I asked a nutritionist this question and she said they don’t actually burn anything any more. Depending on what it is, they usually either make it into a liquid and separate out the fats/carbs/proteins or just add up known standards for the ingredients.

Less fun to imagine, but makes sense.

Edited to add a link – http://www.scientificamerican.com/article/how-do-food-manufacturers/ which breaks it down a lot better but basically follows the same thinking.

According to the National Data Lab (NDL), most of the calorie values in the USDA and industry food tables are based on an indirect calorie estimation made using the so-called Atwater system. In this system, calories are not determined directly by burning the foods. Instead, the total caloric value is calculated by adding up the calories provided by the energy-containing nutrients: protein, carbohydrate, fat and alcohol.