Someone I know has been talking about how there is a huge factory in China that supplies the US with a significant fraction of its annual caffeine import, and that supposedly no one is allowed to enter the factory to inspect what's going on.
She claims that because no one is allowed into the factory to inspect it and because testing of imported food chemicals in the US is infrequent, we can't know for sure how much caffeine is going into our energy drinks, or whether they are lacing it with impurities. As evidence for this, she offered anecdotal evidence of people reacting differently to energy drinks with the same caffeine content, as well as referencing a supposed historical laxity in chemical assaying of imported chemicals in the food industry.
I have been having trouble verifying or denying her claims. The best I have been able to find is a reference to Caffeinated: How Our Daily Habit Helps, Hurts, and Hooks Us, where a book review mentions that
"Refused access to the world’s largest synthetic caffeine factory in China, Carpenter notes that the industry is far from transparent; inspections are rare, and conditions are not always sanitary.
Her claims strike me as incredibly farfetched, primarily for the reason that as far as I was aware, synthetic caffeine is usually supplied in near-100% purity, so it would be incredibly easy to detect any deviation above a few parts per thousand (let alone deviations on the order of ~500 parts per thousand like she is claiming) by a routine chemical assay.
However, I have no experience in how chemical assaying is done in the United States, and in particular how often (and how randomly-scheduled) chemical testing is done on imported synthetic food additives, and so I have not been able to get any definitive answers on whether these claims are plausible or not.
So, in essence:
What are the regulatory guidelines imposed by the FDA on the frequency and scope of chemical assaying of imported pure synthetic food additives like caffeine?