Future Development

Food manufacturers are well aware of the acceptance of microwave cooking and heating by consumers because of the microwave oven sales boom over the last 20 years in the United States and in recent years in other nations throughout the world. There is already a large market for foods designed and packaged for microwave oven use. A substantial increase in this market can be expected if microwave pasteurization and sterilization of portion packed foods is successful. These two processes could account for a dominant share of the microwave equipment business and in the case of sterilization possibly cut deeply into the conventional equipment market for these processes.

Although practically all of the microwave sterilization of food has been carried out at 2,450 MHz and the first production installations have also been at this frequency, eventually major installations will shift to 915 MHz simply because of the power demand (i.e., hundreds of kilowatts of power). Experiments at this lower frequency indicate more uniform temperature distribution. All of the results are not yet in, but this has been the case with other major processes, including tempering and cooking bacon.

The first installations of microwave sterilization of ready-meals went on stream a few years ago. Total installed power is only a few hundred kilowatts, at 240 MHz. Process time is about 30 min, including cooling time. Interesting progress at the U.S. Army Natick Research and Development Laboratories (Natick, Mass.) has been made in development of a chemical method to assess process adequacy. Several chemical markers have been identified that are produced during normal processing of foods and correlate well with microbiological lethality. Thus, it is possible with a rapid analytical technique to determine the effectiveness of a process in minutes instead of the several days required for microbiological methods.

Microwaves may also find use in aseptic processing of flowable materials containing particulates, a process that has proven elusive because of the difficulty in ensuring that the fastest-moving particulates have received an adequate sterilization.

The adoption of microwave-based processes could influence the design of food plants of the future. Not only is space savings made possible by a reduced process time, but the reduced space requirements could have a powerful effect on process economics when one considers the cost of real estate. The labor requirement may also be reduced. A microwave tempering operation could be automated from freezer storage at least through the tempering stage. Pasta dryers already have reduced the space requirement three to four times with no reduction in production. Microwave bread baking could mean smaller bakeries located closer to their markets and have a significant impact on transportation costs. Microwave-assisted baking is another process that could find a niche in the supermarket bakeries to provide just-in-time baked products and thereby eliminate the daily loss of excess product to ensure latecoming shoppers of the same choice available to early shoppers.

Existing processes may be improved by including a microwave capability. Depending on the process, microwave heating could be applied as a preheating step, be built into existing equipment for simultaneous application of two or more forms of heating, or added on at the end of conventional equipment to increase the production rate.

Tools are available to evaluate new microwave processes without resorting to empirical methods alone. Microwave-compatible temperature-measuring systems are being used today. Infrared detectors are being used to follow surface temperature changes that can be translated into meaningful feedback data to control subsequent microwave input. Microwave radiometry, which has been demonstrated in measurement of temperature in human muscle tissue undergoing diathermy, could conceivably be used to noninvasively monitor product temperature during microwave pasteurization and sterilization to provide data for FDA approval. Continuous weighing systems have been used to monitor weight changes during microwave processing. There also is a large body of dielectric property data in the literature that can be useful in mathematical modeling and computer simulation in process development. All in all microwave processing has a promising future.

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