I had the opportunity recently to ask someone who knows the brewing profession what generates that distinctive smell. You can see from the answer (reproduced below with kind permission) that the brewing process shares much in common with whiskey manufacture. Indeed it's often said that whiskey is just distilled beer.
On the Guinness smells, there are actually a few different ones:
- Smell of the roasting. These are the coffee /roasty / smoky smells that come from the roasting of the barley.
- Smell of the mashing. This is when they add the ground malt into the mash vessel with hot water (liquor). You get a pleasant sweet cooking smell - biscuity / sweet / grainy (like digestive biscuits but more the retronasal smell while you are eating them).
- Smell from the boiling of the wort. Wort is the liquid extract from the malt containing all the sugars and the malt flavours. When it is boiled these flavours (similar to those described above) as well as flavours from the hops evaporate. The hops give green / herby / floral notes to the overall malty and roasty flavours.