How Do They Figure Calories?

I've always wondered the same thing

Physicists measure the energy content of food by burning the food. To a physicist, a calorie is the heat flow needed to raise the temperature of one gram of water by one degree Celsius. To measure the calorie content of food accurately, scientists use a “bomb calorimeter”. The food sample to be measured is dried and ground into a powder (important point – all water content eliminated). Then it is placed into the bomb calorimeter, a strong metal container surrounded by a water bath. The metal container is pumped full of pure oxygen at 30 atmospheres pressure and the food is ignited. The resulting energy release is fast and violent, just like a bomb. The steel container holds in the explosion. Pure oxygen promotes combustion, and high-pressure oxygen greatly enhances combustion. All of the burnable parts of the dried and powdered food will burn in a calorimeter, leaving just a touch of ash. The calorimeter turns the energy stored in the food into heat flow. The temperature increase of the water and metal of the calorimeter reveals how many calories the food contained.

In order to answer this question, it helps to define a calorie. A calorie is a unit that is used to measure energy. The Calorie you see on a food package is actually a kilocalorie, or 1,000 calories. A Calorie (kcal) is the amount of energy needed to raise the temperature of 1 kilogram of water 1 degree Celsius. Sometimes the energy content of food is expressed in kilojoules (kj), a metric unit. One kcal equals 4.184 kj. So the Calorie on a food package is 1,000 times larger than the calorie used in chemistry and physics.

The original method used to determine the number of kcals in a given food directly measured the energy it produced.The food was placed in a sealed container surrounded by water--an apparatus known as a bomb calorimeter. The food was completely burned and the resulting rise in water temperature was measured. This method is not frequently used today.

The Nutrition Labeling and Education Act of 1990 (NLEA) currently dictates what information is presented on food labels. The NLEA requires that the Calorie level placed on a packaged food be calculated from food components. According to the National Data Lab (NDL), most of the calorie values in the USDA and industry food tables are based on an indirect calorie estimation made using the so-called Atwater system. In this system, calories are not determined directly by burning the foods. Instead, the total caloric value is calculated by adding up the calories provided by the energy-containing nutrients: protein, carbohydrate, fat and alcohol. Because carbohydrates contain some fiber that is not digested and utilized by the body, the fiber component is usually subtracted from the total carbohydrate before calculating the calories.

The Atwater system uses the average values of 4 Kcal/g for protein, 4 Kcal/g for carbohydrate, and 9 Kcal/g for fat. Alcohol is calculated at 7 Kcal/g. (These numbers were originally determined by burning and then averaging.) Thus the label on an energy bar that contains 10 g of protein, 20 g of carbohydrate and 9 g of fat would read 201 kcals or Calories. A complete discussion of this subject and the calories contained in more than 6,000 foods may be found on the National Data Lab web site at http://www.nal.usda.gov/fnic/foodcomp/. At this site you can also download the food database to a handheld computer. Another online tool that allows the user to total the calorie content of several foods is the Nutrition Analysis Tool at http://www.nat.uiuc.edu.

Posted from Ask the Experts - Scientific American

It's to do with the heat energy it can give off I think. Or something.

The large calorie, kilogram calorie, dietary calorie or food calorie (symbol: Cal)[2] approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C. This is exactly 1,000 small calories or about 4.2 kilojoules.

http://en.wikipedia.org/wiki/Calorie