If you click on their "Sources" link it shows a lot of links but then has this note: "This list is intended as a guide to sources of further information. The inclusion of an item in this list does not necessarily mean that its content was used as the basis for any specific Wolfram|Alpha result."

Basically trying to obfuscate where they are really getting the info.

Been thinking more on this...I think I have an idea how to do it: Wikidata has the USDA ID for a lot (but not all) of items. From there we can get the rest.