Calories are a outdated unit used to measure heat. One calorie is the amount of heat required to raise the temperature of 1 gram of water 1⁰C. This is a unit of energy. (The Joule is a more universal unit for this.) Igniting the food and measuring the amount of heat given off will give a reasonable value for the energy (calories) in the food. The heat given off is measured by a calorimeter. This is a container of a known mass of water whose initial temperature and final temperature are known. You also need to know the mass of the food that is burned up. The heat in calories is equal to the specific heat of water (1 cal/(g x C⁰) times mass of food times temperature change. This value in calories is divided by the mass of the food burned up to get the calories/gram of the food.
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