How do we know that burning a dehydrated amount of food and measuring the temperature change is an accurate way of determining a calorie?

Question

I understand that we dehydrate the food and put the known amount of dry food into a known amount of water then ignite the food then measure the change in water temperature to determine the number of calories in it.

What I don’t understand, is how we know that that is an accurate measurement of our body’s ability to digest food and absorb the energy from it.

Moisture can’t be the only variable for determining flammability and energy output, right? If I ball up a piece of paper before igniting it, it’s going to burn at a different rate than if I don’t, with no change in moisture. I feel like the cellular/molecular/macro structure, flammability, ignition point, etc. all have to be taken into account. Is it? Because it’s never mentioned on any article about how calories are measured. What about if something takes longer to burn at a lower temperature vs something that erupts into flame at a high temp and is gone instantly? Even if it’s the same amount of energy being released, if it’s over a different amount of time, wouldn’t that affect the relative temperature of the water at the time it’s being measured?

The body absorbs trace amounts of iron and needs it for survival, but if I try to set a 1 gram iron ball on fire, wouldn’t i need a higher ignition point, and so by the very virtue of that be increasing the minimum temperature of the water before I even begin to receive energy from the burn?

I imagine there’s some part of this mechanism that I’m just not aware of, or misunderstand, but I can’t think of that it would be.

Also, yes, i realize that nowadays we use a different method that is basically just math derived from what we already know about calorie content, but how did we determine *that*? By setting it on fire, right?

In: 2

Answer 1

Yeah, there are complications. Combusting a sample down to H2O, CO2, etc. will tell you the total amount of energy in a food, which tells you the upper limit on how much *you* could get from it.

But how much you *will* get from food depends on more than that. A gram of sugar will be consumed much, *much* more efficiently than a gram of cellulose. And cooking can make food more digestible, e.g. converting collagen into gelatin.

https://en.wikipedia.org/wiki/Food_energy