Microwave Oven

History & Development

Cooking food with microwaves was discovered accidentally in 1945. Percy Spencer, an American self-taught engineer from Howland, Maine, was building magnetrons for radar sets with the American company Raytheon. He was working on an active radar set when he noticed that a peanut chocolate bar he had in his pocket started to melt. The first food to be deliberately cooked with microwave was popcorn, and the second was an egg, which exploded in the face Spencer’s colleague.

On October 8, 1945, Raytheon filed a U.S. patent for microwave cooking process. In 1947, the company built Radarange, the first microwave oven in the world. It was almost 1.8 meters tall, weights 340 kg and cost about US$5,000 each. It consumed 3 kW of electricity, about 3 times as much as today’s microwave ovens, and was water-cooled.

The initial models were not very successful. By the 1970s, technological advances led to a microwave oven that was gaining more acceptance among consumers. More and more people were finding the benefits of microwave cooking to overweight the possible risks, and none of them were dying of radiation poisoning, going blind, sterile or becoming impotent. Myths were melting away, and doubts were turning into demand.

By 1986, roughly 25% of households in the U.S. owned a microwave oven. Current estimates hold that over 90% of American households have a microwave oven.

Percy Spencer continued at Raytheon as a senior consultant until he died at age of 76. At the time of his death, Spencer held 150 patents. On September 18, 1999, Percy Spencer was inducted into the National Inventors Hall of Fame.



Principles

A microwaves oven works by passing non-ionzing microwave, usually at a frequency of 2.45 gigahertz through the food.

Water, fat and other substance in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules are electric dipoles, meaning that they have a positive charge at one end and a negative chare at other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. This molecular movement represents het which is then dispersed as the rotating molecules hit other molecules and put them into motion.

Benefits

1. Time Saving

Microwaves ovens are most suitable to reheat pre-cooked frozen dishes. On the other hand, defrosting using a microwaves also cut the waiting time tremendously.

Microwaves are also useful for the ease in which they can perform some traditionally cumbersome kitchen task, such as softening butter or melting chocolate

2. Energy Saving

A microwave oven consumes 1100W of electricity in producing 700W of microwave power, an efficiency of 64%. The other 400W 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. Whereas a conventional convection oven consumes 1600W or electricity, with cooking energy efficiency of 65%.

When heating water for a coffee, a microwave heats just the mugful of water itself.

When using a kettle, an element heats the kettle itself plus the water plus extra water which is then left unused.

Cooking in conventional ovens entails heating in the internal structure of the oven and the air it contains to cooking temperature, and additionally, it involves maintaining that temperature against convective and radiative losses of heat for a longer time that is usual with a microwave oven.

3. Less Burn-hazard

Microwave ovens heat food without getting hot themselves. Only the food is heated directly, the cookware is indirectly heated by the food, thus cookware’s temperature is much cooler than the food. Comparing to a conventional oven, one’s arms are exposed to the very hot walls of the oven and the hot cookware.

Food and cookware taken out of a microwave oven is rarely much hotter than 100°C.

4. Overcooked/Charred

A typical cooking temperature is 180°C. Cooking with microwave ovens eliminates the formation of tars and chars, which are carcinogenic.

5. Even Heating

Microwave radiation also penetrates deeper than direct heat. So that the food is heated by its own internal water content. In contract, direct heat can fry the surface while the inside is still cold.



Misused / Disadvantage

1. Uneven Cooking/Heating

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.

Uneven heating may partly due to the different rates of energy absorption in different parts of the food.

Microwave ovens are better for reheating previously cook.

2. Food Poisoning

Bacterial contamination may not be killed if the safe temperature is not reach, resulting in food borne illness, as with all inadequate reheating methods.

3. Burn-hazard

Any metal or conductive object placed into the microwave will act as an antenna to some degree, resulting in an electric current. This causes the object to act as a heating element.

Metal cookware or cutlery put inside a working microwave oven may thus be heated, and caused scalding of an unguarded users, when removing the contain from the oven.

4. Hazardous Gas

Any object containing pointed metal can create an electric arc ( sparks ) when microwaved. This includes cutlery, aluminum foil, ceramics decorated with metal, or almost any metal formed into a poorly conductive foil or thin wire; or into a pointed shape.

This is because the pointed metals resonate with the microwave radiation and produce high voltage at the tips. This has the effect of exceeding the dielectric breakdown of air, about 3 megavolts per meter. The air forms a conductive plasma, which is visible as a spark. When dielectric breakdown occurs in air, some ozone and nitrogen oxides are formed, both of which are unhealthy in large quantities.

Hazards

1. Superheat

Liquids can superheat when heated in a microwave oven in a container with a smooth surface. That is, the liquid reaches a temperature slightly above its normal boiling point without bubbles of vapour forming inside the liquid. The boiling process can start explosively when the liquid is disturbed, such as when the user takes hold of the container to remove it from the oven or while adding solid ingredients such as powdered creamer or sugar. This can result in spontaneous boiling (nucleation) which may be violent enough to eject the boiling liquid from the container and produce severe scalding.

It is commonly, but wrongly, thought that only distilled water exhibits this behaviour.

2. Explosion

Closed container and eggs can explode when heated in a microwave oven due to the increasing pressure of steam.

3. Fire

Products that are heated too long can catch fire.

Though this is inherent to any form of cooking.

However, because the microwave oven’s cavity is enclosed and metal, fires are generally well contained. Simply switching off the oven and allowing the fire to consume available oxygen with the door closed will typically contain damaged to the oven itself.



Safety
1. Microwave leakage

The microwaves emitted by the sauce in a microwave oven are confined in the oven by the material out of which the oven is constructed.

According to USDA’s Center for devices and Radiological Health, the amount of microwaves that can leak from an oven throughout its lifetime to 5miliwatts of microwave radiation per square centimeter at approximately 2 inches from the surface of the oven. This is far below the exposure level currently considered to be harmful to human health.

2. Non-ionizing Microwave

The radiation produces by a microwave oven is non-ionizing. It therefore does not have the cancer risks associated with ionizing radiation such as X-rays, ultraviolet light, and high energy particles.

Long term rodent studies to assess cancer risk have so far failed to identify any carcinogenicity from 2.45 GHz microwave radiation even with chronic exposures level.

However, the radiation may cause damage by heating; as with any cooking device

3. Fail-safe Interlock

Microwave oven sold in the market has a protective interlock so that it cannot be run when the door is open or improperly latch.