Given this importance of the flavors we learn to like, it seems to me remarkable, and unfortunate, that most people are unaware that the flavors are due mostly to the sense of smell and that they arise largely from smells we detect when we are breathing out with food in our mouths. (…)
The role of retronasal smell in flavor was finally put on the map by Paul Rozin, a psychologist at the University of Pennsylvania, in an article in 1982. As he phrased it, we need to recognize that smell is not a single sense but rather a dual sense, comprising orthonasal (breathing in) and retronasal (breathing out) senses. He devised experiments to show that the perception of the same odor is actually different depending on which sense is being used. Subjects trained to recognize smells by sniffing them had difficulty recognizing them when they were introduced at the back of the mouth.