Final answer:
Sour taste is the perception of H* concentration. When we taste sour foods, the increasing hydrogen ion concentrations trigger depolarization of gustatory cells. For example, orange juice tastes sour because it contains citric acid.
Step-by-step explanation:
Sour taste is the perception of H* concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For example, orange juice—which contains citric acid will taste sour because it has a pH value of approximately 3. Of course, it is often sweetened so that the sour taste is masked.