How do we know how much protein, fat, and carbs are in the foods we eat?
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The easy answer is that we’ve determined the macronutrient levels in a lot of basic ingredients. When you make a new food, you just take those values and add them together.
The more complex answer requires some science.
Fats dissolve in different chemicals compared to proteins and carbs. The first step would be to break any bonds between fats and these other molecules, using something like hydrochloric acid. Then, you can use some ethers (a kind of chemical) that will extract the fat molecules. This takes advantage of their electrical charge.
Proteins are largely nitrogen. This is super useful, because we can then just extract the nitrogen content and use that to calculate the amount of protein. We do this by heating the food in sulphuric acid. This breaks down food molecules and creates ammonium sulphate. This is a gas that has nitrogen in it. Through a purification process, you can turn that gas into a nitrogenous gas (ammonia gas, to be exact) that can be measured.
Carbs are the remainder of the food. Once you’ve got fats and proteins, you calculate carbs by subtracting the fat and protein masses from the total mass of the food, minus water.
Now, there are also more modern technologies that take advantage of the fact that different molecules respond to radio waves in a different manner (for example).
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