Reheating can't make stale bread fresh.

Many people love the smell of fresh-baked French bread, but sometimes it goes stale faster than they can eat it. Now, Gerhard Zehentbauer of the University of Minnesota in St. Paul has analyzed why French bread smells the way it does and why its aroma quickly turns old. Moreover, his study shows that reheating stale bread doesn't restore its fragrance to its former mouth-watering glory.

For his analysis, Zehentbauer baked French bread in his lab, using both a common industrial recipe and a traditional one. He then captured and identified the melange of volatile chemical components in the bread's aroma.

One hour after baking, the aroma of the industrial bread contained a higher concentration of 2-acetyl-1-pyrroline, a compound associated with a roasted smell, than the traditional bread did. The traditional bread, however, gave off higher concentrations of malty-smelling compounds called Strecker aldehydes than did the industrial bread.

Four hours after baking, the industrial bread had lost more than half of its malty-smelling components and increased its concentration of fatty substances--a combination that results in a stale odor. The traditional bread also had lost much of its maltiness, but it didn't smell as stale because it had more Strecker aldehydes to begin with. "If there's enough of a malty aroma, it can suppress the fatty aroma," Zehentbauer says.

He also reheated the bread after a day, let it sit for an hour, and tested to see if it had regained that fresh-baked fragrance. It hadn't. Nevertheless, he says, "reheating must have some benefit or else people wouldn't do it." He speculates that most people eat the bread 10 to 15 minutes after reheating, which might improve the flavor "within limits."