the same cells that pick up sour flavors on the tongue make enzymes that turn CO2 into bicarbonate ions—as in baking soda—and protons, which are really just hydrogen atoms stripped of their single electron. Protons are one of the hallmarks of an acid; the tongue perceives them as sour. So what’s really going on is the conversion of CO2 into carbonic acid, in a backward sort of way. That’s why flat soda pop tastes overly sweet. The sour CO2 isn’t there to balance it out.
