Coffee contains over 800 aroma compounds. Wine has around 200. That is why a single cup can smell like chocolate, citrus, and toasted nuts at the same time. Most of those aromas are created during roasting, in a chain of reactions called the Maillard reaction, the same chemistry that browns bread and sears steak. Heat rearranges sugars and amino acids into hundreds of new molecules. So when you smell your coffee before you taste it, you are smelling chemistry that did not exist in the raw bean an hour earlier.