Fresh pasta is a product that by its very nature is not easily pasteurized by conventional thermal methods. The loose condition of the product in the package makes conduction heat transfer impractical. Until recently the product was being marketed in modified atmosphere packaging that provides about 30 days of refrigerated shelf life. The combination of modified atmosphere packaging and microwave pasteurization extends the shelf life to 90-120 days.

The process is carried out by filling the fresh product into plastic trays, gas flushing, lidding, and passing the trays through a microwave tunnel oven to raise its temperature to 80°C, then cooling. Processing time is 3-5 min followed by a holding time at 80°C that varies with the product but is typically about 7 min. Cooling may take an additional 45 min. Thus far, this process has been carried out only at 2,450 MHz. A typical plant producing fresh gnocchi processes 3,000 lb/h. The microwave process is effective against molds, yeasts, and thermolabile microorganisms.

There are a number of full-scale systems in operation today for the microwave pasteurization of sliced packaged bread. The purpose of this process is twofold: (1) to increase the shelf life so that specialty breads, in particular, do not have to be baked as frequently and (2) to eliminate the need for chemical preservatives.

The most recently installed systems carry out pasteurization by conveying baskets of prepackaged bread through a microwave tunnel. The density of the product is sufficiently low that heating occurs deep within the product. As the baskets exit from the tunnel they are stacked to allow the temperatures to equilibrate. This may take 1-2 h. This practice appears to have no harmful effect on product quality and shelf life is substantially increased.

A wide variety of prepared foods are being pasteurized in package by two different microwave processes and sold at retail from refrigerated cabinets. One process involves hot filling the product in microwavable plastic dishes, lidding them, followed by a series of passes through a controlled-temperature tunnel and a microwave-hot air tunnel before being cooled. A refrigerated shelf life of six weeks at 4°C is claimed. The microwave system is operated at 2,450 MHz using 48-1.2 kW magnetrons. The other process involves microwave heating the products while they are immersed in a shallow, controlled-temperature water bath. This technique claims to provide more uniform control of product temperature.

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