Calorie
The calorie is a unit of energy widely used in nutrition.
For historical reasons, two main definitions of calorie are in wide use. The small calorie or gram calorie is the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius. The large calorie, food calorie, or kilocalorie is the amount of heat needed to cause the same increase in one kilogram of water. Thus, 1 kilocalorie = 1000 calories. By convention in food science, the large calorie is commonly called calorie. In most countries, labels of industrialized food products are required to indicate the nutritional energy value in calories per serving or per weight.
Calorie relates directly to the metric system, and therefore to the SI system. It is regarded as obsolete within the scientific community, since the adoption of the SI system, but is still in some use. The SI unit of energy is the joule, with symbol "J": one small calorie is defined as exactly 4.184 J; one large calorie is 4184 J.
History
The calorie was first introduced by Nicolas Clément, as a unit of heat energy, in lectures during the years 1819-1824. This was the "large" calorie, viz. modern kilocalorie.The term entered French and English dictionaries between 1841 and 1867. It comes.
The "small" calorie was introduced by Pierre Antoine Favre and Johann T. Silbermann in 1852.
In 1879, Marcellin Berthelot distinguished between gram-calorie and kilogram-calorie. Berthelot also introduced the convention of capitalizing the kilogram-calorie, as Calorie.
The use of the kilogram-calorie for nutrition was introduced to the American public by Wilbur Olin Atwater, a professor at Wesleyan University, in 1887.
The modern calorie was first recognized as a unit of the cm-g-s system in 1896,
alongside the already-existing cgs unit of energy, the erg.
Already in 1928 there were serious complaints about the possible confusion arising from the two main definitions of the calorie and whether the notion of using the capital letter to distinguish them was sound.
Use of the calorie was officially deprecated by the ninth General Conference on Weights and Measures, in 1948.
The alternate spelling calory is archaic.
Definitions
The modern calorie is defined as the amount of energy needed to increase the temperature of 1 gram of water by 1 °C.The definition depends on the atmospheric pressure and the starting temperature. Accordingly, several different precise definitions of the calorie have been used.
| Name | Symbol | Conversions | Definition and notes |
| Thermochemical calorie | calth | ≡ ≈ ≈ ≈ | The amount of energy equal to exactly 4.184 J and 1 kJ = 0.239 kcal.. |
| 4 °C calorie | cal4 | ≈ 4.204 J ≈ ≈ ≈ | The amount of energy required to warm one gram of air-free water from 3.5 to 4.5 °C at standard atmospheric pressure. |
| 15 °C calorie | cal15 | ≈ 4.1855 J ≈ ≈ ≈ | The amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure. Experimental values of this calorie ranged from 4.1852 to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J. |
| 20 °C calorie | cal20 | ≈ 4.182 J ≈ ≈ ≈ | The amount of energy required to warm one gram of air-free water from 19.5 to 20.5 °C at standard atmospheric pressure. |
| Mean calorie | calmean | ≈ 4.190 J ≈ ≈ ≈ | Defined as of the amount of energy required to warm one gram of air-free water from 0 to 100 °C at standard atmospheric pressure. |
| International Steam table calorie | ≈ 4.1868 J ≈ ≈ ≈ | Defined as "international" watt hours = "international" joules exactly. | |
| International Steam Table calorie | calIT | ≡ 4.1868 J ≈ ≈ | Defined as 1.163 mW⋅h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam. |
The two definitions most common in older literature appear to be the 15 °C calorie and the thermochemical calorie. Until 1948, the latter was defined as 4.1833 international joules; the current standard of 4.184 J was chosen to have the new thermochemical calorie represent the same quantity of energy as before.
The calorie was first defined specifically to measure energy in the form of heat, especially in experimental calorimetry.
Nutrition
In a nutritional context, the kilojoule is the SI unit of food energy, although the calorie is commonly used. The word calorie is commonly used with the number of kilocalories of nutritional energy measured.In the United States, most nutritionists prefer the unit kilocalorie to the unit kilojoules, whereas most physiologists prefer to use kilojoules. In the majority of other countries, nutritionists prefer the kilojoule to the kilocalorie. US food labelling laws require the use of kilocalories ; kilojoules are permitted to be included on food labels alongside kilocalories, but most food labels do not do so. In Australia, kilojoules are officially preferred over kilocalories, but kilocalories retain some degree of popular use. Australian and New Zealand food labelling laws require the use of kilojoules; kilocalories are allowed to be included on labels in addition to kilocalories, but are not required. EU food labelling laws require both kilojoules and kilocalories on all nutritional labels, with the kilojoules listed first.
To facilitate comparison, specific energy or energy density figures are often quoted as "calories per serving" or "kcal per 100 g". A nutritional requirement or consumption is often expressed in calories or kilocalories per day.
Food nutrients as fat contains 9 kilocalories per gram, while carbohydrate or protein contains approximately 4 kcal/g. Alcohol in food contains 7 kcal/g.. Food nutrients are also often quoted "per 100 g".