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microwave oven is one of the great inventions of the twentieth century since millions of homes in America have one. Microwave ovens are popular because they cook food incredibly quickly. They are also extremely efficient in their use of electricity because a microwave oven heats only the food and nothing else.
A microwave oven uses microwaves to heat food. Microwaves are radio waves. In the case of microwave ovens, the commonly used radio wave frequency is roughly twenty-five megahertz, which is 2.5 gigahertz. Radio waves in this frequency range have an interesting property: water, fats and sugars absorb them. When they are absorbed they are converted directly into atomic motion in other words, heat. Microwaves in this frequency range have another interesting property: most plastics, glass or ceramics does not absorb them. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.
People often hear that microwave ovens cook food "From the inside out." An example of that would be if someone wanted to bake a cake in a conventional oven. Normally you would bake a cake at 350 degrees F. Or so, but let's say you accidentally set the oven at 600 degrees instead of 350. What is going to happen is that the outside of the cake will burn before the inside even gets warm. In a conventional oven, the heat has to migrate (by conduction) from the outside of the food toward the middle People also have dry, hot air on the outside of the food evaporating moisture. So the outside can be crispy and brown while the inside is moist.
In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. There is no "heat having to migrate toward the interior by conduction." There is heat everywhere all at once because the molecules are all excited together. There are limits of course. Radio waves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference, but you get the idea. The whole heating process is different because you are "exciting atoms" rather than "conducting heat."
In a microwave oven, the air in the oven is at room temperature, so there is no way to form a crust. That is why foods like "Hot Pockets" come with a little cardboard/foil sleeve. You put the food in the sleeve and then microwave it. The sleeve reacts to microwave every by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.
Microwave technology evolved out of the development of radar (Radio Detection And Ranging). Because microwave pulses can be very short, they can be used for distance and time measurement. The simplest form of radar measures the time for an echo to return from a certain direction. Microwaves penetrate fog and clouds, travel in straight lines, and give distinct shadows and reflections.
Microwave ovens provide an effective way of heating many nonconductive materials. They penetrate the material; whether or not heat is generated is determined by the specific dielectric properties of the material itself. In most materials, the microwave-power absorption is proportional to the water content of the material. The frequency of commercial microwave ovens (2.45 GHz) was selected so that a standard portion of food would be heated uniformly. Because the heat does not have to be conducted thermally through the food, but is generated inside the materials, microwaving reduces the time needed for heating the food to a uniform temperature.
Microwaves cause heating within a material by exciting molecules to rotate. This rotation produces energy in the form of heat. Unlike conventional heating, this effect occurs simultaneously throughout the whole material being microwaved. This has important implications for microscopy, because the basis of much specimen preparation is the effective diffusion of fluids in and out of tissue blocks or sections. Heat increases the rate of diffusion, and microwave (internal) heating can enhance it even more effectively.
As an example, two 2 x 2 cm3 cubes of beef (striated muscle) were dehydrated. One cube was heated externally at 70 C. In 100% ethyl alcohol for five minutes, the other kept at that temperature by microwave exposure. In the case of external heating, only the outer part of the cube was slightly dehydrated (hard and gray), but the microwaved cube was completely dehydrated (hard and gray all the way through), illustrating the more effective diffusion of alcohol into the interior of the material.
These same properties of microwave heating will dictate the choice of which processing fluids to use. Different substances subjected to the same amount of microwave energy heat up at different rates. For example, 100 ml of water needs 2.2 times more heat to warm up than 100 ml of alcohol. The materials that heat up fastest are comprised of non-symmetrical polar molecules, which are easily rotated by microwave energy. This can have important implications for the microscopist. For example, xylem has been the clearing agent of choice for most conventional histology because of its fast diffusion rate, despite the fact that it is flammable, causes dermatitis and can shrink tissue. With microwave processing, however, isopropanol penetrates faster than xylem; and isopropanol is much less harmful, and causes less shrinkage of specimens.
In controlling the temperature of microwaved materials can be achieved through use of a temperature probe which is connected to power-level control. After a cycle of exposure, temperature is checked against a preset value. When this value is reached, the exposure pattern is adapted to maintain this temperature. However, this control is still very imprecise with kitchen microwave ovens. Often, temperatures can only be set in multiples of 5 C. Or over a limited range of temperatures. More important, when after a cycle, the desired temperature has almost, but not quite, been reached, the next cycle may overshoot the preset temperature. The lack of fine control becomes especially dramatic in the case of small laboratory samples, which can easily overheat and become damaged. Conversely, in the case of relatively microwave-transparent materials (like paraffin), this pattern does not usually suffice to maintain the desired temperature.
A further way of controlling temperature is through the use of a "dummy load"- a vessel of tap water placed in the back of the oven which functions as a heat sink, and thereby reduces the power absorbed by other specimens in the oven. In general, the rate of temperature rise slows in proportion to the size of the dummy load, but the shape of the container, its location, and the initial temperature of the water, all have an effect
The great advantage of microwave stabilization is that there is no chemicals involved which would extract important components from the tissue. Researchers have found that up to 40% of protein can be lost after formaldehyde fixation. However, other researchers have found significant disadvantages in this method (shrinkage, sponginess of tissue, and breakdown of red blood cells). Kok and Boon recommend a "hybrid" method, in which chemical post fixation is done after the initial microwave stabilization. In this case, the "poaching" effect of microwave stabilization seems to create channels through the tissue, permitting subsequent enhanced diffusion of fixatives into the cell.
Microwave exposure can be used to enhance diffusion of fixation reagents into the tissue, and to accelerate the chemical process by which the fixative cross-links with the protein of the tissue. The most common histological fixative, formalin, is a solution containing methylene glycol and a little formaldehyde. Normal formalin fixation takes place in three steps: first, the methylene glycol quickly penetrates the tissue (formalin will penetrate a 5 mm block in 4 hours); second, some methylene glycol is slowly converted to formaldehyde by dehydration; third, formaldehyde binds very slowly to the proteins in the tissue by cross-linking. All three of these steps can be accelerated by microwave exposure.
However, simply microwaving tissue in formalin produces disappointing results, because the outside of the tissue fixes so rapidly and well that it effectively prevents further diffusion of fixative into the central part of the biopsy. For that reason, a "hybrid" procedure is recommended. First, tissue blocks are soaked in formalin for 4 hours at room temperature (longer, if blocks thicker than 5 mm are used); next, the blocks are microwaved for 1.5 minutes at 55 C. Some researchers use shorter soaking times in diluted formalin solutions. Excellent immunostaining has been achieved using this hybrid method of fixation.
All electromagnetic energy can be characterized as waves with a specific wavelength and frequency distributed over a continuous range known as the electromagnetic spectrum. For example, some radio waves have a wavelength of 6 feet (2 meters) and a frequency of 50 million hertz (Hz-cycles per second). Visible light waves have a wavelength of 400 to 700 mill microns, and typical X-rays have a length of 0.01 mill microns and a frequency of 30…[continue]
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