FUNDAMENTALS OF FOOD TECHNOLOGY (1+1)

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 = 24^o C.
• Final temperature of water = 26^oC
• Rise in temperature = 2^oC\
• 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