I wonder if corn is a red herring. Everything has corn. Everything. Perhaps it's something they use to wash the corn grains and then it spread everywhere. She is allergic to other seeds, but it's not clear if the problem is in the other seed, the traces of corn they add to the other seeds, or something they add to corn and the other seeds.

From https://cornallergygirl.com/2017/05/24/the-water-post/

> I frequently have a problem with borosilicate glass causing water that was safe to become unsafe after it sits in the glass for a few hours. All soda-lime glass (such as mason jars and modern Pyrex) are fine.

This looks like a promising starting point. Clean water and borosilicate glass have very few stuff and they can try to analyze the difference. Even heating the glass to a few hundred degrees to destroy any organic contamination and test if the problem is some mineral.

(In other post they discuss jerky. It's impossible to guess what is the problem with jerky. It has a very long list of additives, species and all of them change in every lot, so it's impossible to guess the origin of the problem. But glass and water looks like a good starting point for debugging.)