How do you test a food for vitamins?

August 24, 2009 5:35 AM

How do they figure out what kind of vitamins and nutrients are in food? I understand how the figure out calories, but how did they ever figure out that a banana had potassium or oranges were full of vitamin C in the first place?
posted by Caravantea to science & nature (7 comments total)

The various forms of chromatography were a great help, as they are with most analytical chemistry problems.
posted by paulsc at 5:43 AM on August 24

I don't know the details at a practical level, but for the organics, it's a pretty trivial task to pull them out of things (there's a whole field--natural product isolation--devoted to this). Basically, you puree the item in question, then extract it with a series of solvents. You end up with a couple of compounds in each solvent. You can then separate them by chromatography (or if you're lucky, recrystallization), and identify them by NMR, Mass spectrometry, IR absorption, etc.

The biggest challenge in the process is when you're looking for something new, which is often only found in trace amounts anyway. When you know what you're looking for, it's pretty simple. We have undergrads isolate beta-carotene from spinach or carrots, for example.
posted by Dr.Enormous at 5:55 AM on August 24

Another awesome analytical techniques is FT-IR, which measures how well particular wavelengths of infrared light are absorbed by a sample. If you have one substance in solution, this can help you determine its structure. If you have a mixture of various things, it can tell you how much of each there is. It's also been used to directly determine the saturated fat content of foods.
posted by James Scott-Brown at 6:06 AM on August 24

Some Experiments You Can Do at Home to test levels of iron, vitamin C, etc. in fruits and vegetables.

There are different tests for each vitamin/substance you care about, but there's always SOME way to test for it.
posted by rokusan at 6:07 AM on August 24 [2 favorites]

Metals are usually done by atomic adsorption spectoscopy.
posted by Fiery Jack at 6:11 AM on August 24

Are you asking how it was originally discovered? By a combination of luck and scientific curiosity. For example Frederick Hopkins around 1912:

Whilst teaching in Cambridge, Hopkins was surprised when a student's protein samples failed to turn blue in the standard Adamkiewicz test. This led him to the discovery of the amino acid tryptophan, present in many, but not all, proteins, and necessary for the Adamkiewicz reaction. He found that mice fed a diet without tryptophan became sick, and, that certain other amino acids were also dietetically essential. The body needs these components to build its own proteins, but cannot make them for itself.

Hopkins now suspected that minute amounts of other, yet unknown, chemicals were necessary in the diet. He fed mice solely on fat, starch, salts, and purified milk protein (containing all the essential amino acids). They became sick and ceased to grow. However, when also given a very small quantity of whole milk, they recovered. This led to the isolation of vitamin C, and what Hopkins called "fat-soluble A" (actually two vitamins: A and D).

posted by Rhomboid at 6:17 AM on August 24 [1 favorite]

For example, it was known that something in oranges prevented scurvy before anyone knew what it was. In a sense, vitamin C is what is in oranges and not the other way around.
posted by a robot made out of meat at 7:41 AM on August 24