Eli5 – How do we know what vitamins are in foods?
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Not an expert but as far as I know it’s a “simple” matter of sending food ingredients to a chem lab.
The lab will clean and pre-process the sample (i.e. by grinding down into mush and juice) to get at all the chemicals (including vitamins).
One procedure I’ve done in a lab is chromatography. You put a drop of food juice on a strip of very absorbent paper, like paper towel but specially made for the purpose. The juice soaks into the paper and the wet spot spreads out. But different chemicals from the juice will travel a different distance on the paper before the juice dries up. So you end up with different stain marks on the paper at different distances from where you put the drop. There are charts for how fast which chemicals move, and this gives you a way to separate the original substances. The size of the stains can also help tell how much of each was there in the original sample.
There are other methods. For example, some vitamins won’t dissolve in water, only in oil. So you mush up some of that food with oil and you do the same procedure with a drop of oily food juice rather than wet food juice.
A lab will have a decent bag of similar tricks on hand that they’ll apply to the food until they have a list of chemicals, including vitamins, found in the food, including the amounts.
Lab tests like that were done a long time ago for most foods ingredients, and the results were published. Now anybody can look up typical vitamin content for various food on the Internet.
Companies that make canned or packaged foods will also send their product in to a lab for testing so they can tell customers what’s in specifically their food.
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