What do you mean by calorific value of foods and fuels?

Coal, petroleum, natural gas etc. are regarded as fuels for many machines and industrial purposes. The oxidation or combustion of all these fuels releases energy. This energy is measured in terms of calorific value. It is defined as the amount of heat produced by the complete combustion of one gram of the substance (food or fuel) in oxygen. It is generally expressed in kilocalories per gram (k cal g^–1) or kJ g^–1. The calorific value of fuel or food is measured by bomb calorimeter. Greater the calorific value, better is the fuel.
Food is regarded as fuel for the animal or human body. It has been found that fats and carbohydrates are the main sources of energy which have calorific values of about 9 k cal g^–1 and 4 k cal g^–1 respectively. Proteins are also the main constituents of our food but have a low calorific value of 4 k cal g^–1.