What are burps made of?

Burps are made from air. The stuff that is constantly around you and the stuff you breathe in. You get burps when drinking a carbonated/fizzy drink very fast like drinking a pint in one go. You swallow lots of air fast and the air escaped out of your stomach up to your mouth and out. But what makes it smell? When the air is in your stomach it absorbes and takes the small of the acid in your stomach and the undigested food making a foul smell.