@WhiteRaven I think you have to use your own nose or tastebuds to tell you. With all of the different flavors, manufacturers, and different storage techniques that we all have or use, I don’t think their is a stead fast rule as far as when to toss out a flavor. If it’s dark enough to make you nervous, then you can toss it. If it tastes bad, toss it. If not, I have more than a few that have darkened a bit, but still work fine, and taste great.
Was just wondering if said “Bavarian Cream” (et al) was “sweet” via direct tongue contact - or (whether you instead intended to mean) exclusively following the application of elevated temperatures to your juices.
One’s senses of smell and taste to some extent can serve as “chemical detection” (though probably not completely reliably, as some molecules do not “register”). Nevertheless, it seems that chemical reactions taking place over time may often alter the smell/taste perceptual impressions (to the extent remembered).
Even knowing all of the specific chemical contents of a particular flavoring/juice, specific toxicology related answers have not been, and likely cannot easily be, arrived at. In addition to chemical reactions, pathogen growth (may) be another possible concern - making assessments of potential dangers more complicated.
One particular study that I have seen (abstract only; behind pay-wall) that might possibly interest you is: