For connoisseurs of fine beers, there is little more highly prized than a crisp, brewery-fresh flavor. And there is nothing worse than a beer that has gone stale or sickeningly sweet from being stored too long on the shelf. For centuries the chemicals behind these flavors have been a mystery, but new research is now revealing the elusive compounds. As beers lose their freshness they first take on bready flavors, which later shift to flavors known as cardboard, sweet, caramel, and wine.