Teaching Kids the Science of Calories

A Cheese Curl

How do you calculate the calories in a cheese curl? Joe Cifelli's students can tell you. IStockPhoto hide caption

itoggle caption IStockPhoto

How Calories Are Measured

You don't have to burn your food to figure out the calories, the professionals do it for you.

  

Scroll down to find out exactly how food scientists determine the amount of calories — and what kind — are in store-bought foods.

Joseph Cifelli, professor of Education at Philadelphia's St. Joseph's University

Joseph Cifelli, professor of Education at Philadelphia's St. Joseph's University, teaches nutrition class once a week to fourth graders at nearby Gompers Elementary. hide caption

itoggle caption

Questions on Kids and Obesity

Pediatrician Matthew Gillman, director of Harvard Medical School/Harvard Pilgrim Health Care's Obesity Prevention Program, and pediatrician Thomas Robinson, director of the Center for Healthy Weight at Lucile Packard Children's Hospital, answer your questions and kids and weight control.

A year ago, the playground at Gompers Elementary in Philadelphia was nothing but a slab of asphalt. No swings, no slides, no monkey bars.

But last spring, all that changed.

Joe Cifelli, a professor of education at nearby St. Joseph's University, and his colleagues decided to build a playground. The educators had received a $700,000 grant from the Centers for Disease Control and Prevention to put forth a program to fight obesity. The playground was just one of many projects covered under the grant.

Cifelli also started teaching nutrition in the classroom. Once a week, he turns a fourth-grade classroom into a science lab. And he uses techniques that won't bore kids to tears or make them feel guilty about their favorite foods.

"How many people like cheese curls?" Cifelli asks the class of 9-year-olds. When every hand shoots up into the air, he opens up a bag and asks them how many they'd like to eat.

"Three," says one child.

"The whole bag!" says another.

The point of bringing cheese curls into the classroom, Cifelli explains, is to figure out whether they have energy.

The lesson Cifelli aims to teach is that food is the equivalent of energy. And calories are simply a unit of measurement for the energy. These concepts are new to the students.

"How many of you think cheese curls have energy?" he probes. About half the kids raise their hands.

"OK, half of you say yes and half say no. Good," Cifelli says.

Cifelli knows that if he just blurts out the answer, the kids will lose interest. He wants them to figure it out. In order to bring the lesson alive, he sets the cheese curl on fire.

"I'm just going to put this flame under it," Cifelli tells students, "and we're going to watch what happens." A beaker of water sits over the burning cheese curl, and Cifelli uses a thermometer to measure the temperature.

As the flame grows, Cifelli asks them to describe what they see. They talk among themselves about the smoke and the flame, then Cifelli shows them that the curl is burning on its own. He's no longer lighting it with a flame from below.

"What does this tell us?" Cifelli asks.

"It shows us that the cheese curl has energy," answers a student.

In order to figure out how much energy, Cifelli dips the thermometer in the beaker of water and takes a second reading.

"When we look at the new temperature, we can see that it's 45 degrees Celsius. We can see it's changed," says Cifelli. He asks them to use their calculators to compute the change. He explains that the difference between the old temperature and the new temperature is the key to calculating calories.

Cifelli doesn't expect the student to walk away with the precise understanding that a calorie is the energy needed to increase the temperature of 1 gram of water by 1 degree Celsius. Rather, he wants them to grasp a broader understanding. When he asks for questions, student Tiantia Willis says she know cheese curls are junk food, so she was surprised to see they have so much energy.

"When I saw all that energy and fire, I was thinking maybe that is good for you," says Willis.

This point leads to Cifelli's next lesson, which will focus on what kinds of energy are good for the body.

"I firmly believe in the notion that learning is mostly talking, and teaching is mostly listening," Cifelli says. "So the idea is that we can get these kids to talk to each other about this stuff. They're actually using words like calories and energy and saturated fats."

Cifelli's hope is that the knowledge empowers students to make healthy choices. But he knows this won't happen overnight. He learned this last year when he invited a few students who'd just finished his course to join him for lunch at St. Joseph's cafeteria.

"They just filled up their plates with everything they could. They had big fries and hamburgers. One girl said she was watching what she was eating, so she just took one hamburger instead of two," Cifelli says with a laugh.

Cifelli says he takes comfort in seeing that kids are spending lots of time on the new playground — exercising.

Twenty-Six Calories Can Lift an SUV

Hershey's Kiss

A typical chocolate kiss contains 26 calories and provides enough mechanical energy to lift a large SUV. Lawrence Lucier / Getty Images hide caption

itoggle caption Lawrence Lucier / Getty Images
Bomb Calorimeter

To measure calories, scientists use a device called a bomb calorimeter. The incineration of food causes the temperature of the water to rise. Because the amount of energy in the system remains the same, the difference in temperature indicates how much heat (or energy) is released in the food. Gary Lane Bertland / University of Missouri-Rolla hide caption

itoggle caption Gary Lane Bertland / University of Missouri-Rolla

You'd never think that eating one chocolate kiss could provide enough energy to lift an 8,600 pound SUV over six feet into the air. After all, according to the label, there are a mere 26 calories in that one piece of chocolate. But there is a different way of looking at calories and energy. And the end result illustrates that there's way more energy in your food than what you may think.

(At this point, if you're not an aspiring chemist, know that what follows is potentially your worst nightmare: dense scientific explanations. Feel no shame, and move quickly down to the bold header. All aspiring chemists and friends, onward!)

"When you lift 2.2 pounds [a small book] one meter [that's about forty inches] off the ground, you expend 10 Joules of energy," explains Joe Cifelli, who teaches science education at St. Josephs University in Philadelphia, Pa. He uses analogies to illustrate how much energy food contains.

Cifelli notes that Joules are used to measure the expenditure of energy, such as determining the amount of effort it takes to lift a large car. Calories, on the other hand, are used to measure food energy. A calorie equals 4.18 Joules.

Confused yet? Wait, there's more. We're all used to thinking about "small c" calories — the calories on American food labels. A calorie is the energy needed to increase the temperature of 1 gram of water by 1 degree Celcius. But there is another type of calorie — with a "capital C" — which equals 1,000 "small c" calories. An apple containing 50 Calories actually contains 50,000 calories. When you exercise and burn 200 Calories, you're burning 200,000 calories. But almost every nutrition reference in the United States refers to Calories but calls them calories. (The process is different in Europe, where food labels list things in kiloJoules.) It's terribly tricky. I burn calories just thinking about it.

So the 26 calories in your piece of chocolate (from the label) actually contain 26 Calories –- or 26,000 calories.

Convert that to Joules and you wind up with more than 108,000 Joules. And if you expend 10 J every time you (theoretically) lift a small book one meter off the ground, you'll suffer through 10,800 book-lifts to burn 26 Calories. Or you can try lifting one large SUV once, about 6 feet in the air. So get cracking.

(Of course, you shouldn't be too concerned –- the SUV scenario assumes two things: first, that no calories are burned through normal human digestion and two, that all of the energy is available to lift the car. In fact, most of the calories you consume are used by the human body to maintain normal bodily functions. The SUV scenario just illustrates how much energy is available in your everyday calorie consumption.)

How Scientists Measure 26 Calories

To measure calories, scientists use a device called a bomb calorimeter, which calculates the change in heat in a closed container. In a high school chemistry lab, students might use a simple calorimeter made out of a covered coffee can (with two compartments) and a thermometer, says Cifelli.

"If you introduce an electrical element that causes the food in one compartment to burn," says Cifelli, "and if you have water in the other container, you can measure the temperature before and after the food burns to determine the amount of energy lost by the food."

Cifelli explains that the amount of energy in a closed, covered container won't change. Because the energy stays the same, the water must gain the heat lost by the food. If you measure the difference in temperature, you can determine how many calories are in the food.

For decades, scientists used this container method for crude calorimeter calculations. But food scientists and chemists now use a more precise measuring process to determine caloric content in commercial grocery-store items.

William Ikins, chemistry director of Silliker Labs, a food testing laboratory, explains that there are specific methods to quantify the amount of moisture, fat and protein in any food substance.

Take a chocolate kiss, for example.

Moisture and Fats:

First, scientists use an oven to determine the amount of moisture in a chocolate kiss. After removing the moisture, the kiss goes through a process called gas chromatography, which essentially separates the fat from the rest of the chocolate kiss. Using a mathematical equation, scientists can then measure the amount of fat, in grams, in a chocolate kiss.

Proteins:

To determine the amount of protein in a chocolate kiss, the kiss is analyzed using the Kjeldahl method: The sample is chemically digested, and then converted into a substance containing ammonium ions. The amount of ammonium — which contains one of the building blocks for all protein — is then isolated and measured.

Minerals:

Finally, the mineral content of the chocolate kiss is analyzed. After incinerating the kiss in a high heat oven, scientists can calculate the amount of ash (essential minerals remain in the ash; everything else burns.)

Carbohydrates:

Carbohydrates are determined through a process of elimination. Assume what's left over equals the amount of carbohydrates.

Final Calculation:

Ikins explains that each gram of fat contains 9 calories and each gram of protein and carbohydrates contains 4 calories. Therefore, the amount of calories in any food item is equal to the amount of (fat * 9) + the amount of (protein * 4) + the amount of (carbohydrate * 4) (in grams).

Going back to our imaginary chocolate kiss, which contains 1.3 grams of fat, 0 grams of protein, and 3.6 carbohydrates.

(1.3 g fat)(9cal/g) +(3.6 g)(4cal/g) + (0 g)(4 cal/g) = 26 calories. Ta da!

But with the amount of work I've just done, I think I'll have that chocolate kiss. No, make it two.

Related NPR Stories

Comments

 

Please keep your community civil. All comments must follow the NPR.org Community rules and terms of use, and will be moderated prior to posting. NPR reserves the right to use the comments we receive, in whole or in part, and to use the commenter's name and location, in any medium. See also the Terms of Use, Privacy Policy and Community FAQ.