Microwave Safe and Efficient Materials
Date: Summer 2012
What materials would be most microwave efficient, in addition to being safe? I know it is best to only use items labeled "microwave safe" in the microwave, but which ones will also be best at heating the food and not the container? I have heard that porcelain and glass are the best, but why? Also, is it all glass that is safe, or just some glass, or does glass have to be "microwave safe" too?
Microwave ovens operate at a frequency (typically 2.4 Gigaherz) that
is the resonance frequency of the water molecule. They cause the
water molecules in food to vibrate, and the resulting molecular friction
generates heat in the food. The container does not (or should not)
have anything to do with this heating process. A proper microwave
safe container is transparent to, and completely unaffected by, the
To be microwave safe, a container must have several requirements.
First, it must be made of a non-metal and/or have no metallic coatings.
A metal container is electrically conductive and such a container can
result in arcing inside the oven, and cause damage to its magnetron
(the microwave generator).
Second, the container must be made of a material that withstands heat
from the hot food it contains. Many plastic contain melt or soften too
easily when in contact with hot food to be microwave safe. The
container itself is not being heated by microwave energy, but the food
can get very hot.
Finally, as stated above, a microwave safe container must not itself be
heated by microwave energy. The most efficient use of microwave
energy in a microwave oven, occurs when all the microwave energy is
absorbed by the food and this heats the food directly, and no energy is
wasted heating the container.
I have heard that porcelain and glass are the best, but why? Also, is it all glass that is safe, or just some glass, or does glass have to be "microwave safe" too?
2. Let us talk more about containers;
A) Glass/ceramics: typically they are microwave safe. One thing is if there is some air bubbles inside glass/ceramic containers, this could expand to crack or break them. Another is whether they are efficient for microwave heating. To test it, you can run microwave ovens with empty containers for a short time. If they are hot, they absorb microwave radiation and are not efficient.
B) Containers with metals or color: metals are usually not suitable for microwave. Even containers are made of glass/ceramics, metals can cause sparking or focus energy to a certain point which provides uneven heating. This could break containers.
Colorful containers usually have some amount of dyes or pigments (even stains) which also absorb microwave radiation. Any type of organic molecules on the surface of containers could absorb radiation, sometimes they break down to release chemicals which might not be good for food, too.
C) Plastics: now this is easy to explain plastics as we mentioned above. Some plastics contain chemicals such as dyes, pigments, and additives. They might be released during microwave heating, so you should use them carefully. There are certain types of plastic containers for microwave, but don't put anything in plastic containers which you use for microwave. As plastics have high affinity for fats and organics (you can see it's harder to clean organic stains on plastics than on glass/ceramics), one they contact to non-food items such as detergent or paint, they can adsorb on the surface of containers and release during the heating.
Usually materials that are considered “microwave safe” are invisible to microwaves. In other words, they do not absorb microwaves and do not heat up from exposure to microwaves. Some glasses and ceramics also are heat-resistant in that they can be heated (by the food) and will not shatter or crack. All glasses and ceramics are not created “microwave safe,” however. They might have some added elements (to color the material, for example) that absorb microwaves. These containers can be dangerous as they can heat rapidly and shatter. They can also become hot enough to burn you if you think they are “microwave safe,” and you pull them from the microwave without gloves. My own experience has taught me that not all coffee cups are “microwave safe,” even though they may be ceramic.
Now, as for “microwave efficient.” Efficiency would imply that microwaves heat the food using the least amount of energy. If we consider only the absorption of microwaves by materials, then we would assume all materials that do not absorb microwaves to be the same. We can consider, however, that any container should not only let microwaves in but also retain heat. In this case, heat insulating materials might be considered better than those that conduct heat away quickly. I would consider this effect to be a rather small contributor to efficiency, though, given the rapid rate food is heated in a microwave.
I would turn your question into another direction and ask, “What is the best way to heat food using microwaves?” Much of the difficulty of using microwave ovens is not that they do not efficiently heat food, but that they can unevenly and too rapidly heat food. Conventional ovens will brown food. Many packaged food for use in microwave ovens try to heat food in just the right way to get the speed of microwave cooking while still retaining the desirable qualities that we get from conventional ovens. Some ovens have turntables to make heating more even in the food. Some packages have absorbing materials that heat the surface of foods at a higher temperature to get the browning of conventional ovens. Some packages separate foods to allow them to heat differently. Most ovens have a “percentage power” setting to heat at lower rates. These cycle the microwaves on and off to get an average power that heats food less quickly. In all these cases, what is more important is the cooking result rather than the efficiency.
Kyle Bunch, PhD
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Update: November 2011