Chapter 3
Energy Value of Food Stuffs
3.0.1 The Energy-Yielding Food Factors
The energy yielding food factors are
1. Carbohydrates.
2. Fats ; and
3. Proteins.
Within the body, these units are oxidised in the cells. The process is one of the continuous
utilisation of oxygen and production of CO2 , water and heat.
Oxidation
Carbohydrates and Fats −−−−−→ CO2 + Water + Heat
Oxidation
Proteins −−−−−→ CO2 + Water + Urea
3.1 Calorific Value
Energy value of food stuffs are usually expressed in terms of a term known as calorific value.
The calorific value is defined as the quantity of heat liberated in calories by the complete com-
bustion of a unit mass of the food stuff in excess air or oxygen under specified standard conditions.
The calorific value depends on the nature of the food and relative proportion of proteins,fats and
carbohydrates present in that food. It is usually expressed in kilo calories and the standard mass
taken is 100 g.
This term is used to grade different food stuffs or fuels. Greater the calorific value higher the
quality of the food stuff or fuel.
Different fuels as well as food stuffs are graded on the basis of their calorific values.
3.1.1 Energy Units
The energy value of foods can be expressed in terms of kilo calories (KCal) or megajoules (MJ).
The International Union of Nutritional Sciences had suggested the use of Mega Joule(MJ) as the
energy unit in place of KCal.
Kilo Calorie: One kilo calorie is the quantity of heat required to to raise the temperature of 1
kg of water through 10 C. It is one thousand times the small calorie used in physics measurements.
Mega Joule : One kilo calorie equals 4.186 kilo joules. Hence thousand kilo calorie equals 4.186
× 103 kilo joules or 4.186 mega joules.
3.1.2 Determination of Energy Value of Foods
The energy value of foods is usually determined using the instrument called bomb calorimeter.
It consists of a heavy steel bomb, with a cover held tightly.
1
, 2 CHAPTER 3. ENERGY VALUE OF FOOD STUFFS
A weighed amount of food sample is placed inside the calorimeter, in a crucible. It is filled with
oxygen under pressure. The calorimeter is immersed in a known quantity of water. The sample is
ignited by means of electric fuse and heat liberated is measured by the rise in temperature.
For example, consider the evaluation of calorific value of 2 g of wheat measured with with a
calorimeter containing 3 kilograms of water.
W eight of wheat taken = 2g
W eight of water in the outside vessel = 3000 g
W ater equivalent of calorimeter = 500 g
Initial temperature of water = 240 C
F inal temperature of water = 260 C
Rise in temperature = 20 C
Heat gained by water and calorimeter = 3500 × 2 = 7000 calories
2 g of wheat produces 7 kilo calories
1 g wheat produces 3.5 KCal
Calorific value of 100 g of wheat = 350 KCal.
Gross Energy Value of Foods
The average gross energy value of pure carbohydrates, fats, and proteins determined with the
bomb calorimeter are given below.
1g Carbohydrate yields 4.1 KCal
1g Fat ,, 9.45 ,,
1g Protein ,, 6.65 ,,
Physiological Energy Value of Foods
In the utilization of carbohydrates, fats and proteins in the body, a certain percentage of the
above nutrients is lost in digestion and the nitrogen of protein is excreted in urine as urea which
still contains some energy value. The average losses in digestion in human subjects have been
estimated to be 2.0 percent for carbohydrates, 5.0 percent for fats and 8.0 percent for proteins.
The loss of energy in urea has been estimated to be 1.2 KCal per gram of protein oxidised. The
physiological energy value of foods calculated from the gross energy values after allowing for the
above losses in digestion and metabolism are as follows: carbohydrates 4.0; fats, 9.0 and proteins 4.0.
These values are known as Atwater-Bryant values.
Energy Value of Food Stuffs
3.0.1 The Energy-Yielding Food Factors
The energy yielding food factors are
1. Carbohydrates.
2. Fats ; and
3. Proteins.
Within the body, these units are oxidised in the cells. The process is one of the continuous
utilisation of oxygen and production of CO2 , water and heat.
Oxidation
Carbohydrates and Fats −−−−−→ CO2 + Water + Heat
Oxidation
Proteins −−−−−→ CO2 + Water + Urea
3.1 Calorific Value
Energy value of food stuffs are usually expressed in terms of a term known as calorific value.
The calorific value is defined as the quantity of heat liberated in calories by the complete com-
bustion of a unit mass of the food stuff in excess air or oxygen under specified standard conditions.
The calorific value depends on the nature of the food and relative proportion of proteins,fats and
carbohydrates present in that food. It is usually expressed in kilo calories and the standard mass
taken is 100 g.
This term is used to grade different food stuffs or fuels. Greater the calorific value higher the
quality of the food stuff or fuel.
Different fuels as well as food stuffs are graded on the basis of their calorific values.
3.1.1 Energy Units
The energy value of foods can be expressed in terms of kilo calories (KCal) or megajoules (MJ).
The International Union of Nutritional Sciences had suggested the use of Mega Joule(MJ) as the
energy unit in place of KCal.
Kilo Calorie: One kilo calorie is the quantity of heat required to to raise the temperature of 1
kg of water through 10 C. It is one thousand times the small calorie used in physics measurements.
Mega Joule : One kilo calorie equals 4.186 kilo joules. Hence thousand kilo calorie equals 4.186
× 103 kilo joules or 4.186 mega joules.
3.1.2 Determination of Energy Value of Foods
The energy value of foods is usually determined using the instrument called bomb calorimeter.
It consists of a heavy steel bomb, with a cover held tightly.
1
, 2 CHAPTER 3. ENERGY VALUE OF FOOD STUFFS
A weighed amount of food sample is placed inside the calorimeter, in a crucible. It is filled with
oxygen under pressure. The calorimeter is immersed in a known quantity of water. The sample is
ignited by means of electric fuse and heat liberated is measured by the rise in temperature.
For example, consider the evaluation of calorific value of 2 g of wheat measured with with a
calorimeter containing 3 kilograms of water.
W eight of wheat taken = 2g
W eight of water in the outside vessel = 3000 g
W ater equivalent of calorimeter = 500 g
Initial temperature of water = 240 C
F inal temperature of water = 260 C
Rise in temperature = 20 C
Heat gained by water and calorimeter = 3500 × 2 = 7000 calories
2 g of wheat produces 7 kilo calories
1 g wheat produces 3.5 KCal
Calorific value of 100 g of wheat = 350 KCal.
Gross Energy Value of Foods
The average gross energy value of pure carbohydrates, fats, and proteins determined with the
bomb calorimeter are given below.
1g Carbohydrate yields 4.1 KCal
1g Fat ,, 9.45 ,,
1g Protein ,, 6.65 ,,
Physiological Energy Value of Foods
In the utilization of carbohydrates, fats and proteins in the body, a certain percentage of the
above nutrients is lost in digestion and the nitrogen of protein is excreted in urine as urea which
still contains some energy value. The average losses in digestion in human subjects have been
estimated to be 2.0 percent for carbohydrates, 5.0 percent for fats and 8.0 percent for proteins.
The loss of energy in urea has been estimated to be 1.2 KCal per gram of protein oxidised. The
physiological energy value of foods calculated from the gross energy values after allowing for the
above losses in digestion and metabolism are as follows: carbohydrates 4.0; fats, 9.0 and proteins 4.0.
These values are known as Atwater-Bryant values.
