The calorie content of food can be determined using a simple calorimetry experiment. The food just needs to be placed inside a calorimeter and burned to determine its calorie content. These days, however, the calorie content of a food can also be determined by simply using standard values of calories in each gram of protein (4 calories/gram), fat (9 calories/gram), and carbohydrates (4 calories/gram).

The trick is just to accurately determine the quantities of protein, fat, and carbohydrates in the food, and that’s where analytical chemistry comes in particularly handy. The basic idea is that the fat, protein, or carbohydrates need to be extracted from the food. This can be done by choosing an appropriate solvent in which the fats/proteins/carbohydrates are soluble, and then using spectrophotometry, or one of a variety of other methods, to determine the content of each species in a solution.