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 a 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 is 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. On account of the large amount of oxygen present; it undergoes rapid and complete combustion and generates heat. The heat liberated is absorbed by the water in which the bomb is immersed resulting in rise in temperature which 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.
Wt. of wheat taken = 2g.
Wt. of water in the outside vessel =3,000g.
Water equivalent of the calorimeter =500g.
Initial temperature of water =240 C
Final temperature of water = 260 C
Rise in temperature = 20 C
Heat gained by water and calorimeter = 3,500 x 2 =7,000 small calories or 7 Kcal.
2 g. wheat produces 7 Kcal.
1 g. wheat produces 3.5 Kcal.
Calorific value of 100 g. of wheat = 350 Kcal.
Gross Calorific value of Carbohydrates, Fats and Proteins
Physiological energy value of Foods
In the bomb calorimeter, carbohydrates and fats are completely oxidized to Co2 and water. Protein is oxidized to Co2, water and nitrogen. Another important error in the use of bomb calorimeter for determining the calorific value of foods of vegetable origin is that the fibre present in foods is burnt and yields energy, while it is not utilized by human beings. But, 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 in proteins is excreted mainly as urea which contains some energy value.
Loss in Digestion
The average loss of carbohydrates, fats and proteins during digestion in human beings on diets consumed in Western countries have been estimated to be as 2 percent for carbohydrates, 5 percent for fats and 8 percent for proteins.
Loss of Energy in Metabolism due to Incomplete Oxidation
Another apparatus used for determination of the energy value of foods is the oxy-calorimeter, devised by Benedict and co-workers. This instrument measures the volume of oxygen required to burn a known weight of the food. The apparatus consists of a combustion chamber, in which the weighed sample is burnt, a soda lime container for absorption of carbon dioxide, a spirometer for measuring the oxygen used and a motor-blower unit for circulating the gas mixture. Using this instrument, the amount of oxygen consumed in burning 1 g of pure carbohydrate, fat or protein can be determined.