A glance at 2001 supermarket shelves reveals a new surge of foods intended for microwave reheating. It appears that food processors/packagers have rediscovered the world of products for microwave ovens that seemed to have ebbed during the 1990s.
Developed during the 1950s as an offshoot of radar, microwave food heating was the result of a confluence of radar’s use of 2,450 MHz frequency to detect objects and a peculiar characteristic of this frequency to penetrate deeply into aqueous foods. Development of solid-state electronics during the 1970s provided the impetus for low-cost microwave ovens affordable to hotel/restaurant/institutional operators, homeowners, and apartment dwellers.
By the 1980s, virtually all fast foods were labeled as microwavable, and virtually every paper, glass, plastics, and metal supplier offered its products as microwavable or at least microwave safe. Many of these folks had not done their homework, however, as the foods in microwavable packaging did not cook at all and usually did not heat uniformly.
Simultaneously with “microwavable” came “dual-ovenable” packaging for use in either microwave or conventional ovens. Food processors/packagers did not want to risk losing those few customers who elected not to join in the microwave oven stampede. And, of course, almost every packaging supplier declared its structure to be dual ovenable.
Consumers and restaurant operators soon learned that beverages could be heated rapidly in microwave ovens and that, with care, reheating of chilled particulate foods could be effected. Microwave ovens became another in the arsenal of kitchen appliances, effective for only a limited number of applications.
But into this mix came the singular popcorn package—its challenges solved in part by susceptor technology. A direct descendant of paper-bag popcorn—microwave popping employed by 1950s food technologists to produce their lunches—microwave popcorn became a major product success of the 1980s. Unfortunately, susceptor technology that was a basis for functionality did not transfer as well to other foods requiring surface heat. Relatively few products with susceptor packaging—frozen hand-held sandwiches and frozen pizza—survived the gauntlet of consumer use.
During the mid-1980s, a dramatic new entry emerged from the cooperative efforts of industry suppliers and processor/packagers: “Lunch Bucket”® cans and their analogs. This extraordinary package originated with attempts to develop plastic cans to displace metal. Can and plastic makers tried to develop the perfect plastic can, some by laminating aluminum foil, some through insert injection forming, and some by thermoforming laminations or coextrusions. Finally, coinjection blow molding was developed for multilayer structures containing ethylene vinyl alcohol (EVOH) as the oxygen barrier key to functionality.
The main structural component, polypropylene, is a heat-resistant moisture barrier that protects the core moisture-sensitive EVOH, especially during the crucial retorting phase. The actual development was, of course, much longer and more complex than this brief paragraph might imply. To indicate the complexity of the many issues involved, engineering changes are continuing in the development of the plastic can: hourglass shapes to better distribute microwave energy; incorporation of desiccants into the multilayers to reduce the transfer of moisture (and, incidentally, the basis for incorporation of oxygen scavengers into many multilayer plastics); and the use of oxygen scavengers in spin-welded closures.
Examination of the plastic can should reveal a basic tapered shape that fosters entry of microwave energy with relatively little edge overheating and reduction of internal steam pockets that could otherwise force the cans to “walk” across the oven floor. Each can has a plastic overcap with die cuts to be placed over the opening to retain steam generated in heating and contribute faster heating. A not insignificant proportion of the food temperature rise results from reducing heat losses.
The structural design of the plastic can represented such a major advance that members of the food packaging community predicted the demise of the metal can as well as the rise of the retort tray for place-packed foods such as salisbury steak. One major variable was minimized: quality of contained product on the consumer’s table. Despite the potential to reduce the thermal input to reduce heat damage in retorting, few recognized this opportunity, and the contents received the full treatment. As a result, they had the mediocre flavor and mouthfeel of canned food. Ditto with retort trays. Because of their relative ease of preparation, marketing focused on after-school latchkey kids with products such as macaroni and cheese. Microwavable packaging was a cornerstone of the 1990s for this concept.
Meanwhile, in another seemingly unrelated trend, Asian ramen soups were becoming the rage among the younger set enamored with ethnic flavors, convenience, and economics. Some of these dry products were packaged in beaded foamed polystyrene tubs into which consumers poured hot water, stirred to reconstitute, then ate directly from the package. Despite a package that could not be heated directly in the microwave oven, ramen soups quickly cannibalized canned soups and dry soups in barrier pouches.
Ramen soup packaging inspired another variant: coated paperboard tubs containing dry entrees and side dishes such as macaroni and cheese, again to be filled with hot water for reconstitution. Like their ancestral beaded plastic cups, paperboard was not ideal for microwave reheating because tapered shapes alone did not permit uniform distribution of energy, and, with salty products having a high depth of penetration and low-surface-tension components, boilovers were common and hazardous to consumers.
Enter the notion of using thermoformed, heat-resistant, rounded corners instead of the sharply angled shapes of paperboard that led to corner overheating and transient steam geysers. When consumers followed instructions to fill with water only to a low level, with reclosure to retain steam, the package and its contents could be conveniently reheated directly in microwave ovens with little hazard. Plastic tubs were friendly for consumer consumption of soups, cereals, and entrees.
A recent variation is the use of a toroid-shaped plastic tub base to permit microwave energy to enter through the base from all sides and thus distribute the energy broadly and heat more uniformly. Food technologists have been employing the bagel shape for decades to heat food in microwave ovens because of quantitative evidence of significantly enhanced heating triggered by greatly increased surface-to-volume ratio.
Currently, ambient-temperature shelf-stable moist pasta and sauce have been introduced whose preservation depends on use of hurdle technology. The sauce is distributed over the upper surface of the pasta in the polypropylene tray shaped with a dished-in bottom. In effect, the surface area of the system is increased by the toroidal base plus spreading of high-salt tomato sauce in a ring shape over a larger area on the top. With a relatively thin 1-in-thick pasta mass spread over a 48-sq-in surface area, microwave energy penetrates and is absorbed from all angles. As the pasta sauce is heated, it tends to flow to the pasta below, heating and remoisturizing this component that is suboptimal in water and salt content so important to effective microwave reheating.
Even newer are ambient-temperature, shelf-stable soft pretzels preserved by reduced oxygen enhanced by oxygen scavengers. As every Philadelphia native knows, soft pretzels must be consumed hot with scads of salt. To heat this naturally toroid-shaped product, microwave reheating is recommended to consumers, another reflection of the return of foods and integral packages designed for microwave heating.
Intruding into the realm of microwave food heating is the introduction of combination ovens which complement microwave energy with infrared radiant and/or forced-convection air heating, some of which may be faster and more uniform than microwave alone.
Today’s “microwavable” packages are the product of considerable radiation and thermodynamic engineering inputs that have contributed to delivering uniformly heated quality foods. This ongoing episode in food history demonstrates that consumption of food must integrate the final preparation steps. Microwave ovens have been a part of two generations of eaters, but only now, with food packaging technology, is it entering the mainstream of the food delivery system.
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The Journal of Food Science is a publication of the Institute of Food Technologists, a not-for-profit society for food science and technology.
by AARON L. BRODY