It is also very easy to just do. Buy fries, extract fat in hexane, evaporate hexane and submit the fat you obtained for analysis.
Edit: It might even be possible to DIY the analysis if it is not commercially available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4609978/. (IR spectroscopy and AgNO3-DLC look somewhat accessible though I would have to look deeper into the topic to be sure.)
What you are missing here is that S. mutants often lives in pockets between tooth an epithelium or between teeth with direct permanent contact to epithelium. Due to the geometry of these spaces access to saliva is very poor so metabolites can enrich to levels way beyond those you suggest here.
This mechanism is also a big problem with the pH study above.