FAQ's
No one thought of cooking food with microwaves until the 1940s, when a self-taught engineer named Percy Spencer was building radar equipment in a lab for Raytheon, and noticed that a chocolate bar he had in his pocket started to melt. He had been building magnetrons, and realized that microwaves can be directed at food to heat it up rapidly. He tested his idea by popping popcorn and exploding an egg. Not long after we were all happily scarfing down TV dinners.
Microwave radiation is a form of non-ionizing radiation (meaning it can’t directly break up atoms or molecules) that lies between common radio and infrared frequencies. So it is not thought to damage DNA of living things, the way X – rays and gamma-rays do. Still, microwaves can obviously cause heating effects, and can harm or kill at high energies. That’s why microwave ovens on the market must operate at or below strict limits set by the International Standards. Most microwave ovens heat food with microwaves at a frequency of 2.45 gigahertz (GHz) (a wavelength of 12.24 centimetres OR 4.82 inches). The molecules in the food, particularly water, absorb energy from the waves through dielectric heating. That is, since water molecules are polar, having a positive end and negative end, they begin to rotate / vibrate rapidly as the alternating electromagnetic field passes through. This rotation / vibration generates heat due to frictional and dielectric hysterisys loss. For materials to be heated in microwaves the materials must be dielectric with polar molecules and a considerable dielectric loss factor.
Although many people believe this to be the case, microwaves actually work on the complete bulk of the material, heating it by exciting the polar molecules there. However the intensity of microwaves goes on reducing as the waves penetrate deeper inside. Thus there is more heating rate outside than the core of a material. there is also a possibility of microwave not being able to heat the core of a bulk material when the material dimensions are larger than the wavelength of the microwaves. This is why a microwave can only cook a big hunk of meat to a depth of about one inch inward. So microwave heating is still outside to inside heating, but with a great advantage of a deep penetration microwaves deep inside the material.
Metals reflect microwaves, whereas plastic, glass and ceramics allow them to pass through. So metals dont heat in microwave ovens…Not really. We now need to be more specific that only “bulk metals” dont heat in microwave ovens. However, powder metals and powder metal compacts are proven to heat in microwave ovens mostly due to Eddy current losses and magnetic hysterisys losses. Also, thin pieces of metal, such as foils or the tines of a fork, can act as antennae, and the waves can arc off them, forming dramatic sparks and creating heat.
A complete analysis of heating efficiency depends on a number of factors, including what you are trying to prepare and the cost and “greenness” of your local supply of electricity, gas or other fuel. Typically though, a microwave uses less energy to heat materials than conventional ovens, because it works faster and more of the energy is focused directly on the material, versus heating containers or surrounding air. In fact, Energy Star calculated that cooking or re-heating small portions of food in the microwave saves as much as 80% of the energy needed for an oven. This gives a great opportunity to save energy in domestic and more importantly industrial heating applications.
Oils such as olive oil do not heat well in microwaves because their molecules lack the polarity found in water. It’s also true that frozen butter is hard to thaw in a microwave, because the bulk of the substance is oil, and the portion of water present is in the form of ice, which keeps the molecules locked up in crystal form, making oscillation more difficult and hence making it difficult for the object to heat.
One potential danger of microwave ovens is getting scalded by over heated water. What can happen is that when plain water is heated in a microwave in a clean ceramic or glass container for too long, it can prevent bubbles from forming, which normally cool it down. The water can become superheated, past its boiling point. So when it is disturbed, say by moving it or dropping something in it, the heat releases violently, erupting boiling water out of the cup.To avoid this risk heat water only the minimum amount of time needed. Or place a wooden spoon or stick in it (you should be fine with a metal spoon too, as we discussed above. Don’t use a metal fork though, which could spark.). This also puts us on an alert to use some stirring mechanism in case of industrial water heating applications.
For decades scientists and consumers have been debating over the possible effects of non-ionizing electromagnetic radiation on living tissue. Since we can’t very well grow people in controlled lab experiments, it’s very difficult to sort out the various risks we might get from fields emitted from power lines, cell phones, airplane flights, computers, clock radios, and of course microwave ovens. We know strong fields raise cancer rates and other problems, but what about the cumulative effect of small exposure, or effects on children? No one knows, although we can take heart that the International Standards limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts (mW) of microwave radiation per square centimeter, at approximately 2 inches from the oven surface. “This limit is far below the level known to harm people.” It’s also true that microwave energy decreases dramatically as you move away from the source of radiation. A measurement made 20 inches from an oven would be approximately one one-hundredth of value measured at 2 inches. This standard also requires all ovens to have two independent interlock systems that stop the production of microwaves the moment the latch is released or the door opened