Why does Kaneki's perception of taste change?
The ghoul DNA "infected" his whole body (look at his eyes) so it´s not too far fetched it changed the tongue as well.

Apart from physical change there is also a mental change. There is a phenomenon where patients who obtained an organ transplant notice effects on what they like or dislike, e.g. if the donor was a pianist the patient might like listening to piano more than before or even want to learn it. Kaneki's tongue could still have the same structure as before but different associations to food based on his digestive system.