Well, you might want to ask that in the food zone, but I can guess the answer if I put on the right science hat.
Nurition is the study of molecules that we eat. It’s all molecules. Sugar, fat, vitamins. These are just words for different kinds of molecules. Fats are long and oily, sugars are full of oxygen and made of rings. Vitamins are very varied in their structure and are needed to help our biochemical processes go faster.
But it’s all molecules.
So what you do is you take the food, weigh it, and then work out how much of each of the different kinds of molecules are in the food.
Weighing is easy. What about the molecules?
Well that’s what chemistry is good at. We dissolve the food in water, and look at that. Anything that doesn’t dissolve, we try to dissolve in other liquids that aren’t water, like alcohol, or more exotic things like ether. Stuff that still doesn’t dissolve we react with stuff like acids, and look at the products. We take all this stuff and use machines to work out what the molecules are. That’s not too hard because we know what vitamin C and glucose look like – we’ve examined them before. So you inject your food sample into a machine and get a signal that says “Yes, there’s vitamin C, and you have so many nanograms in that sample you injected”
The machines are all different and tell you different things. No one machine gives you the answer.
On CSI they often inject some liquid they found at a crime scene into one machine, like an infra-red, or a mass-spec, and the technician says to Horatio Cane (who’s always standing there looking cross) “Hey, Horatio, that sample you gave me was dynamite” and Horatio takes off his sunglasses and goes and arrests someone.
That’s stupid – no one machine will tell you that. You need to use a bunch of different things and get a picture of what you’re looking at. One of the key things we use is chromatography – which separates out things into the separate pure molecules. Foods have a lot of stuff in, and you need to separate things out. Ever done that thing where you draw a black spot on a bit of filter paper and use water to separate out the colours – it’s EXACTLY that, except things aren’t coloured in foods, usually, and you have to take the separate bits and examine them all.
But that’s all a lot of technical detail you don’t need. The answer is that you just measure all the different molecules in there by a gradual process of separating out all the component bits.
I actually have no idea. I would guess that a sample of the food can be run through some sort of chemical analyser which can break it down to molecular level and sort it into protein, fat, sugar etc. and determine how much of each thing is in it. It can probably be analysed for vitamins and minerals in the same way, or perhaps they have to be tested for by a different method.
The energy content of a quantity of protein, fat, or sugar is already known. I don’t know how this is done either, but I think it’s something to do with burning (rapidly combining with oxygen) the particular energy source and measuring how much energy it releases (we need to ask someone over in the Food Zone about how it all works!)
A calorie is the amount of energy required to heat 1 gram of water by 1 degree Celsius. So the simplest way to find the approximate amount of energy in a particular food would be to set in on fire and see how much it heats up a known quantity of water (there’s a good science experiment! Here’s someone who’s tried it: http://www.123helpme.com/view.asp?id=121153).