The way we cook with a frying pan is quite different from with an oven or a stew pot. Recipes must be specific for the cooking technique and its characteristics, and microwave cooking is no exception.
The first, and dominating, characteristic of microwave cooking is its high speed. In all other cooking techniques, heat is delivered to the surface of the food, e.g. in contact with the hot cooking vessel, and must reach the interior of the food by conduction or convection. In contrast, microwaves penetrate into the food, and deliver their heat there, cutting the required cooking time considerably. It is not true that microwaves cook the food from the inside out. The strength of the electromagnetic wave decreases as it penetrates into the food, so the heating rate is still faster closer to the outside surface. However, the surface of the food loses heat by conduction to the surrounding air, so that the highest temperature might be inside. Given that the microwaves only penetrate a few cm’s into the food, the short cooking time advantage will mostly be realized for food which is no thicker than a few cm’s. Thick food items will require cooking at lower power, and allowing time for thermal diffusion; the relative advantage of microwave cooking will not be so great when cooking thick items.
Furthermore, in all other cooking techniques, maximum heat is not delivered to the food until after some warm-up time, which may range from a few minutes for frying, to 15 minutes for oven baking. In contrast, the warm-up for microwave cooking is but a few seconds (needed to warm up the magnetron tube that generates the microwaves). Therefore, maximum heat is delivered almost immediately. Thus with almost no warm-up time, and much high heat flow, cooking times are often a fraction of that required using other techniques.
Spatial variation in the heating rate
Microwaves with a wavelength of 12.2 cm are directed into the food chamber of microwave ovens, and bounce off the various walls in such a way that there is “constructive and destructive interference”. This means that at some locations, the various reflected waves reinforce each other, and the electric field is very strong (“hot spots”), and others where the various waves cancel each other, and the electric field is very weak (“cold spots”). Food that happens to be located near a hot spot will be heated much faster than food located at a cold spot.
Microwave oven manufacturers use various techniques to alleviate this uneven heating. The most common technique is to provide a rotating turntable. Food placed on the turntable passes through hot spots and cold spots, so that the heating is much more uniform. In a few microwave ovens, the locations of the hot spots and cold spots are varied with time, using a technique called “mode stirring”
Variation from oven to oven
Many microwave oven models are available in the market, and the heating characteristics will vary from model to model. Typical household microwave ovens vary with power, from about 0.7 kW to about 2.5 kW, and volume, from about 17 liters to about 60 liters. Furthermore, the performance of a given oven may vary with the age of the oven, and the type, quantity and location of the food placed in the oven.