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DEBBIE ELLIOTT, Host:

Still feeling stuffed with Christmas cookies and the eggnog? Right about now, you may be resolving to diet in the New Year. So today, for our Science Out of the Box series, we learn how to calculate those calories you'll be counting.

ELLIOTT: to find out how many units of energy or calories are packed into that holiday perennial, the fruitcake.

We start out with little, freeze-dried samples of fruitcake.

It almost looks like it could be granola.

THOM CASTONGUAY: Yeah, that's true.

ELLIOTT: Professor Castonguay weighs the samples.

CASTONGUAY: And we're going to write that number down. That's .961 grams.

ELLIOTT: Then he puts the fruitcake flakes in little plastic trays. They don't know it yet, but they are about to meet their doom.

CASTONGUAY: Let's take them down the hall and blow them up.

(SOUNDBITE OF MIXER)

ELLIOTT: Okay. Now it looks like we're in a kitchen. There are 1970s-style countertops and stoves. And then on the other side here, you have what looks like some pretty high-tech equipment.

CASTONGUAY: Well, in fact, this is an adiabatic bomb calorimeter.

ELLIOTT: The bomb calorimeter is a vibrating metal box about the size of a copy machine, but with a hole in the top like a well. It's designed to contain miniature food explosions. And what we're about to explode is the fruitcake sample. First, Professor Castonguay compresses it and loads it into what looks like an artillery shell.

CASTONGUAY: So I'm going to take this sample here, the little bits that look like granola, and I'm going to carefully transfer them into this little die. We'll then take this material and then compress it.

ELLIOTT: What pops out is a fruitcake pellet. He puts it inside the canister.

CASTONGUAY: And then we'll take a small piece of fuse wire.

ELLIOTT: It really looks like you are constructing a bomb.

CASTONGUAY: Well, I honestly don't know what a bomb looks like. I do know that this is going to happen in a flash.

ELLIOTT: He submerges the fruitcake pellet in water.

CASTONGUAY: And say goodbye to it, because this is the last time it will be seen. And if this wasn't sealed properly, it would go right through the roof.

ELLIOTT: Next, he pressurizes the canister with oxygen, sets the canister inside the calorimeter, fills the calorimeter with water, shuts the lid, and then...

CASTONGUAY: Would you like to do the honors and ignite the sample, if you would?

ELLIOTT: Certainly.

CASTONGUAY: Go ahead and push that button.

ELLIOTT: Here I go. Okay, so we didn't hear fireworks. But Professor Castonguay assures us that inside the canister, the fruitcake has been blown to smithereens.

CASTONGUAY: Now, watch the temperature. This rapid oxidization of the sample, the explosion, if you will, is going to create a lot of heat that will warm up the canister. And what we're doing here is we're measuring the amount of heat that actually gets transferred into the water.

ELLIOTT: And you can take that rise in the water temperature and use it to calculate how many calories were in the fruitcake.

CASTONGUAY: Exactly.

ELLIOTT: Our fruitcake explosion heated up the water by about two degrees Celsius. Professor Castonguay does some calculations; then comes the moment of truth.

Okay, we've just blown up our gram of fruitcake. How do we compare to what the manufacturer thought?

CASTONGUAY: Well, according to the manufacturer, one gram will provide about 4.42 calories. We measured closer to 4.98 calories.

ELLIOTT: And that's just in a pellet the size of a Rolaid. Imagine how much you ate over the holidays. But even though our number of calories was higher, that doesn't mean the manufacturer conned us. They probably did this experiment many times, taking samples from all over the fruitcake and averaging the results. Maybe our piece just had a few extra nuts in it.

P: Or perhaps a piece of cherry or two.

ELLIOTT: Well, thank you for having us in your lab today.

CASTONGUAY: My pleasure.

ELLIOTT: Thom Castonguay is a professor of nutrition and food science at the University of Maryland. You can find out more about bomb calorimetry at npr.org.

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