Geometry

The shape of food items is critical to good microwave heating results. The sphere is the ideal shape as energy tends to be focused to give heating at the center of the sphere. Obviously as the diameter is increased it may be impossible for center heating to occur except by conduction.

Figure 1. The effect of thermal conduction on the temperature profile of a large roast beef after 45 min standing out of a microwave oven.

Heating can be concentrated in the center of spheres with diameters (8) that measure between 20 and 60 mm. Such concentrated heating can be a disadvantage in that conduction heating may not be able to dissipate the temperature gradient and the mass would erupt, as in the heating of an egg in its shell. At lower microwave heating rates, more time is provided for conduction heating to take place. In the example of beef roasting the postmicrowave heating time could have been reduced if a lower heating rate were used, as the temperature gradients would not have been as great.

Computer simulation studies as well as actual heating of spheres and cylinders of various diameters have been carried out (8). A phantom food mixture with dielectric properties similar to food materials was used in these studies and temperature patterns were obtained by color thermography. For cylinders, thermography indicated maximum core heating occurred at diameters of 35 mm. At 50 mm, the core and surface heating occurred, while at 75 mm only surface heating was observed. Calculations support maximum center heating for diameters of 20 to 35 mm, while surface heating was more prominent at 40 to 50 mm and was dominant for larger cylinders.

While spherically shaped foods are commonplace among many natural foods eg, beets, onions, potatoes and some prepared foods (meatballs), the majority of foods are not spherical in shape. The cylinder is the next best shape in terms of heating performance.

For many foods it is the food container or dish that determines the shape and, therefore, affects the heating performance. A rectangular shape is common, but it is clear that food in the corners of a rectangular container will be overcooked before the remainder of the food. Overcooking in the corners also occurs when using a conventional oven, but because conduction cooking is much slower the temperature differences are not as pronounced. Where this shape is unavoidable, the corners can be shielded with alu minum foil to reduce the heating rate in these areas. Even when corners are unavoidable they should have generous radii to minimize the overheating effect. Also vertical or near vertical sides are preferable to sloping sides. Excessive sloping gives shallow areas where dehydration can occur.

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