartmented trays.
"Advances in technology, such as the microwave oven, along with changing demographics and lifestyles, have created opportunities for Westvaco and its customers over the years to create a new market segment within the frozen-food industry," says Andy Luke, manager of Westvaco's Food Packaging Systems.
"Our innovative package designs and our packaging system, which form and seal the containers in customers' plants, have helped users of Westvaco Ovenware increase sales, market share and profits while lowering system costs."
Molded paper trays also are used for many microwavable products and are supplied by companies such as Van Leer (Ovenware II®) and International Paper (Pressware®).
Multilayer barrier containers for microwavable shelf-stable entrees were introduced in 1987 with products such as Top Shelf™ from Hormel and Lunch Bucket from Double Tree Foods. The latter came in a unique tub container with a pull-ring removable aluminum lid and a vented, splatter-shield plastic cap.
Standup pouches are appearing for microwavable foods. An early entry in this market is Oscar Mayer's refrigerated Hot Wiches® Sandwich Makers. After tearing off the top strip, the pouch is stood vertically in the microwave oven and can be heated in about one minute.
The aluminum foil tray also might make a comeback. In 1984, Aluminum Company of America, Pittsburgh, tried introducing coated shallow aluminum trays, but they failed despite good product performance. In the United Kingdom, Microfoil®, a new tray design developed by U.B. (Ross Young's) Ltd. for use in its Mediterranean Ocean Pie, has won endorsement from leading microwave oven manufacturers.
With aluminum trays, microwave energy can only enter through the open top, since aluminum reflects microwave energy. By contrast, microwave-transparent paper and plastic containers can be heated from top, sides and bottom. Aluminum trays create a somewhat slower and more uneven heating, especially with frozen foods. The Microfoil® technology utilizes a large hole in the container bottom, covered with PET film laminate. The hole directs microwave energy to the center of the product, while the metal walls prevent overheating and burning of the edges.
The containers previously described can be considered "passive": they hold products during and after heat, and can be eaten from. During the late 1970s and early 1980s, active containers -- best exemplified by microwave susceptors -- were developed.
Active containers have microwave interactive films which usually supply surface or focused heat, in addition to that supplied directly to the foods. The microwave susceptor was finalized by a patent granted in 1986 (U.S. patent 4,641,005) to Oscar Seiferth, then vice president of engineering for Austin, MN-based Hormel Inc.
Usually, susceptors are ultra-thin films of metals, such as aluminum, deposited via vacuum evaporation (or other means) upon a polyester substrate. This is bonded to paper or paperboard to provide a structure, which can heat rapidly to a temperature of approximately 400°F. This hot surface, in contact with a properly designed food system, provides some crisping and browning. In the case of microwave popcorn, it can increase popping efficiency. Susceptors have been used with microwave pizza, French bread pizza, pie crusts, and numerous other products.
One of the earliest entries in the field was Richmond, VA-based James River Corporation's Quik Crisp®. Several new developments have occurred in recent years. James River's Focus® technology and Qwik Check® provides patterned susceptors through a patented deactivation process, allowing selective heating at different locations on the food.
Taunton, MA-based A.D. Tech, Inc.'s Safety® susceptor automatically optimizes itself to a particular food product and prevents overheating or scorching. Also, A.D. Tech's Accu-WaveTM diffuser film shields microwave energy away from areas prone to overheating.
Micro-Rite® from Beckett Technologies Corporation, Mississauga, Ontario, makes patterns of aluminum films through a delamination process. This gives it antenna properties, focusing the microwave electric field to cause heating while remaining cool. This improves browning and crisping of pastry and dough products. It improves microwave power delivery for products exceeding 500 grams and heats them more evenly, quickly and without hot spots. The company also is using technology developed at Alcan International Ltd., Kingston, Ontario, to provide shielding and directed microwave energy transmission for more uniform heating.
Where are microwave foods, packaging and ovens heading? In early 1997, the Food Industry Report predicted a microwave-industry rebound by the year 2020 with replacement of many of the 40% of ovens that are at least 10 years old at time of replacement.
The food industry also seems to be gearing up for a return to the microwave market -- but with a more cautious approach aimed at meeting consumer needs with better quality products. As Lytle stated in January at The Food and Drink Industry Training Advisory Council, Northern Ireland: "With the cost of new product introductions reaching $100 million, companies can't afford to be wrong."
What are the opportunities? In its 1995 Microwave Industry Report: A Survey of Consumer Usage and Attitudes Regarding Microwave Products, the International Microwave Power Institute identified the most frequent consumer use of microwave ovens as: heating leftovers, defrosting, and popping popcorn. The report also examined product categories more closely, providing a list of the frequency of preparation of a wide variety of foods in the microwave oven.
Various new product gurus -- such as Lytle; New Product News publisher Lynn Dornblaser; and Elizabeth Sloan, president of North Palm Beach, FL-based Applied Biometrics -- have recently described "hot button" opportunities well-suited to microwave products, such as:
For ingredient and packaging suppliers, the challenge will be meeting the requirements for such products. "With demand by consumers increasing for products that are quick and convenient to prepare, we continue to see exciting opportunities in the frozen-food market," Westvaco's Luke says, "and are working on new technologies and packaging formats to help our customers meet this challenge. We also see significant potential for our products in the expanding home-meal replacement market."
It's doubtful the large United Kingdom market for "ready meals" -- refrigerated prepared meals -- is unique to that nation. These complete entrees or side dishes are ready to pop into the microwave oven and serve. Crystalline PET trays dominate this market, although a line of fresh soups in microwavable stand-up pouches has existed there for some time. Code dates on products are short (a three-day maximum); consumer purchase and, therefore, turnover is high.
The barrier to adoption of this technology on this side of the Atlantic has been largely due to the longer shelf-life requirements of the largest U.S. food processors. Perhaps a regional manufacturing system could grab a significant share of the take-out meal category.
The U.S. trend during the past few years has been to reduce excess packaging and overall packaging cost. Some interesting technologies we might see in the next few years include:
· The printed susceptor. Instead of using aluminum, steel or the alloy, Inconel, which require expensive processing to form susceptors, films can be printed with carbon-based coatings. Companies such as James River and Westvaco have patented various technologies using this technique. Temperature control still remains a problem. But once solved, this technology could provide less expensive and more controlled or focused heating to meet challenging product geometry.
· Doneness indicators. Temperature sensors indicating doneness from St. Paul, MN-based 3M Corporation and Morris Plains, NJ-based LifeLines Technology, Inc. have been used on some microwavable foods. They essentially responded to steam development inside a package -- not always the best indicator of doneness when uneven heating occurs.
Yet, difficulty in determining when a product is done is high on the consumer complaint list. One Japanese company produced a package with a tuned notch in the lid that whistled -- just like a tea kettle -- once the product was done. A microchip could possibly be used to provide an audible or visual signal that acts as an indicator.
· Crisp coated foods. This will require a joint effort between packaging and food-ingredient suppliers. The Japanese-style bread crumb has merit in microwavable coated foods. Combined with a susceptor-type container able to reduce energy transmitted through it --reducing the water-pumping effect -- crisp-coated foods would be possible.
Improvement to come
Oven manufacturers will employ still more sensors, fuzzy logic and automatic features, taking much of the guess work out of microwaving. Digital displays are likely to become more informative as they tell the consumer when to stir, uncover, add salt, etc. Sharp produces and markets an oven exclusively in Japan that uses bar codes. These can be scanned from a recipe book, aiding the microwave cook. The purchaser places the product into an oven containing a bar-code reader, pushes the "start" button, and the oven heats the product per the coded instructions.
One oven development yet to transform into reality is the all solid-state oven. Large numbers of transistors would replace the magnetron, eliminating the heavy power supply and creating a light, truly portable oven. It also might have outstanding cooking uniformity because transistors could be placed strategically around the cavity to disperse the microwave energy from many points rather than a single source. Today, such technology is too expensive and limited to the lower frequency 915 MHz microwave band, rather than the current 2,450 MHz microwave oven frequency; a problem for currently used cavity sizes.
Orlando, FL-based Apollo USA, Inc. (the U.S. subsidiary of Apollo Enterprises Ltd. in the United Kingdom) is working on what might prove the ideal cooking appliance: a microwave-assisted convection hot-air oven. Totally computerized, the oven can automatically cook, bake and roast almost any food. All the consumer has to do is push the button for the proper food category; answer a question or two, such as "refrigerated or frozen?" from a pleasant computerized voice; and push "start."
The oven does everything else -- all in one-third to one-half the normal cooking time, with microwave uniformity never before achieved. Consumers will be able to tell the oven when they intend to eat, and the food will be ready at that time. While currently being developed for a major appliance manufacturer, it can soon be viewed at Epcot Center, Orlando, FL.
Percy Spencer's chance encounter with microwaves certainly was a significant step in modern cooking. But it was more than a single person who moved this field along and continued sustaining and nurturing it. Many innovative individuals and companies have continually improved microwave ovens, foods and packaging. If Spencer could only have known where it would all lead since those pioneering days in 1945.
Robert F. Schiffmann, president of New York-based R.F. Schiffmann Associates, Inc., has been involved in microwave product and process development for more than 35 years. A Fellow of the International Microwave Power Institute and its chief operating officer for 12 years, he was recently made an Honorary Member of the U.K. Microwave Association. Contact the author at: R.F. Schiffmann Associates, Inc., 149 West 88 Street, New York, NY 10024; telephone: 212/362-7021; e-mail : email@example.com.
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