Why does phlegm taste sour?

Answer 1

In the mouth, there is the enzyme called salivary amylase, which digests carbohydrates into sugars, such as glucose. When this enzyme mixes with the carbohydrate in phlegm, they are digested into sugars, hence tasting sweet in the mouth. In addition, there may already be sugars in the phlegm.

Answer 2

Phlegm tastes sour because it is a combination of basal nutrients and enzymes, many of which are acids, which coat the lower parts of the stomach and the upper small intenstine.