Specifically, we’re talking volatile compounds – organic compounds that easily evaporate at room temperature and pressure. Compounds have to be airborne in order for our nose to be able to detect their smell, so it follows that any compounds that are particularly large (for example, the melanoidins that contribute to coffee’s colouration) will have low volatilities, and won’t contribute to the aroma. That leaves us with the slightly more modestly-sized molecules shown in the graphic. But where do these come from?