How is the calorie content of foods determined?

Dear Cecil:

The label of almost any food package today will tell you the calorie content. But how are calories determined? Is it very technical?

Cecil replies:

Oh, sure, you’ve got your beakers and test tubes and stuff like that. But when you get right down to it, they measure calories in food the old-fashioned way — they burn it. At least they did years ago, in a wonderful device known as a bomb calorimeter, presumably so called because the centerpiece of it was a thick-walled metal can with wires leading out of it that would send them running for the exits if somebody found it in an airport locker. You put the food inside, torched it, and measured the total heat output to figure the food’s calorie content. You thought maybe “burning off the calories” was just a figure of speech? Uh-uh. Your body burns food just as the calorimeter did, admittedly in a less dramatic manner.

Researchers don’t use calorimeters much today because years of experiment have reduced calorie calculations to a simple formula: protein and carbohydrate each have four calories per gram and fat has nine, regardless (more or less) of what food it’s found in. Aha, you’re thinking, I know my times tables, I’m qualified to be a food scientist. Not so fast. The trick is figuring out the amount of protein, fat, and carbohydrate in each food. That’s where the beakers and test tubes come in. You want to hear about the oxidation of sugars by an alkaline solution of trivalent bismuth in the presence of potassium-sodium-tartrate? I didn’t think so. But the underlying premise of calorie computation is simplicity itself.

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