Microwaving is a method of cooking where food is bombarded by microwaves, usually within an appliance called a microwave oven which excite the water, fat and sugar molecules, thereby heating (cooking) both the outside and center of the food at the same time. Conversely the microwaves do not heat glass, ceramic and plastic containers. (A common myth is that a microwave oven cooks from the center of the food outward. This appears to happen because heat generated at the surface escapes more readily from the surface of the food into the surrounding air.) One advantage of microwaving is that small amounts of food can be heated very quickly, making it useful for reheating leftovers.
The disadvantage is that food which is microwaved does not undergo some of the chemical reactions, such as browning, which makes the food visually attractive. Primitive microwave ovens often do not cook evenly, leading to a concern that bacteria easily killed by more traditional cooking methods may survive the quick cooking time in "cold spots", though the food item as a whole is cooked to a safe average temperature. This can be overcome by leaving the food to stand for a few minutes when cooking is completed. Some high-end microwave ovens are combined with a convection oven which basically cook the food using microwave and hot air simultaneously to achieve both the fast cooking time and browning effect.
Using a microwave for cooking a large food mass is difficult due to limited penetration of microwaves. However microwave ovens are used in some fast food chains and special microwave bags are available for cooking fowl or large joints of meat.
Professional chefs generally recommend using microwaves for a limited set of tasks, including: melting fats (such as butter) and chocolate, cooking grains like oatmeal and grits, cooking rice, thawing frozen meats and vegetables before cooking by other methods and quickly reheating already-cooked foods.
Using a microwave to boil water is potentially dangerous, due to superheating. In a microwave, water can be raised quickly to a temperature above the boiling point before major bubbles form, especially if it is purified and in a very clean glass vessel. When water in that state is disturbed, it can suddenly and unexpectedly boil violently, leading to a risk of scalding. This effect is rare, even for scientists who try to deliberately recreate it, and any seed whatsoever for boiling is likely to prevent the problem. Boiling water with, for instance, a teabag already in it will prevent any dangers by providing a seed, as will using a mug that is not perfectly clean.
The risk greatly increases when water has already been boiled once in the same container. This situation can occur if the user of the oven boiled the water once, forgot about it, then came back later to boil it again. The first time the water boils, the seed bubbles (microscopic bubbles of air around which larger steam bubbles grow) are used up and largely eliminated from the water as it cools down. When the water is heated again, the lack of seed bubbles causes superheating, and a risk of a steam explosion when the water's surface is disturbed.
Placing something in the water before heating can mostly alleviate this risk. If you are planning to mix something with the water, say tea or hot chocolate, adding it before heating will ensure that the water boils. Otherwise, placing a wood object, for instance a chopstick, in the water before heating will also work.
Care should be taken when removing heated water from a microwave. Make sure that the hands are protected from possible liquid boil-over, place the container on a level, heat-proof surface and stir liquid with a warm spoon. Also, never add powdered substances (such as instant coffee or cocoa mix) to the container taken from the microwave, due to the addition of all those seed bubbles and the potential for violent, spontaneous boiling. It is advised that the water should be poured slowly into another container that already contains the powder.
Metal objects, such as metal utensils, in a microwave oven can lead to dangerous situations. Metals do not absorb microwaves effectively. Instead, metals reflect microwaves, thereby preventing the latter from reaching the food. If the microwaves are not absorbed inside the oven, the oven can be damaged by electrical arcs and overheating of the microwave source. Thin metal layers, such as metal foil and mugs with metal trim can melt or burn due to the strong electrical currents that are generated in metal objects. However, small solid metal objects, such as spoons, in combination with a large amount of absorbing food or liquid, normally do not lead to problems.
With wireless computer networks gaining in popularity, microwaves can disrupt wireless network transmissions due to the fact that the microwave creates radio waves at about 2.4 GHz. This is about the same frequency that wireless networks use, so microwaves in use will interfere with network signals.