Gail Horta, age 11, of New Warwick, Rhode Island, for her question:
How do they measure the calories in different foods?
A dieter's Calorie Chart loops as neat as a pin. The so called fattening and nonfattening foods are listed in ounces, grams or cups. Beside each item is a number of calories in easy to figure round numbers. The picture looks perfect, but such precise neatness might make a person suspicious. Actually, there are several reasons why the calorie value of this or that food sample cannot be precisely accurate.
Calorie counting concerns the possible energy that digested foods can produce in the body. This is translated into units of heat. The calorie itself is the amount of heat required to raise the temperature of one gram of water one degree C. This unit, however, is not the one used by the calorie counting experts. They use the kilocalorie, alias the Large Calorie, which equals 1,000 small calories.
The unit used on a Calorie Chart should start with a capital letter and it represents the heat required to raise a gram of water 1,000 degrees C, or 1,000 grams by one degree. A heat unit is used because heat is a form of energy released when foods are digested by the body. These highly complex processes are far from fully understood and they differ from person to person. Hence, it is not easy to figure precisely how much energy certain foods produce fox one and all.
However, experts can measure the possible heat energy that different foods are capable of yielding outside the body. This is done with an instrument called the calorimeter. It works because when different foods are burned they yield different amounts of heat energy. This heat can be measured in calories or kilocalories.
The simplest calorimeter is a copper pot containing a measured amount of water. The heat content of the pot and the water are known. The sample substance is measured for weight and temperature, then dunked into the pot. If it is hot, it loses heat and if it is cold it gains heat. The loss or gain in temperature is measured in calories.
A more complex calorimeter is a sturdy metal chamber called a bomb. The food sample is weighed and measured and placed inside. The bomb is dunked into water and an electric current ignites the sample and burns it.
This heats the bomb and the bomb heats the water. By measuring how much the temperature of a given amount of water is raised by the burning of the sample, we can determine its calorie content.
The system is not perfect but neither is it slipshod. It tells in a general ways, how much energy the average person gets from certain amounts of different foods. Fog people who need special diets, a Calorie Chart is essential. For ordinary folk it is a helpful guide to foods that tend to form surplus fat.