So adapted are dogs to smelling the outside world that they have little ability to smell food in their mouths. When humans chew food, we force gases escaping from the food up into our nasal cavity via passageways in the back of the throat. This process, called retronasal olfaction, partially reverses the flow of air in the nose, which normally comes in through the nostrils. [...] But the internal anatomy of a dog’s head allows little of this backward airflow. Dogs mostly lack the combined feeling of smell and taste that makes up flavor. As a result, they care little about the flavor of their food once it is in their mouths. Detecting the smells of edible things before opening their mouths is enough for them to take a bite.

— Read more in Do Dogs “See” Smells? at Psychology Today.