JotBlog

The instructions for my microwave tell me how long to reheat one serving of this or that. But sometimes I want to reheat two or more servings at the same time. To reheat "x" number of servings, should I set the timer for "x" times the number of minutes for a single serving?

Forgive me, but before I answer your question I must dispose of two pet peeves.

In algebra, "x" stands for an unknown quantity. It is a number. It therefore does not need to be followed by the word "number." For an unknown number of servings, we should simply say "x servings." If, for example, we wanted to talk about two servings (i.e., x=2), we wouldn't say "two number of servings," would we? Will everybody within the sound of my pen please spread this word to all those who are going around these days saying and writing "x number of" this or that? Thank you.

Advertisement

Also, what we put our food into is a microwave oven, not a microwave. A microwave, if you'll pardon the techspeak, is a photon of electromagnetic radiation. I'm quite resigned to losing this battle, however, being mindful of how the original "trunk compartment" of an automobile became simply "the trunk." But "glove compartment" has for some reason survived.

Now about heating things in your microwave. (If you can't lick 'em . . . ) To heat two servings of something, it takes less than twice the time required to heat one. Much less. Here's how the arithmetic works out.

First of all, the microwave generator, called a magnetron, doesn't know or care what's inside the oven. It puts out a constant level of microwave power, no matter what. When you set the "power" for 50 percent, for example, the magnetron cycles on and off, so that it is on at full power for half the time. So the "power" setting doesn't affect our problem.

Advertisement

But different foods absorb microwaves to different extents. Let's say for the sake of argument that one of your servings absorbs -- and turns into heat -- 40 percent of the microwaves that hit it. If that serving is alone in the oven, 60 percent of the magnetron's microwave output is not being absorbed.

Let's now imagine a second, identical serving in the oven along with the first. It will absorb its own 40 percent of the microwaves available to it, namely, 40 percent of the 60 percent that are not being absorbed by the first serving, or 24 percent of the output of the magnetron. The total amount of microwave power being absorbed by the two servings is then 40 percent by the first serving plus 24 percent by the second serving, or a total of 64 percent of the magnetron's output, instead of the expected 80 percent (2 x 40). To bring them both up to the desired temperature, then, the heating time must be lengthened by the factor 80/64. So if a lone serving would require 10 minutes, then the two together would require 10 x 80/64 or 12.5 minutes. That's a lot less than twice the time.

(The situation isn't really that simple, because the second serving perturbs the energy absorption of the first one and vice versa. But you probably feel that this "simplified" explanation is complicated enough.)

Advertisement

I tested these ideas with my own "smart" microwave oven, which has preprogrammed cycles for various common heating and cooking chores. For "Heating a Beverage," for example, the oven first wants me to press a button to tell it how many cups of liquid I want to heat. It then begins its preprogrammed heating cycle for that amount. Here's how long the cycles lasted: for 1/2 cup, 30 seconds; for 1 cup, 50 seconds; for 1 1/2 cups, 70 seconds; for 2 cups, 90 seconds. You can see that the first 1/2 cup required 30 seconds, but that each additional 1/2 cup requires only 20 additional seconds.

Another example: For "Baked Potatoes" (they're not being baked, of course, but I'll let that one go), the oven cooks one potato for 4 minutes, 30 seconds; two potatoes for 7 minutes, 40 seconds; three potatoes for 10 minutes, 50 seconds; and four potatoes for 14 minutes. That's 4 1/2 minutes for one potato plus an additional 3 minutes, 10 seconds for each additional one. Putting it another way, two potatoes take only 1.7 times as long as a single potato; three potatoes take only 2.4 times as long; and four potatoes take only 3.1 times as long.

Lacking a "smart" oven, all you can do is guess. For two servings, your first guess should be about 1 1/2 times the time required for a single serving.

Advertisement

Bromate update

In a recent column, I said that the potassium bromate used in bleaching and conditioning flour turns into harmless potassium bromide. That's true, and in the almost 100 years that flour has been bromated, no one had been able to detect any residual bromate in baked goods that used bromated flour. But chemists now have such sensitive detection methods that very low levels of residual bromate can be detected. Because high levels of bromate have been found to cause cancers in rats, the U.S. Food and Drug Administration has asked bakers to stop using it and find a substitute. Canada and the United Kingdom have banned the chemical altogether.

Labelingo : The tag that came with my wife's new kitchen apron says, "Washing Instructions: Before washing, turn garment inside-out." Now please picture a kitchen apron and tell me how to turn it inside out.

Advertisement

(Have you noticed any silly things on food labels? Send your Labelingo contributions, along with your name and town, to Food 101, Food Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or to the e-mail address below.)

Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Barber: More Scientific Answers to Everyday Questions" (Dell Publications, $12.95). His next book, "What Einstein Told His Cook," is to be published by W.W. Norton next spring. Send your kitchen questions to wolke@pitt.edu.

ncG1vNJzZmivp6x7uK3SoaCnn6Sku7G70q1lnKedZK6zr8eirZ5nnJ6zpr%2FTsqOeZ5akvKV7kWlnamdgbHxyfY6tn55lnZ6ws7vWmq2eZZWmwqLAyKilaHFhmLN2gpVxZJ9vkZh6da3FnGSbm2VreqeCkGxomm5hZq55sY4%3D