Calories measure energy and can be used to describe any fuel from petrol to bread. One calorie is the amount of energy required to raise the temperature of one gram (0.035 ounces) of water by one degree Celsius (1.8 degrees Fahrenheit). Food labels often quote energy content in kilocalories (kcal), because food is so rich in energy that it makes more sense to label 1,000 calories at a time.
The number of calories in any given item of food is calculated by measuring how much energy is released when a substance is burned. Inside our bodies, molecular machinery is responsible for burning the fuel we eat, but in the lab, using a spark gives the same result. The traditional method of calorie calculation is to put the food inside a sealed unit known as a bomb calorimeter.
The food is surrounded by an atmosphere of oxygen to ensure it will burn well, and the container is sealed and surrounded by a known volume of water. A spark ignites the food inside and allows it to burn until it is reduced to charcoal, releasing all of the energy contained inside.
The energy is converted to heat, which in turn raises the temperature of the water. By measuring the water’s temperature change, you can then find out exactly how much energy has been released and calculate the calories from there.
Today, many food manufacturers use a different system to create nutritional labels; instead of burning the food item whole, they simply add up the calories of the different components, such as fats, carbohydrates and proteins.