## Bomb Calorimeter

 Determination of energy value of food
Bomb Calorimeter
The energy value of foods is usually determined using the instrument called bomb calorimeter.
• It consists of a heavy steel bomb, with a platinum or gold-plated copper lining and a cover held tightly in place by means of a strong screw collar.
• A weighed amount of sample, usually pressed into pellet form, is placed in a capsule within the bomb which is then closed except for the oxygen valve, charged with oxygen to a pressure of about 300 pounds to the square inch.
• The oxygen valve is then closed and the bomb immersed in a weighed amount of water.
• The water is constantly stirred and its temperature taken at intervals of one minute by means of a differential thermometer, capable of being read to one thousandth of a degree.
• After the temperature of the water has been determined, the sample is ignited by means of an electric fuse and on account of the large amount of oxygen present; it undergoes rapid and complete combustion.

The heat liberated is absorbed by the water in which the bomb is immersed and the resulting rise in temperature is accurately determined.
• The thermometer readings are also continued through an ‘after period’, in order that the ‘radiation correction’ may be calculated and the observed rise of temperature corrected accordingly.
• This corrected rise, multiplied by the total heat capacity of the apparatus and the water in which it is immersed, gives the total heat liberated in the bomb.
• From this, the heat arising from accessory combustions (the oxidation of the iron wire used as a fuse etc.) must be deducted to obtain the number of calories arising from the combustion of the sample.
Example
• Weight of sample taken = 2 g.
• Weight. of water in the outside vessel = 3,000 g
• Water equivalent of the calorimeter = 500 g
• Initial temperature of water = 24o C.
• Final temperature of water = 26oC
• Rise in temperature = 2oC\
• Heat gained by water and calorimeter = 3, 500 x 2 = 7,000 calories or 7 Kcal
• 2 g sample produces 3.5 Kcal.
• 1 g sample produces = 3.5 Kcal.
Gross fuel value of foods
• The amount of energy release from the nutrients in bomb calorimeter or oxy calorimeter is known as gross fuel value.
• 1 g carbohydrate 4.1 Kcal
• 1 g fat 9.45 Kcal
• 1 g protein 5.65 Kcal
Physiological fuel value of foods
• The amount of energy actually available in the body from a given amount nutrient is known as physiological fuel value.
• 1 g carbohydrates 4.0 Kcal
• 1 g fat 9.0 Kcal
• 1 g protein 4.0 Kcal
Last modified: Monday, 4 June 2012, 4:36 AM