Microwave energy

Microwave oven has become a important appliance in modern day kitchens. So in this content we are going to study about the construction of microwave ovens.

At first the microwave oven uses microwave waves to cook the food

MICROWAVE ENERGY:
Microwaves are electromagnetic energy, which is same as  that carries Radio and Television signals. Note that while you read this information you are surrounded by such energy. If you were to operate a transistor radio you would expect to be able to listen to radio stations, therefore radio frequency energy must be present to enable this to happen. The major difference between the electromagnetic energy that brings you your favourite radio station and the energy used in a microwave oven is the frequency. Microwave ovens use radio frequency (RF) at 2450MHz. A typical TV broadcast frequency is about 400MHz. The higher the frequency, the shorter the wavelength, hence the term 'microwave'. From the diagram below it can be seen exactly where each frequency lies, and how the rest of the electromagnetic spectrum is allocated.

                The whole of the energy spectrum can be split into two broad areas:

  • frequencies in the ultraviolet section 
  • above and frequencies that are below ultraviolet
                              Energy in and above ultraviolet is Ionising in nature. If our bodies are exposed to this type of energy permanent tissue damage will occur.Due to radiation even cancer may form in the body. The effect of Ionising radiation is cumulative, each time we are exposed to this type of energy the level within our bodies is increased, which will eventually lead to sickness.

                                Energy at frequencies below ultraviolet light are non-ionising and the effects are not cumulative, that is to say there would be no long-term problems if exposed to energy in this part of the spectrum. There is no permanent tissue damage or change in molecular structure. For example when we are over exposed to sunlight while sun bathing and get burnt, the skin quickly heals and returns back to normal. However there may be long term problems due to the sun's rays containing ultraviolet energy.
                                Energies at microwave frequencies and of sufficient power have a heating effect and this principal is exploited in the use of microwave energy for cooking.

MOLECULAR AGITATION:

                                  All matter is made up of atoms and atoms of different elements combined in a long strings to form molecules.  These molecules are affected by magnetic force. Molecules of water, fats and sugars in food are affected by microwave energy. If these molecular structures were examined under a microscope, under normal conditions, the molecules were arranged in random form i.e., not aligned in any particular pattern. If the food when placed in a strong electromagnetic field, it would be seen that the molecules had aligned themselves in the direction of the force of the field. If the field were then reversed, the polarity change would cause the molecules to be reoriented to the existing field direction. It is possible to cause the electromagnetic field to alternate continuously, producing a continuous movement of the molecules. This movement produces heat due to the friction caused as the molecules move against each other.

                                                 As from the above picture we can see the molecules arrange themselves according to the direction of force.  Since direction of force are in a alternate manner the molecules are in a random motion and agitation.  Microwave ovens use RF energy that causes the molecules in substances being cooked to move 4,900,000,000 times a second (2 X 2450MHz).
HEATING EFFECT AND PENETRATION DEPTH:
                                      All microwave oven manufacturers follow a particular microwave frequency which is a standard value.  This frequency is kept for speed cooking with large penetration depth for high efficiency of the oven.
                                               This graph denotes the adequate frequency to be set. An increase in the microwave frequency would give an increase in the speed of heating, but lower penetration depth. Lowering the frequency would allow much deeper penetration, but would have a poor heating effect. The frequency of 2450MHz is a compromise giving the best heating effect against penetration depth, as the chart opposite shows. Typically the penetration depth in meat is approximately 2.54cm (one inch) and approximately 6cm (two and a third inches) for most other types of food.

CHARACTERISTICS OF MICROWAVE ENERGY:
Microwave energy has three main characteristics they are
  • Reflection
  • Penetration
  • Absorption
1.REFLECTION:

                                Microwave energy will be reflected by the surfaces of metal objects. This characteristic is useful when designing microwave ovens. The cavity and wave guide are made of metal and this keeps the energy enclosed within these areas, while directing the energy onto the food.                                
                                 Reflection from metal objects however can be a problem. When cooking, energy would be unable to penetrate a metal vessel leaving the food inside uncooked. Large metal objects can also have a adverse effect on the wave pattern within the cavity, which affect the cooking. It should be noted that if a metal object is present in the cavity while cooking, this object will act as a receiving aerial and therefore a potential difference will exist between the object and the oven walls. If the metal object comes close to or in contact with the cavity then arcing will occur. This will result in damage to the surface of the cavity and the metal object. Thus the metal vessel aren't advised to use as a cooking pan in microwave oven.

2.PENETRATION:

                                      Certain substances absorb very little or no microwave energy.  The energy is able to pass through them without having an effect. This characteristic is helpful for oven design and oven use. Waveguide covers and stirrer covers are made of materials that allow energy to pass through without loss. For cooking purposes chinaclay and glass are good examples of non-absorbent materials. They are therefore ideal for holding food whilst still allowing the energy to reach the food from all sides.

3.ABSORPTION:
                                         There are many substances that absorb microwave energy and are therefore heated by it. However it should be noted that different substances absorb energy at different rates which in turn will affect their required cooking times. The chart on below this paragraph shows the absorption rates of some different types of food/materials.

LOSS ANGLE CHART

                                        Substances will absorb microwave energy in differing degrees, which is usually dependent upon the molecular structure of the material.  A diagrammatic view of this is shown on the right. High loss materials will heat up faster then low loss materials because the molecules have more space to move around and thus more heat is generated by friction. Note that water and ice are at opposite ends of the Loss Angle Chart.  This is because the molecular lattice structure of ice is very ridged and therefore they not move when vibrated by the microwave energy.  This may seem confusing as microwave ovens are often used for defrosting.  Refer to the section below for an explanation on how defrosting is achieved.

The Absorption Phenomenon:
                                           The loss angle chart shows just a few examples of substances that absorb microwave energy at different rates. It can be seen that water is one of the most absorptive of substances to microwave energy and therefore heats up most rapidly. However when frozen it becomes one of the least absorptive.
                                            If a mixture of food substances is heated together, perhaps a meal being reheated on a plate, then it is possible that certain items will heat more rapidly than others. Fish for instance, would heat up quickly where as a mashed potato would take longer. This effect is perfectly normal, although some customers new to microwave cooking may find this strange at first.
                                             A slightly different technique is required for microwave cooking when compared with conventional cooking methods. Purchasers of new ovens receive a cook book and instruction leaflet with their oven.
                                             One technique that is used, and will help with our mixed meal example discussed previously, is to allow 'standing time'.
                                              Standing time is an additional cooking time after the heating has finished. If the mixed meal were covered after the heating period, the cover would help retain the heat generated within the foods that heat up the most rapidly. This heat is then available to assist the heating of the other food in the area using the heat exchange principle.
                                               Therefore if at the end of the microwave heating time the food is allowed to stand, the heat exchange effect will continue the cooking process and the result will be an evenly heated meal. Standing time is not used to allow microwave energy to escape from the food before it is eaten. This is just one of the myths that have circulated over the years regarding microwave energy. The other effect considered previously was the way in which water absorbs microwave energy much more readily when in its liquid state compared to ice. This could cause problems when using a microwave oven to defrost some food. Since ice is not affected by microwave energy, the defrosting process will not begin until the ice begins to melt naturally.
                                                 Only when some water has formed, will the microwave energy start having an effect and the defrosting cycle begins. The delay caused is likely to result in the food not being completely defrosted when the defrosting cycle ends.
                                                  The way to overcome this is to remove the food from the freezer about five minutes before using the oven in defrost mode. This will allow some ice to melt and the microwave energy will have an effect on the food immediately the oven is turned on. Most ovens will have at least one defrost setting and usually a range of settings is available.
                                                   Sharp ovens use a cyclic system of power control. This involves running the magnetron at full power for a pre-set time, the amount of time depending on the mode selected, and then the magnetron is turned off for the remainder of the cycle. Cycle time is usually about 30 seconds. The timing diagrams for the electromechanical ovens can be seen in the Variable power control section of this book.
                                                   Changing the on time of the magnetron in this way allows some parts of the food to be warmed while the magnetron is on, then when the magnetron is off, heat is conducted into other parts of the food. This process is repeated until the food is defrosted.


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