microwave oven Efficiency,Safety and controversy

Efficiency
A microwave oven only converts part of its electrical input into microwave energy. A typical consumer microwave oven uses 1,100 W AC and produces 700 W of microwave power, an efficiency of 64%. The other 400 W are dissipated as heat, mostly in the magnetron tube. Additional power is used to operate the lamps, AC power transformer, magnetron cooling fan, food turntable motor and the control circuits. This waste heat, along with heat from the food, is exhausted as warm air through cooling vents.

Safety and controversy
Template:Original research

The dominant view has been that microwaved food is safe to eat. Microwave ovens have become a fairly standard item in most western homes. Microwaving food is fast, energy efficient, and arguably is healthier than traditional means of cooking. Even those who take this view still concede a number of potential safety issues.

Safety benefits
Commercial microwave ovens all use a timer in their standard operating mode; when the timer runs out, the oven turns itself off. This allows forgetful or unreliable people to use the device safely. If a person puts some food in a microwave and then forgets about it, the most likely scenario is that they will find some cold food in the microwave the next day. If the same scenario occurred with a stove or conventional oven, it would likely cause a fire. It is possible to incorporate a timer into a traditional cooking appliance, but this is not the usual design for such devices. However, some digital microwave oven designs make it easy to accidentally set the timer for much longer than intended. “2:30” may accidentally become “23:00” if the user’s finger accidentally presses the 0 button twice. This can become a hazard if dry foods such as popcorn are being cooked, as they will catch fire if overheated.

Microwave ovens heat food without getting hot themselves. Taking a pot off a stove, with the exception of an induction cooktop, leaves a potentially dangerous heating element or trivet that will stay hot for some time. Some stoves have indicator lights to warn people about the danger, but these do not eliminate the threat. Likewise, when taking a casserole out of a conventional oven, one's arms are exposed to the very hot walls of the oven. A microwave oven does not pose this problem.

Food and cookware taken out of a microwave oven is rarely much hotter than 100 °C (212 °F). Cookware used in a microwave oven is often much cooler than the food because the microwaves heat the food directly and the cookware is heated by the food. Food and cookware from a conventional oven, on the other hand, are the same temperature as the rest of the oven; a typical cooking temperature is 180 °C (356 °F). That means that conventional stoves and ovens can cause more serious burns.

Uneven heating, deliberate and otherwise
In a microwave oven, food may be heated for so short a time that it is cooked unevenly, since heat requires time to diffuse through food, and microwaves only penetrate to a limited depth. Microwave ovens are frequently used for reheating previously cooked food, and bacterial contamination may not be killed if the safe temperature is not reached, resulting in foodborne illness.

Uneven heating in microwaved food is partly due to the uneven distribution of microwave energy inside the oven, and partly due to the different rates of energy absorption in different parts of the food. The first problem is reduced by a stirrer, a type of fan that reflects microwave energy to different parts of the oven as it rotates, or by a turntable or carousel that turns the food. It is also important not to place food or a container in the center of a microwave's turntable. That actually defeats its purpose. Rather, it should be placed a bit off-center so that the item travels all around the area of oven's cooking cavity, thus assuring even heating.

The second problem is due to food composition and geometry, and must be addressed by the cook, who should arrange the food so that it absorbs energy evenly, and periodically test and shield any parts of the food that overheat. In some materials with low thermal conductivity, where dielectric constant increases with temperature, microwave heating can cause localized thermal runaway. As an example, uneven heating in frozen foods is a particular problem, since ice absorbs microwave energy much less well than liquid water, leading to defrosted sections of food warming faster due to more rapid heat deposition there. Due to this phenomenon, microwave ovens set at too-high power levels may even start to cook the edges of the frozen food, while the inside of the food remains frozen. The low power levels which mark the "defrost" oven setting are designed to allow time for ice in a food to melt by conduction from food volumes where melting has occurred, without temperatures of the ice-free volumes rising too high. Another case of uneven heating can be observed in baked goods containing berries. In these items, the berries absorb more energy than the drier surrounding bread and also cannot dissipate the heat due to the low thermal conductivity of the bread. The result is frequently the overheating of the berries relative to the rest of the food. This can also be addressed with lower power settings.

Microwave heating can be deliberately uneven by design. Some microwavable packages (notably pies) may contain ceramic or aluminum-flake containing materials which are designed to absorb microwaves and heat up which aids in baking or crust preparation. Such ceramic patches affixed to cardboard are positioned next to the food, and are typically smokey blue or gray in color, usually making them easily identifiable. Microwavable cardboard packaging may also contain overhead ceramic patches which function in the same way. The technical term for such a microwave-absorbing patch is a susceptor.