[PACKAGING] by Aaron L. Brody

Advances in Microwave Pasteurization and Sterilization

he history of microwave food heating is sprin- kled with wonderfully intertwined tales of Tserendipity and genius that enrich and educate us to a deeper appreciation of the bearing of the past upon us tomorrow. In the beginning was Raytheon’s legendary wizard Percy Spencer, whose chocolate bar melted as he wandered by a radar antenna in his laboratory, a development that led him to the invention of the microwave oven. Apocryphal or not, the water-cooled white Radarange®, the first commercial microwave oven, made its debut before 1950. Perhaps the pop dreams were fueled by the amusing newspaper cartoons depicting the explod- ing eggs in microwave oven cavities or the popcorn popping in paper bags (which much later led to the microwave popcorn package) or the odd coupling of while being commercialized. Multitherm was Second-generation microwave sterilization microwave energy with ionizing radiation that derived from the concept that microwave radiation system at Washington State University. would sterilize foods with infinite ambient-tempera- does not necessarily have to function alone to gen- Photo courtesy of Washington State University ture shelf lives. Massachusetts Institute of erate the desired uniform heating patterns that Technology’s Team Goldblith (Sam) blitzed the could simultaneously destroy enzymes and micro- notion with a carefully contrived and conducted organisms with minimum heat and maximum study that demonstrated that the microbiological quality retention. effects of microwaves were wholly thermal, a mes- Stenström noted in his Multitherm thesis that sage that remains unheard by many through this solid foods could receive energy from all surfaces, day. Recognizing the power from the electromag- but not uniformly. This known deficiency of micro- Microwave radiation does not necessarily have to function alone to generate the desired uniform heating patterns that could simultaneously destroy enzymes and microorganisms with minimum heat and maximum quality retention. netic spectrum, Raytheon’s Dave Copson engaged wave heating challenged its ability to sterilize the me to test the thesis that the quick transfer of food content because some of the food—just the microwave energy could destroy enzymes in portion containing microorganisms—would invari- orange juice within seconds. Thermodynamics ably not be heated to sterilization conditions. He proved the point, which lay latent for decades. therefore proposed combination (hence, “multi”) with water and steam on the exterior to moderate Multitherm Processing the several energy sources to deliver uniform heat During the 1960s, the brilliant Swedish scientist throughout the food mass. Thus, to spread the heat, Lennart Stenström probed deeply into the geome- the product (in its shallow, brick-shaped micro- try of microwaves in and out of cavities and wave-transparent barrier plastic package) is developed several alternative solutions, one of immersed in hot water, which removes heat from which he dubbed Multitherm and had patented the corners that would otherwise overheat, and

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Advances in Microwave Pasteurization and Sterilization continued...

Microwave Pasteurized Foods product temperature to sterilization lev- Capturing the multiple heating ideas, sev- els of about 130ºC. The last section of the eral companies manufactured about 100 system was another pressurized section multiphase microwave plus hot air cham- in which the product temperature was bers that were initially applied for cooled to ambient. Packaging was, in pasteurization combined with chilled effect, the same as that for retort trays distribution for safe delivery of products but always requiring microwave trans- such as wet pastas. In such applications, parency: shallow barrier plastic trays the high quality product in microwave- with hermetically heat-sealed barrier transparent barrier trays heat sealed closures. Total thermal time was in the with barrier flexible lidding entered a vicinity of 10 minutes, still much less than microwave tunnel and was exposed to that dictated for canning or even retort 10–200 kw of 2,450 MHz energy. tray, but with much less thermal input. Microwave generators are situated near The system applying 2,450 MHz was mar- the entry to heat the interiors, and steam/ keted in the United States under the hot air is present following the micro- Classica trade name about five years ago wave radiation to temper the total food with no notable success. mass temperature. Most of the food product mass in its Microwave Sterilized Foods hermetically sealed barrier plastic trays in the United States has been heated to near 100°C when it • Post-packaging sterilization. In addi- leaves the proximity of the microwave tion to the more than 100 million energy (meaning that there is some level microwave ovens operating in American of microwave radiation in the cavity homes and foodservice outlets, hundreds throughout). The external hot environ- of larger-size units are finishing bacon, ment heats by conduction to ensure that tempering frozen meat, and precooking Segment of the UltrAseptics continuous flow microwave sterilization installation at North Carolina State University pilot plant. the entire interior food mass is tempered foods for industrial and institutional Photo courtesy of Josip Simunovic (heated uniformly) to a pasteurization applications using both 2,450 and 915 level. The final section of the cavity does MHz frequencies. The three most impor- energy is distributed more uniformly not contain any injected steam/hot air to tant applications in 2011 are the Yamco throughout the food mass. The basic arrest the active heating and begin the continuous flow machine for sweet notion was reduced to practice in a sin- cooling process. In this system, the nebu- potato puree, the continuous flow partic- gle-cavity unit and later, during the lous reduction in vegetative (but not ulate sterilizer at North Carolina State 1980s, in a continuous motion commer- necessarily all microbial) cells occurs at University, and the post-packaging unit at cial unit that produced a variety of high the post-packaging stage with a finite Washington State University. quality seafoods and vegetables mar- probability of the presence of pathogenic Winner of a major IFT award, the in- keted in Sweden. microorganisms so that the product must package sterilization system is one This great physicist comprehended be distributed under refrigeration. Shelf outcome of the enhancement of the thermodynamics in both solids and liq- life of such products depends on the Swedish Multitherm system over an uids and earnestly drove a continuous product and the oxygen concentration in eight-year period by a consortium of flow fluid microwave sterilization pro- and entering the package. Thus, under industrial and academic organizations cess, which he called Achilles for the present commercial practice, shelf life is (www.microwaveheating.wsu.edu) speed element. probably in the range of six weeks. under the direction of Washington State Throughout this series of develop- University’s Jimmy Tang. Among the ments from the 1940s through to the Microwave Sterilized Foods in Europe foods whose quality retention is so beginning of the new millennium, only Related to microwave pasteurized foods improved by the reduced (one-quarter to 2,450 MHz frequency microwave energy are microwave sterilized foods, which one-tenth the time of conventional can- was applied, even though 915 MHz, which initially took a different pathway than the ning to achieve sterility) thermal input had greater depth of penetration, was Swedish Multitherm. The European Omac resulting from exposure to Ferrite’s sin- available from regulatory authorities. systems used by Belgium’s Tops Foods on gle mode 915 MHz microwave energy Although many in the United States may prepared refrigerated ready meals was (www.ferrite.com) are macaroni and not have knowledge of the processes effectively a blast—a pressurized cham- cheese, mashed potatoes and beef (think sparked by Stenström’s research, micro- ber into which microwave energy was Shepherds’ pie), and solid-pack salmon wave heat sterilization processing has input for a short distance plus pressur- and rice. experienced a comfortable commercial ized hot air applied for an extended Packaging for the pilot operations is success in Europe. period of time in order to elevate the multilayer plastic trays sandwiching pg 84 02.11 • www.ift.org ethylene vinyl alcohol for oxygen barrier frequency microwave energy and packaged is stirring. Based on the integration of between layers of polypropylene for struc- applying aseptic technology. In the process, numerous single disciplines, holistic hurdles ture, moisture barrier, and microwave which has been recognized with an IFT are driving microwave heating, pasteuriza- transparent heat resistance plus a hermeti- Industrial Achievement Award, product is tion, and sterilization into the mainstream of cally heat-sealed, peelable flexible barrier preheated to 170ºF in a screw heater and processing, preservation, and distribution. closure. To assist in the sterilization heat brought up to sterilization temperatures of By radically reducing the input heat required transfer, the trays are relatively thin, which, 290ºF in 10 seconds in a double-tube continu- to achieve microbiological control in combi- not coincidentally, aids in rethermalization of ous flow microwave unit supplied by nation with other heat sources, oxygen the ambient-temperature-distributed Industrial Microwave Corp. (www.industrial- reduction, gas barrier packaging, and asep- product. microwave.com/industries). In commercial tic packaging and the ever-crucial Although the group projects an ambient operation, the cooled product is aseptically distribution system, microwave processing temperature shelf life of more than two years, filled into industrial/institutional-sized bag- and its related radio frequency sources the postulation from this perspective is that in-box packages on a LiquiBox system (www. have emerged from their shroud to offer absent the removal of almost all of the oxygen liquibox.com). powerful new means to satisfy consumer from the system, shelf life would not be Meanwhile, again extending the reach of desires for safe prepared food products that extended nearly that much. Chilled distribu- microwave heating in pumpables, particu- are high in quality. Once again in the chro- tion would prolong the shelf life, but the cost/ lates up to ¾ inch in size are being moved nology of all food science and technology, a benefit return is debatable: We believe that applying progressive-cavity Marlen multiplicity of food disciplines are weaving extended-shelf-life considerations would Research (www.marlen.com) pumps and into an intricate web that promises to figu- warrant the controlled distribution, but that microwave sterilized in the North Carolina ratively explode on the commercial would have to be demonstrable in commer- State University pilot plant managed by Josip scene—finally. FT cial practice. Simunovic ([email protected]). • Prepackaging sterilization. In today’s Aaron L. Brody, Ph.D., Contributing Editor expanding spectrum of technology, low-acid Envisioning the Future • President and CEO, Packaging/Brody Inc., Duluth, Ga., and Adjunct Professor, University of Georgia sweet potato puree is sterilized at Yamco After too many years of fits and starts on • [email protected] (www.yamco.net), also with 915 MHz the American market, a technological giant