We often add a little salt to desserts. Ions penetrate the microvillus. React directly to any electrically charged molecules by altering electrical properties of the membrane 17. Also they vary across the tongue. Sour is most detectable on BACK AND SIDE of tongue.
Caffeine involves a LOT of variability.
Weber fractions in taste vary from .1 to 1.00, showing LACK of sensitivity. The exponent may be less for the old. Likely our impressions are a fairly direct 1 to 1 correlate of the rate of neural firing. Sweet or sour.