Calculate the calorific value of a food from enthalpy of combustion data.

The energy content of a food can be found by burning the food in a food calorimeter. A known mass of the food is heated electrically and burned in a supply of oxygen. The heat produced is transferred through a copper spiral to water and the temperature increase of the water recorded. The 'water equivalent' of the whole system is calibrated and the calorific value of the food determined. Calorific values are typically recorded as kcal per 100 g or more commonly as kJ per 100g. 1 kcal = 4.18 kJ/g.

Proteins and carbohydrates have a calorific value of 4 kcal per 100 g
Fats have a calorific value of 9 kcal per 100 g.
Alcohol has a calorific value of 7 kcal per 100 g.

Formulas:
Heat produced / evolved = mass of water (kg) x specific heat capacity of water (4.18 kJ kg-1 ºC-1) x temperature change (°C or K)
[∆H = m . c . ∆T (kJ)]

Calorific value or energy content per 100 g = ∆H ÷ grams of food burned (kcal or kJ/g)

To find calorific value in kJ/mol = ( ∆H (kJ) ÷ m (g) ) x M (g/mol)

Sample Calculation If 180 g of combusted pizza dough raises the temperature of 800 g of water by 5.80 °C, what is the energy content per 100 g?
∆H = mc ∆T
∆H = (800 x 4.18 x 5.80)
= 19395.2 J ÷ 1000
= 19.4 kJ (3SF)

Energy content per 100g = ∆H x 100 grams ÷ grams of food burned
= (19.4 kJ x 100) ÷ 180
= 10.8 kJ/g or kcal (3 SF)

## Calculate the calorific value of a food from enthalpy of combustion data.

The energy content of a food can be found by burning the food in a food calorimeter. A known mass of the food is heated electrically and burned in a supply of oxygen. The heat produced is transferred through a copper spiral to water and the temperature increase of the water recorded. The 'water equivalent' of the whole system is calibrated and the calorific value of the food determined. Calorific values are typically recorded as kcal per 100 g or more commonly as kJ per 100g. 1 kcal = 4.18 kJ/g.Proteins and carbohydrates have a calorific value of 4 kcal per 100 g

Fats have a calorific value of 9 kcal per 100 g.

Alcohol has a calorific value of 7 kcal per 100 g.

Formulas:Heat produced / evolved = mass of water (kg) x specific heat capacity of water (4.18 kJ kg-1 ºC-1) x temperature change (°C or K)

[∆H = m . c . ∆T (kJ)]

Calorific value or energy content per 100 g = ∆H ÷ grams of food burned (kcal or kJ/g)

To find calorific value in kJ/mol = ( ∆H (kJ) ÷ m (g) ) x M (g/mol)

Sample CalculationIf 180 g of combusted pizza dough raises the temperature of 800 g of water by 5.80 °C, what is the energy content per 100 g?∆H = mc ∆T

∆H = (800 x 4.18 x 5.80)

= 19395.2 J ÷ 1000

= 19.4 kJ (3SF)

Energy content per 100g = ∆H x 100 grams ÷ grams of food burned

= (19.4 kJ x 100) ÷ 180

= 10.8 kJ/g or kcal (3 SF)