The product should already have a UPC code that is trivial to capture. From there, it would be a simple matter of looking up that code to map it to its nutrition information. There may even be databases (eg: FDA) where this is available. MyFitnessPal seems to have the nutritional info on just about every product I've scanned, and I can't tell if they crowdsourced all that data input and/or if they seeded it with some database.
I believe it's crowdsourced. There are many items I eat that they don't have in their database. The app allows you to submit the nutrition info to their database and it gets recognized afterwards. MyFitnessPal is very popular. They should have one of the biggest databases out there.
