10 Common Questions
And Their Answers

Microwave ovens have become as common in the American home as refrigerators, dishwashers and the TV remote control. Almost as common as the appliance are consumer questions about microwave cooking, most of which can't be answered off the cuff.

Swamy Anantheswaran, associate professor of food science in Penn State's College of Agricultural Sciences, who has done research on microwave technology, gives these answers to his top 10 most common microwave cooking questions.


  1. Why does my coffee taste bad and show an oily film when I reheat it?
    "Coffee and tea are very sensitive to the time and temperature used to reheat it," Anantheswaran says. "Because microwaves may heat unevenly, coffee may heat at different rates, which burns some elements of the coffee, leading to off-flavors. The film comes from oils within the coffee or tea which are responsible for its flavor."


  2. Why do buns and breads turn soft and chewy when heating a hot dog or a sandwich?
    "The hot dog or lunch meat in a sandwich releases moisture very rapidly as it's cooked," Anantheswaran says. "The moisture has to go somewhere, and it becomes absorbed by the bread. Overheating and rapid heating also will make bread chewy."


  3. Do meals cooked in a microwave cool faster than those cooked conventionally?
    Anantheswaran says the reverse is true. "Microwaves cook from the inside out, so the heat on the outside is dissipating and the food seems cooler, but in reality it is hotter on the inside," he explains. "The principle is seen easily by heating a jelly doughnut -- the outside is relatively cool, whereas the filling may be scalding hot."


  4. Why does spaghetti or other pastas seem to take longer to reheat?
    Anantheswaran says microwave ovens heat more efficiently when foods have a large surface area, so spread the food out on the plate and avoid cooking directly in containers. Sauces are high in salt, which absorbs microwaves, thus causing the sauce to overcook before the pasta is heated. "The trick to reheating leftovers is to decrease power and add a little moisture," says Anantheswaran. "Everyone likes to cook everything on high. What's an extra 60 seconds if the food is cooked more evenly? Don't race your oven. It's as bad for the food as racing a car is for the engine."
  5. Why does my popcorn burn, and why does the "popcorn" button leave so many kernels unpopped?
    Anantheswaran says microwave popcorn pops differently because of such variable factors as brand of popcorn, brand of oven, quality of the kernels, quality of the bag, age of the product and whether the product is salted.

    Joy Daniel, senior manager of product development for Sharp Electronics Corp., explains that popcorn buttons differ in two ways. A "timed" popcorn button is designed to cook for a pre-set time based on the average cooking time for consumer popcorn brands. A "sensor" popcorn button will shut the oven off as soon as it senses a certain level of humidity indicating the popcorn is fully popped.

    "Microwave popcorn may have more unpopped kernels because the manufacturers are less concerned with popping all the kernels than with providing the indicated quantity," Daniel says. "The manufacturers include more kernels to make sure the corn pops to a full bag every time."


  6. What is the best covering for food?
    Daniel says paper towels are best for breads, rolls and muffins. Wax paper works best for vegetables. Lids are best for large quantities of dense foods.


  7. Why do microwaves heat unevenly?
    "Microwaves are absorbed from the outside in," Anantheswaran says. "That heat is absorbed differently depending on the size, shape and composition of the food." He says microwave ovens get the most even heating with small cylindrical or spherical foods, such as hot dogs or potatoes.

    Rectangular foods, such as packaged dinners, may heat less evenly because the microwaves are hitting the top and the sides, causing the corners to overcook.


  8. What role does the glass platform play in the cooking process?
    Removable glass trays and turntables are easy to clean. Carousel platforms allow more even heating as the food rotates within the oven. They also lift the food to allow microwaves to penetrate from below, Daniel explains.


  9. Why does my microwave oven cook meats to a tougher consistency?
    "In most cases meat loses tenderness in a microwave from being cooked too rapidly." Anantheswaran says. "Cooking is a series of chemical reactions, and we have to allow time for each reaction to be completed. I usually cook meat on 50 percent power."


  10. Why do my hot dogs and potatoes explode?
    Anantheswaran explains that foods with skins, such as potatoes, apples and hotdogs, build up steam pressure within the center as the microwaves heat the product. To prevent bursting food, pierce the skin with a fork or make a small slit along the length of the product.

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Comments on "Microwave cooking"

tk June 03, 2010 | 10:05 AM

numbers 3 and 7 are contradictory. 3 says microwaves heat from the inside out, and 7 says microwaves are absorbed from the outside in and that "corners may overcook". how will a corner over cook if #3 is true and things heat from the inside out? oops... Personally, i'd get a better expert. maybe Wikipedia? A microwave oven works by passing non-ionizing microwave radiation, usually at a frequency of 2.45 gigahertz (GHz)—a wavelength of 122 millimetres (4.80 in)—through the food. Microwave radiation is between common radio and infrared frequencies. Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. This molecular movement represents heat which is then dispersed as the rotating molecules hit other molecules and put them into motion. Microwave heating is more efficient on liquid water than on fats and sugars (which have a smaller molecular dipole moment), and also more efficient than on frozen water (where the molecules are not free to rotate).[6] Microwave heating is sometimes explained as a resonance of water molecules, but this is incorrect: such resonance only occurs in water vapor at much higher frequencies, at about 20 GHz.[7] Moreover, large industrial/commercial microwave ovens operating at the common large industrial-oven microwave heating frequency of 915 MHz—wavelength 328 millimetres (12.9 in)—also heat water and food perfectly well.[8] A common misconception is that microwave ovens cook food "from the inside out". In reality, microwaves are absorbed in the outer layers of food in a manner somewhat similar to heat from other methods. The misconception arises because microwaves penetrate dry non-conductive substances at the surfaces of many common foods, and thus often induce initial heat more deeply than other methods. Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast to broiling (infrared) or convection heating, which deposit heat thinly at the food surface. Penetration depth of microwaves is dependent on food composition and the frequency, with lower microwave frequencies (longer wavelengths) penetrating better.

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