Calculation of food calories
Insights
The most accurate method to measure the energy contained in various foods is to burn them in an instrument called a calorimetric bomb.
OPERATING PRINCIPLE OF THE CALORIMETRIC BOMB:
A known quantity of fuel (food) is inserted inside an oxygen saturated chamber. The combustion process is triggered by an internal device and the heat developed is transferred to the surrounding water. The whole is inserted inside an insulating container to avoid heat exchanges with the external environment.
By recording the temperature variation of the water it is possible to calculate the caloric value of the fuel.
To quantify the caloric power, in Italy the Kilocaloria (Kcal) is mainly used, which is commonly called calorie (in reality, an evaluation error is made since one kilocalorie corresponds to 1000 calories).
KILOCALORIA:
represents the quantity of heat necessary to raise the temperature of 1 kg of distilled water from 14.5 ° C to 15.5 ° C.
According to the international measurement system, the calorific value of a food is expressed in Kilojoule (Kj).
A calorie is equivalent to 4.186 joules so to convert the Kilocalories into kilojoules just multiply them by 4.186
Calories of Carbohydrates
Burning one gram of carbohydrates produces an average heat of 4.2 Kcal per gram.
Normally 97% of the carbohydrates introduced with the diet are absorbed.
It follows that carbohydrates provide our body with an average of 4 Kcal per gram.
Calories of Fat
Burning one gram of fat produces an average heat of 9.45 Kcal per gram.
Normally 95% of the fats introduced in the diet are absorbed.
It follows that fats provide our body with an average of 9 Kcal per gram.
Calories of Proteins
Burning one gram of protein produces an average heat of 5.65 Kcal per gram. However, since our body is unable to use the nitrogen contained in them, their energy power is reduced to 4.35 Kcal per gram.
Normally 92% of the proteins introduced with the diet are absorbed (97% of animal ones and 78% of vegetable ones).
It follows proteins provide our body with an average of 4 Kcal per gram.
These caloric values are called Atwater, from the name of the scientist who first measured the energy released by food in the body.
Limits of Calorie Calculation
In reality, the caloric values reported above, even if corrected for the absorption coefficient, represent an average value. For example, 1 gram of carbohydrate in a dessert does not develop the same calories as 1 gram of carbohydrate in an apple. In some cases the variations between the various foods can be as high as 10%.
Let's see some examples:
The energy value of 1 g of fat from pork is 9.5 kcal while that deriving from 1 g of vegetable fat is 9.3 kcal.
The energy value of 1 g of glucose is equal to 3.74 kcal while that of 1 g of starch is 4.2Kcal.
Calculating the exact caloric quantity of a dish is therefore a "difficult task, feasible only in specialized laboratories."
Even if the values shown in the nutritional tables are used, it must be considered that variations in the composition of individual foods still cause evaluation errors (for example an apple can be more or less ripe, a fillet of beef can be more or less fat).
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