Author: Lou Bloomfield

You must be busy since last night’s broadcast (Superheated Water Produced in Microwave Ovens on ABC Primetime 3/15/2001). Very, very scary as we have certainly done exactly what was shown. I have 3 little girls who love to “cook” their own soups, heat their dad’s coffee water, etc. in the microwave. This report terrified me. I am grateful no harm has come to them. My question is if we strictly use microwaveable plastic bowls, ceramic mugs, or other heavy mixing type bowls and avoid the glass, is the potential for the explosion still there?

I’m afraid that there’s no easy answer to this question. You can use a microwave oven to superheat water in any container that doesn’t assist bubble formation. How a particular container behaves is hard for me to say without experimenting. I’d heat a small amount of water (1/2 cup or less) in the container and look at it through the oven’s window to see if the water boils nicely, with lots of steam bubbles streaming upward from many different points on the inner surface of the container. The more easily water boils in the container, the less likely it is to superheat when you cook it too long. (If you try this experiment, leave the potentially superheated water in the closed microwave oven to cool!)

Glass containers are clearly the most likely to superheat water because their surfaces are essentially perfect. Glasses have the characteristics of frozen liquids and a glass surface is as smooth as… well, glass. When you overheat water in a clean glass measuring cup, your chances of superheating it at least mildly are surprisingly high. The spontaneous bubbling that occurs when you add sugar, coffee powder, or a teabag to microwave-heated water is the result of such mild superheating. Fortunately, severe superheating is much less common because defects, dirt, or other impurities usually help the water boil before it becomes truly dangerous. That’s why most of us avoid serious injuries.

However, even non-transparent microwaveable containers often have glass surfaces. Ceramics are “glazed,” which means that they are coated with glass for both sealing and decoration. Many heavy mixing bowls are glass or glass-ceramics. As you can see, it’s hard to get away from trouble. I simply don’t know how plastic microwaveable containers behave when heating water; they may be safe or they may be dangerous.

If you’re looking for a way out of this hazard, here are my suggestions. First, learn to know how long a given amount of liquid must be heated in your microwave in order to reach boiling and don’t cook it that long. If you really need to boil water, be very careful with it after microwaving or boil it on a stovetop instead. My microwave oven has a “beverage” setting that senses how hot the water is getting. If the water isn’t hot enough when that setting finishes, I add another 30 seconds and then test again. I never cook the water longer than I need to. Cooking water too long on a stovetop means that some of it boils away, but doing the same in a microwave oven may mean that it becomes dangerously superheated. Your children can still “cook” soup in the microwave if they use the right amount of time. Children don’t like boiling hot soup anyway, so if you figure out how long it takes to heat their soup to eating temperature and have them cook their soup only that long, they’ll never encounter superheating. As for dad’s coffee water, same advice. If dad wants his coffee boiling hot, then he should probably make it himself. Boiling water is a hazard for children even without superheating.

Second, handle liquids that have been heated in a microwave oven with respect. Don’t remove a liquid the instant the oven stops and then hover over it with your face exposed. If the water was bubbling spasmodically or not at all despite heavy heating, it may be superheated and deserves particular respect. But even if you see no indications of superheating, it takes no real effort to be careful. If you cooked the water long enough for it to reach boiling temperature, let it rest for a minute per cup before removing it from the microwave. Never put your face or body over the container and keep the container at a safe distance when you add things to it for the first time: powdered coffee, sugar, a teabag, or a spoon.

Finally, it would be great if some entrepreneurs came up with ways to avoid superheating altogether. The makers of glass containers don’t seem to recognize the dangers of superheating in microwave ovens, despite the mounting evidence for the problem. Absent any efforts on their parts to make the containers intrinsically safer, it would be nice to have some items to help the water boil: reusable or disposable inserts that you could leave in the water as it cooked or an edible powder that you could add to the water before cooking. Chemists have used boiling chips to prevent superheating for decades and making sanitary, nontoxic boiling sticks for microwaves shouldn’t be difficult. Similarly, it should be easy to find edible particles that would help the water boil. Activated carbon is one possibility.

Last night’s report wasn’t meant to scare you away from using your microwave oven or keep you from heating water in it. It was intended to show you that there is a potential hazard that you can avoid if you’re informed about it. Microwave ovens are wonderful devices and they prepare food safely and efficiently as long as you use them properly. “Using them properly” means not heating liquids too long in smooth-walled containers.

I left a spoon in my food and I put it in the microwave by accident. Is it dangerous to eat the food after it was put into the microwave with a metal object. Does it have any radiation? Could it cause cancer? – SK, Santa Monica, California

The spoon will have essentially no effect at all on the food. Metal left in the microwave oven during cooking will only cause trouble if (a) it is very thin or (b) it has sharp edges or points. The microwaves push electric charges back and forth in metal, so if the metal is too thin, it will heat up like the filament of a light bulb and may cause a fire. And if the metal has sharp edges or points, charges may accumulate on those sharp spots and then leap into space as a spark. But because your spoon was thick and had rounded edges, the charges that flowed through it during cooking didn’t have any bad effects on the spoon: no heating and no sparks.

As far as the food is concerned, the presence of the spoon redirected the microwaves somewhat, but probably without causing any noticeable changes in how the food cooked. There is certainly no residual radiation of any sort and the food is no more likely to cause cancer after being cooked with metal around than had there been no spoon with it. In general, leaving a spoon in a cup of coffee or bowl of oatmeal isn’t going to cause any trouble at all. I do it all the time. In fact, having a metal spoon in the liquid may reduce the likelihood of superheating the liquid, a dangerous phenomenon that occurs frequently in microwave cooking. Superheated liquids boil violently when you disturb them and can cause serious injuries as a result.

If a microwave oven door were to open while it was still on, what would happen? Could it hurt you? – JP

The microwaves would flow out of the oven’s cooking chamber like light streaming out of a brightly illuminated mirrored box. If you were nearby, some of those microwaves would pass through you and your body would absorb some of them during their passage. This absorption would heat your tissue so that you would feel the warmth. In parts of your body that have rapid blood circulation, that heat would be distributed quickly to the rest of your body and you probably wouldn’t suffer any rapid injuries. But in parts of your body that don’t have good blood flow, such as the corneas of your eyes, tissue could heat quickly enough to be permanently damaged. In any case, you’d probably feel the warmth and realize that something was wrong before you suffered any substantial permanent injuries.