Calorie
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The calorie is a pre-SI metric unit of energy. The unit was first defined by Professor Nicolas Clément in 1824 as a unit of heat. This definition entered French and English dictionaries between 1841 and 1867.[1] In most fields its use is archaic, having been replaced by the SI unit of energy, the joule. However, in many countries it remains in common use as a unit of food energy. In the context of nutrition, and especially food labelling, the terms calorie (or Calorie) and kilocalorie are interchangeable. In either case the unit is approximately equal to 4.2 kJ.
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[edit] Definitions
Historically, the calorie has had two major alternative definitions differing by a factor of one thousand. In addition to these two major alternative definitions, minor variants of the definition of this unit also exist differing in the exact experimental conditions used, most notably the start temperature of the water.
[edit] Kilogram and gram calories
The original definition by Clément was based on the kilogram. Other definitions based on the gram have since been made. We thus have the two major variants: the kilogram calorie and the gram calorie. One thousand gram calories equal one kilogram calorie.
In the context of food energy the term calorie generally refers to the kilogram calorie. However, the term kilocalorie (kcal), referring to one thousand gram calories, is also in widespread use especially by professional nutritionists (when speaking in terms of calories rather than joules). To avoid confusion, the prefix kilo- is not used with the kilogram calorie.
- Kilogram calorie
- The kilogram calorie, large calorie, food calorie, Calorie (capital C) or just calorie (lowercase c) is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius.
- Gram calorie
- The gram calorie, small calorie or calorie (cal) is the amount of energy required to raise the temperature of one gram of water by 1 °C. The gram calorie was once commonly used in chemistry and physics.
[edit] Variations
The energy needed to increase the temperature of a gram of water by 1 °C depends on the starting temperature and is difficult to measure precisely. Accordingly, there have been several definitions of the calorie. The two perhaps most popular definitions used in older literature are the 15 °C calorie and the thermochemical calorie.
The following definitions are of the gram calorie, for the kilogram calorie multiply by one thousand. The factors used to convert measurements in calories to their equivalents in joules are numerically equivalent to expressions of the specific heat capacity of water in SI units.
- Thermochemical calorie (calth)
- 4.184 J exactly.[2]
- 15 °C calorie (cal15)
- the amount of energy required to warm one gram of air-free water from 14.5 °C to 15.5 °C at standard atmospheric pressure (101.325 kPa). Experimental values of this calorie ranged from 4.1852 J to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.[2]
- 20 °C calorie
- the amount of energy required to warm one gram of air-free water from 19.5 °C to 20.5 °C at standard atmospheric pressure. This is about 4.182 J.
- 4 °C calorie
- the amount of energy required to warm one gram of air-free water from 3.5 °C to 4.5 °C at standard atmospheric pressure. This is about 4.204 J.
- Mean calorie
- 1⁄100 of the amount of energy required to warm one gram of air-free water from 0 °C to 100 °C at standard atmospheric pressure. This is about 4.190 J
- International Steam Table Calorie (1929)
- 1⁄860 international watt hours = 180⁄43 international joules exactly. This is approximately 4.1868 J.[3]
- International Steam Table Calorie (1956) (calIT)
- 1.163 mW·h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).[2]
- IUNS calorie
- 4.182 J exactly. This is a ratio adopted by the Committee on Nomenclature of the International Union of Nutritional Sciences.[4]
[edit] Notes and references
- ^ Etymology: French calorie, from Latin calor (heat).
- ^ a b c International Standard ISO 31-4: Quantities and units – Part 4: Heat. Annex B (informative): Other units given for information, especially regarding the conversion factor. International Organization for Standardization, 1992.
- ^ Figure depends on the conversion factor between international joules and absolute (modern) joules. Using the mean international ohm and volt (1.00049 Ω, 1.00034 V [1]), the international joule is about 1.00019 J, using the US international ohm and volt (1.000495 Ω, 1.000330 V) it is about 1.000165 J, giving 4.18684 J and 4.18674 J, respectively
- ^ FAO (1971). "The adoption of joules as units of energy". http://www.fao.org/docrep/meeting/009/ae906e/ae906e17.htm. "While the nutritional calorie has not been defined, basically it is the thermochemical calorie. The standards used in calorimetric work in nutrition is ultimately the heat of combustion of an internationally graded standard benzoic acid. This is primarily expressed as joules per gramme mole and secondarily as thermochemical calories per mole derived by dividing by 4.182, a factor which has been approved by the Committee on Nomenclature of the IUNS."
