Microwave

What is a Microwave Oven?
What is the whole process in manufacturing?

What is a Microwave Oven?
The microwave oven is a household appliance that has revolutionized modern home kitchens. It is designed to cook food quickly by using special electromagnetic radiation that heats objects from within. It is designed and primarily used to quickly heat foods, and is most convenient for otherwise time-consuming tasks such as melting butter or heating water. Unlike traditional ovens, microwaves cook food from the inside rather than from the outside. While it was originally used in commercial kitchens, it is now most popular in homes and the workplace.

* The University of Texas Health Science Center at Tyler: Microwaving Tidbits

How a Microwave Oven Works

A microwave works by sending electromagnetic waves at a frequency of 2.24 gigahertz and a wavelength of 12.2 centimeters through food. This in turn causes the water molecules in the food to vibrate, and the resulting friction with other water molecules causes it to heat up. This is how food cooks from the inside in a microwave. It is also why frozen food and dry food (such as rice) doesn’t cook as efficiently: frozen moisture and objects lacking in moisture do not respond quickly or uniformly to microwave radiation. To ensure that food cooks more evenly, many microwave ovens feature a rotating plate to turn the food around during cooking.

* Microwave Wizard: How do microwave ovens work

History of the Microwave Oven

The microwave oven is the result of an accidental discovery made by Dr. Percy LeBaron Spencer, a self-taught electronics engineer working for the Raytheon Corporation. In 1946, while working on magnetron vacuum tubes in pursuit of improved radar technology, he walked by a vacuum tube and discovered that his candy bar had melted in his pocket. He realized that the microwaves from the vacuum tubes could cook food, and experimented by putting popcorn kernels in the same location. This resulted in the world’s first batch of microwaved popcorn. In 1947 Raytheon produced the first Radarange microwave oven, which at over 750 pounds and 5 feet in height, cost over $2000 and was only used in commercial kitchens. Raytheon authorized the Tappan Stove Company to use microwave oven technology in 1952, but the first consumer-grade Tappan oven, released in 1955, cost over $1000, which priced it out of the market. Raytheon then bought Amana Corporation in 1965, who then released a counter-top Radarange microwave oven in 1967 for $600, which still made it quite expensive for households. Initially, microwave ovens were more popular in Japan than America, but by 1975 Americans had become comfortable with microwave ovens to the point that they were outselling gas ranges. By 1976 microwave ovens, which were present in 52 million American households, were more popular than dish washing machines.

Microwave Parts and Accessories

All microwave ovens are built using several basic parts. These include a fuse which is connected to the power cord; this can shut the microwave down in case of overheating or a short-circuit. Then there is a high-voltage transformer which passes electricity to the next device, which is a magnetron; these two parts serve to regulate the microwave radiation levels that are needed to cook the food. The microwave radiation created in the magnetron moves through a wave guide device which is located above the oven's cavity where the food is placed. The microwave radiation then passes through a revolving stirrer blade and bathes the food, causing it to heat up. The walls of the oven cavity are made of metal to reflect the radiation inward toward the food. A safety switch exists in all modern microwave ovens; this shuts off all microwave energy whenever the oven door is opened. Microwaves have timers which automatically stop microwaving after a defined period of time. Most modern ovens have clocks, digital timers and rotating turntable plates to turn food while it's cooking; modern ovens can be programmed by keypads. Microwaves can also have removable racks and sensors that are designed to prevent over-cooking.

* University of Alaska Fairbanks: How Microwave Ovens Work
* Gallawa: How Does A Microwave Oven Work?
* How Produces Are Made: Microwave Oven

Different Types of Microwave Ovens

The counter-top microwave oven was the first to reach the market. Since then, over-the-range (OTR) microwaves and cabinet-mounted microwave ovens have also entered the market. Counter-top microwaves are the cheapest and most mobile; while they take up precious counter space, the advantage is that they can be taken from one house to another, if necessary. OTR and cabinet-mounted microwave ovens save counter space, but they are more costly, immobile, and may need to be installed by a professional. Convection microwave ovens have heating elements and fans which cook the food in external heat, which cooks the food from the outside as well as the inside.

* Consumer Reports: Microwave oven guide

Energy Efficiency

Energy usage rates by microwave ovens typically range between 750 and 1100 watts. This is higher than a refrigerator, but lower than a clothes iron. The kilowatt-hour per unit usage ranks at about 200 kWh, which is more than a color TV but less than an electric oven. To bring a cup of water to a boil, a microwave oven uses 25 % more energy than an electric stove; however, this difference amounts to less than a dollar of energy usage per year. Re-heating food in the microwave, on the other hand, uses 80 % less energy than re-heating in a conventional oven. This confirms the perception that one of the microwave's primary benefits is re-heating food or cooking smaller portions.

Benefits of Using a Microwave Oven

Microwave ovens are faster than conventional ovens for cooking and re-heating. Some vitamins are retained when food is cooked in the microwave than when it is cooked in a regular oven, because longer cooking times destroy beneficial nutrients in food. Microwave ovens only use electricity, not gas, charcoal or oil, which reduces their carbon footprint and eliminates pollution beyond that which is created in the production of electricity. Microwaves do not brown foods, especially proteins. Dangerous organisms have less time to thrive on food that is cooked in the microwave because the food is brought to a high temperature quickly and can be served right away.

* Better Health: Microwave ovens - safety issues

Safety Features

In accordance with US Food & Drug Administration regulations, all modern microwave ovens are designed with redundant shut-off switches that stop the microwaving process as soon as the door is opened. Some come with sensors that detect steam or temperature levels and stop cooking at a certain point to prevent food from being dried out. Microwaves are also required to meet a Federal standard which says they cannot emit more than 5 milliwatts of radiation per square centimeter beyond 2 inches from the unit; this prevents any potential damage that could be done to pacemakers.

Effects on Food and Nutrients

Microwave ovens can reduce the amount of vitamins B, B1, C and E in foods. Microwave ovens can also damage the protein and amino acid contents in meats and reduce the food’s nutritional value. However it is often better than boiling the same foods, which will remove even more of these nutrients because nutrients are lost through prolonged exposure to heat. It is recommended not to use microwave ovens to warm up bottled breast milk, because it can reduce its nutritional value; it is advised to heat up breast milk under hot water instead.

Microwave cooking and nutrition

Microwave Hazards

Microwaves are as well known for their hazards as they are known for their uses. The Food and Drug Administration has found that microwaving certain foods may result in the creation or accumulation of acrylamide, which is considered to be a cancer-causing chemical. In addition, other studies suggest that microwaving generates more acrylamide contamination than frying and higher-powered microwave ovens generate more than lower-powered ones. Water can be superheated in a microwave oven to the point where it will erupt violently and cause burns; this can happen when water is microwaved in a very smooth container with no cracks or scratches. Another known hazard with microwaves is the danger of metallic objects sparking inside the oven cavity, which can lead to damage to the oven. This is caused by microwave energy reflecting chaotically off of metal surfaces, particularly sharp ones like forks and uneven surfaces like crumpled aluminum foil. Properly shaped metal objects such as browning pans that microwave pizzas come packaged in, also reflect this energy up into the food, which results in a browning effect that is not normally achieved in a microwave oven. Direct exposure to microwave radiation can cause severe burns, which is why safety mechanisms exist to stop the oven from cooking when the door is open. Certain plastic containers can melt and be ruined by microwave ovens.

However, there are also perceived hazards that either are untrue or are extremely unlikely to ever occur. For instance the threat to pacemakers has been all but eliminated due to regulations that restrict microwave radiation output, and modern pacemakers have also been designed to be shielded from such emissions. Radiation injuries are almost impossible due to the safety features of modern microwave ovens, and contrary to some rumors there is no nuclear radiation produced by these devices.

Safety Tips for Microwave Ovens

* A microwave oven should not be operated if the oven door is damaged or there are problems with closing it securely.

* Metal objects should not be placed inside ovens unless they are designated as safe for microwaves.

* Liquids heated inside a microwave oven should be handled cautiously, with protective mitts.

* Microwave ovens should be cleaned regularly and should not be operated while empty.

Manufacture

What is the whole process in manufacturing?

Microwaves are actually a segment of the electromagnetic wave spectrum, which comprises forms of energy that move through space, generated by the interaction of electric and magnetic fields. The spectrum is commonly broken into subgroups determined by the different wavelengths (or frequencies) and emission, transmission, and absorption behaviors of various types of waves. From longest to shortest wavelengths, the spectrum includes electric and radio waves, microwaves, infrared (heat) radiation, visible light, ultraviolet radiation, X-rays, gamma rays, and electromagnetic cosmic rays. Microwaves have frequencies between approximately .11 and 1.2 inches (0.3 and 30 centimeters).

Microwaves themselves are used in many different applications such as telecommunication products, radar detectors, wood curing and drying, and medical treatment of certain diseases. However, certain of their properties render them ideal for cooking, by far the most common use of microwave energy. Microwaves can pass through plastic, glass, and paper materials; metal surfaces reflect them, and foods (especially liquids) absorb them. A meal placed in a conventional oven is heated from the outside in, as it slowly absorbs the surrounding air that the oven has warmed. Microwaves, on the other hand, heat food much more quickly because they penetrate all layers simultaneously. Inside a piece of food or a container filled with liquid, the microwaves agitate molecules, thereby heating the substance.

Design

The basic design of a microwave oven is simple, and most operate in essentially the same manner. The oven's various electronic motors, relays, and control circuits are located on the exterior casing, to which the oven cavity is bolted. A front panel allows the user to program the microwave, and the The oven cavity and door are made using metal-forming techniques and then painted using electro-deposition, in which electric current is used to apply the paint. The magnetron tube subassembly includes several important parts. A powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated, while a thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating. An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. Also, a blower motor used to cool the metal fins of the magnetron is attached directly to the tube.

The oven cavity and door are made using metal-forming techniques and then painted using electro-deposition, in which electric current is used to apply the paint. The magnetron tube subassembly includes several important parts. A powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated, while a thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating. An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. Also, a blower motor used to cool the metal fins of the magnetron is attached directly to the tube. door frame has a small window to enable the cook to view the food while it is cooking.

Near the top of the steel oven cavity is a magnetron—an electronic tube that produces high-frequency microwave oscillations—which generates the microwaves. The microwaves are funneled through a metal waveguide and into a stirrer fan, also positioned near the top of the cavity. The fan distributes the microwaves evenly within the oven. Manufacturers vary the means by which they disburse microwaves to achieve uniform cooking patterns: some use dual stirrer fans located on opposite walls to direct microwaves to the cavity, while others use entry ports at the bottom of the cavity, allowing microwaves to enter from both the top and bottom. In addition, many ovens rotate food on a turntable.

Raw Materials

The cover or outer case of the microwave oven is usually a one-piece, wrap-around metal enclosure. The oven's inside panels and doors are made of galvanized or stainless steel and are given a coating of acrylic enamel, usually light in color to offer good visibility. The cooking surface is generally made of ceramic or glass. Inside the oven, electromechanical components and controls consist of timer motors, switches, and relays. Also inside the oven are the magnetron tube, the waveguide, and the stirrer fan, all made of metal. The hardware that links the various components consists of a variety of metal and plastic parts such as gears, pulleys, belts, nuts, screws, washers, and cables. The Manufacturing

Process

Oven cavity and door manufacture

1 The process of manufacturing a microwave oven starts with the cavity and the door. First, the frame is formed using automatic metal-forming presses that make about 12 to 15 parts per minute. The frame is then rinsed in alkaline cleaner to get rid of any dirt or oil and further rinsed with water to get rid of the alkaline solution.

2 Next, each part is treated with zinc phosphate, which prepares it for electro-deposition. Electro-deposition consists of immersing the parts in a paint tank at 200 volts for 2.5 minutes. The resulting coating is about 1.5 mils thick. The parts are then moved through a paint bake operation where the paint is cured at 300 degrees Fahrenheit (149 degrees Celsius) for 20 minutes.

The chassis or frame is mounted in a pallet for the main assembly operation. A pallet is a vise-like device used in conjunction with other tools.

The chassis or frame is mounted in a pallet for the main assembly operation. A pallet is a vise-like device used in conjunction with other tools.v
3 After the door has been painted, a perforated metal plate is attached to its window aperture. The plate reflects microwaves but allows light to enter the cavity (the door will not be attached to the cavity until later, when the chassis is assembled).

The magnetron tube subassembly

4 The magnetron tube assembly consists of a cathode cylinder, a filament heater, a metal anode, and an antenna. The filament is attached to the cathode, and the cathode is enclosed in the anode cylinder; this cell will provide the electricity that will help to generate the microwaves. Metal cooling fins are welded to the anode cylinder, and a powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated. A metal strap holds the complete assembly together. A thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating.

5 An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. The waveguide is connected to the magnetron on top of the protruding antenna, while a blower motor used to cool the metal fins of the magnetron is attached directly to the tube. Finally, a plastic fan is attached to the motor, where it will draw air from outside the oven and direct it towards the vanes. This completes the magnetron subassembly.

Main chassis assembly

6 The chassis assembly work is performed on a pallet—a work-holding device used in conjunction with other tools—located at the station. First, the main chassis is placed on the pallet, and the cavity is screwed on to the chassis. Next, the door is attached to the cavity and chassis by means of hinges. The magnetron tube is then bolted to the side of the cavity and the main chassis.

In a completed microwave oven, the magnetron tube creates the microwaves, and the waveguide directs them to the stirrer fan. In turn, this fan points the waves into the oven cavity where they heat the food inside. In a completed microwave oven, the magnetron tube creates the microwaves, and the waveguide directs them to the stirrer fan. In turn, this fan points the waves into the oven cavity where they heat the food inside.

7 The circuit that produces the voltage required to operate the magnetron tube consists of a large transformer, an oil-based capacitor, and a high voltage rectifier. All of these components are mounted directly on the chassis, close to the magnetron tube.

Stirrer fan

8 The stirrer fan used to circulate the microwaves is mounted on top of the cavity. Some manufacturers use a pulley to drive the fan from the magnetron blower motor; others use a separate stirrer motor attached directly to the fan. Once the stirrer fan is attached, a stirrer shield is screwed on top of the fan assembly. The shield prevents dirt and grease from entering the waveguide, where they could produce arcing and damage the magnetron.

Control switches, relays, and motors

9 The cook switch provides power to the transformer by energizing a relay and a timer. The relay is mounted close to the power transformer, while the timer is mounted on the control board. The defrost switch works like the cook switch, activating a motor and timer to operate the defrost cycle. Also mounted on the control board are a timer bell that rings when the cooking cycle is complete and a light switch that allows viewing of the cavity. A number of interlocking switches are mounted near the top and bottom of the door area. The interlocking switches are sometimes grouped together with a safety switch that monitors the other switches and provides protection if the door accidently opens during oven operation.

Front panelvv 10 A front panel that allows the operator to select the various settings and features available for cooking is attached to the chassis. Behind the front panel, the control circuit board is attached. The board, which controls the various programmed operations in their proper sequence when the switches are pushed on the front panel, is connected to the various components and the front panel by means of plug-in sockets and cables.

Making and assembling the case

11 The outer case of the microwave is made of metal and is assembled on a roll former. The case is slipped onto the preassembled microwave oven and bolted to the main chassis.

Testing and packaging the oven

12 The power cords and dial knobs are now attached to the oven, and it is sent for automatic testing. Most manufacturers run the oven from 50-100 hours continuously as part of the testing process. After testing is complete, a palletizer robot records the model and serial data of the oven for inventory purposes, and the oven is sent for packaging. This completes the manufacturing process.

Quality Control

Extensive quality control during the manufacture of microwave ovens is essential, because microwave ovens emit radiation that can burn anyone exposed at high levels for prolonged periods. Federal regulations, applied to all ovens made after October 1971, limit the amount of radiation that can leak from an oven to 5 milliwatts of radiation per square centimeter at approximately 2 inches from the oven surface. The regulations also require all ovens to have two independent, interlocking switches to stop the production of microwaves the moment the latch is released or the door is opened.

In addition, a computer controlled scanner is used to measure emission leaks around the door, window, and back of the oven. Other scanners check the seating of the magnetron tube and antenna radiation. Each scanner operation relays data to the next-on-line operation so that any problems can be corrected.vv The Future

Because of their speed and convenience, microwave ovens have become an indispensable part of modern kitchens. Many developments in the microwave market and allied industries are taking place fairly rapidly. For example, foods and utensils designed specially for microwave cooking have become a huge business. New features will also be introduced in microwaves themselves, including computerized storage of recipes that the consumer will be able to recall at the touch of a button. The display and programmability of the ovens will also be improved, and combination ovens capable of cooking with microwaves as well as by conventional methods will become a standard household product.

Microwave oven

A microwave oven is a sealed box or chamber with a fan to vent the chamber and a source of microwave radiation. Food is placed in the chamber and is bombarded with microwave radiation. This "excites" water molecules in the food, which become hot generating steam and heating the foodstuff around it. Microwaving is a similar process to steaming and food will not brown.

Illustration of a microwave

Microwaves penetrate deeper into foods than heat (infra-red) energy but do not cook from the inside out. If a block of butter is placed in a microwave oven for a short time, heat builds up in the centre and so the centre melts before the outside. Heat can build up in foodstuffs and is not able to escape and this can cause burning or other undesirable effects so it is important to follow manufacturer's instructions and test cooking processes carefully.

Alert icon

Some products should not be placed in the microwave oven as this may damage the microwave generator. There are many materials that should not be placed into microwave ovens but many glass, ceramic, and some plastic containers are suitable. Melamine crockery should never be microwaved as it may catch alight and may cause a fire.
 

Microwave cookery

Microwave cookery involves the transfer of energy to the food in the form of electro-magnetic radiation. These microwaves penetrate the food and rapidly move the molecules of liquid it contains. This creates friction, which in turn creates heat to cook or reheat food.

Diagram of a microwave at work

Microwave ovens are designed so that the microwaves bounce around inside them, penetrating food from all angles. Microwaves can move through some types of containers as it is only when the waves reach the water molecules in the food that friction is created. Please see the chart below for a full description of the types of containers that can be used in a microwave oven.

 

Limitations

A microwave

Microwave ovens are convenient and useful appliances but they do have some limitations:

  • Microwave ovens don't produce the same results as conventional cooking methods. For example, microwaving foods does not produce the same browning effect that grilling or frying does. This is because microwaves do not use applied heat to cook foods.

  • Pastries and cakes tend to go soggy in a microwave oven because of the steam created by microwaving.

  • In general, microwave ovens are smaller and have limited capacity compared to a standard oven.

 

Advantages

Re-heating food in the microwave

Microwave cookery however is particularly useful for:

  • Defrosting frozen foods

  • Reheating pre-prepared dishes

  • Cooking foods that don't require browning

  • Foods that can be cooked fast at an even temperature

Foods suitable for microwave cookery

Ideally, only microwave foods that have high moisture content, such as:

A range of food suitable for microwave cooking

  • vegetables

  • fruit

  • fish

  • shellfish

  • eggs

  • herbs and breadcrumbs can be dried using a microwave.

Utensils and equipment for microwave cooking

Exposure to the radiated energy can be dangerous so it is important to take care when using microwave ovens. These appliances must be properly maintained - keep the unit clean and check doors and seals regularly to make sure that microwaves can't escape while the appliance is running.

 

Food containers

Food containers made from certain materials are unsuitable for use in a microwave oven as they prevent the waves of energy passing through and reaching the food.

Material

Reason for not using

metals

  • reflects microwaves

  • causes sparking which may damage the appliance

  • cause arcing when come into contact with the appliance's wall

lead crystal, antique glass containing glued handles

Microwaves will affect the glue

paper and cardboard

These containers absorb fat, which may lead to the absorption of undesirable elements from the radiated energy

porcelain or glass with a metallic rim

As for metals

Plasticised cardboard

The plasticised coating must be able to withstand temperatures of over 100°C or they will melt

Materials that are suitable for use in a microwave oven include:

  • porcelain without a metallic rim

  • plastic specially marked "microwave safe".

  • glass.

The shape of the container is also important in microwave cookery. A round shape is desirable as the food will absorb the microwave energy more evenly than in an oval, rectangular or square shaped container.