How are calories determined for packaged food?

Calories are determined by a device called a calorimeter.

a calorie is the amount of energy (heat) released from a food. so that is exactly what a calorimeter does, measures the amount of heat food has given off. A piece of food is massed, ignitted and set inside the calorimeter. a tray of water sits above the ignited food. when the food is burned out, experts analyze the temperature of the water - thus how calories are determined.