The amount of calories within a food is determined by burning the food inside a device called a bomb calorimeter and measuring the heat produced by the food. An alternate method is to use the Atwater system to determine caloric amounts based on the average calories per gram in a specific food group.
A bomb calorimeter is a box-shaped device consisting of two separate chambers: an inner chamber and an outer one. Researchers weigh a sample of a food before inserting it into the inner chamber. This chamber is then filled with oxygen and sealed. The outer chamber houses a container that holds water.
Researchers use an electric spark to ignite the food in the inner chamber. As the food burns, the rising temperature of the water in the outer chamber is observed and recorded. Each degree per kilogram that the water temperature rises represents one kilocalorie within the food sample.
Another method, which may be more commonly used today, is the Atwater system. This system uses average values of calories, based on previous bomb calorimeter experiments, within a gram of protein, carbohydrate and fat to determine the calorie count of a food.
A kilocalorie is defined as the amount of energy needed to increase the temperature of one kilogram of water by one degree on a Centigrade, or Celsius, thermometer at sea level. A kilocalorie represents 1,000 calories. Food labels usually shorten the term to "calories." The energy found in a food may also be represented in the metric unit of kilojoules. One kilocalorie is equal to 4.184 kilojoules.