The food calorie is a unit of energy equal to approximately 4,190 joules. It's only used to measure the thermochemical energy in food and isn't a scientific unit of measure. The specific amount of energy in a food calorie is therefore not formally defined.
Origin
The calorie, also known as the gram-calorie, was the first unit of energy used. It was defined as the amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. A kilocalorie was therefore 1,000 times this amount of energy. The gram-calorie became formally obsolete as a scientific unit of energy in 1960, and the kilocalorie came to be referred to as simply a "calorie" in common usage. The term "food calorie" has gradually been adopted as the name for this measure of energy since its use is now restricted to nutrition.
Specificity
The food calorie is generally defined as the energy needed to raise the temperature of a kilogram of water by 1 degree Celsius. However, the procedure for measuring this quantity of energy isn't standardized and a variety of specific definitions exist. The amount of energy in a food calorie can range from 4,182 to 4,204 joules, depending on the exact procedure used.
Temperature
The primary reason for the lack of precision in the food calorie is due to differences in the initial temperature of the water. The amount of energy required to raise the temperature of water by 1 degree Celsius decreases as the initial temperature increases. An initial temperature of 3.5 degrees Celsius gives a value of about 4,204 joules for the food calorie; an initial temperature of 19.5 degrees Celsius gives a figure of about 4,182 joules.
Procedure
The procedure for measuring the energy in a food calorie requires a gram of pure water that doesn't contain any air. The water is placed in a calorimeter and adjusted to the desired initial temperature and pressure. The water is then heated by 1 degree Celsius and the calorimeter measures the expended energy in joules. This value may then be multiplied by 1,000 to obtain the amount of energy in a food calorie.
15-Degree Celsius Calorie
The 15-degree Celsius calorie is one of the most commonly used definitions of the food calorie. It uses an initial temperature of 14.5 degrees Celsius and a pressure of 101,325 pascals. This definition provides a value between 4,185.2 and 4,185.8 joules for the food calorie.
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