BRENDAN BORRELL / © SCIENTIFIC AMERICAN

Editor's Note: This is the fourth in a series of six features on the science of food, running daily from March 30 through April 6, 2009.

UNIVERSITY PARK, Pa.—Marlene Moskowitz unscrews the cap from an amber vial. Within moments, a roasty scent permeates this food science laboratory at Pennsylvania State University, conjuring in my mind my local bakery in the predawn hours as baguette loaves—golden-brown, crusty and warm—are taken out of massive ovens on rolling steel carts. The aromatic teardrop in her vial was extracted from the crust of a white bread loaf a month earlier and is a crucial part of an experiment that could overturn a tightly held belief about the Maillard reaction, one of the key chemical responses that occurs during cooking. Moskowitz, a grad student in food science, and her faculty advisor Devin Peterson, are trying to figure out why white and whole wheat bread smell, and taste, so different from each other when baked. "We bake things but what happens to them?" Peterson says, referring to the cryptic chemistry of cooking. "Nobody understands."

Slide Show: A look inside the Penn State lab

Peterson, 36, who arrived at Penn State in 2002, had just completed his dissertation and was busy churning out related papers on the aroma in fresh and heated butter when a colleague dropped an article on his desk about phenols: those ring-shaped organic compounds best known for imbuing wine with its taste, color and health benefits, but which are also found in cocoa and wheat bran. As Peterson gazed at the phenol's ring, he began to think about its connection to the Maillard reaction, which causes the browning of bread and the flavor of caramel.

Neither proteins nor sugars have a characteristic smell of their own. But when they are heated together they produce thousands of flavor compounds as a result of this Maillard reaction. Peterson realized that small amounts of phenols had the potential to hijack this process and alter flavor chemistry. The only problem is that no one had really explored the thousands of chemicals that spring from the reaction. "What are these molecules," he wondered, "and why are they being modulated?"

A few days later, he went into his lab and set up a simple model reaction. Just as a theoretical physicist might assume Earth to be a perfect sphere, for Peterson a loaf of bread became one sugar and one protein. He heated the two compounds, filling his laboratory with the odor of pyrazines, a toastlike scent tempered with a hint of chocolate. But when he added phenol to the mix, the pyrazine aroma practically disappeared.

"It was one of those 'wow' moments," Peterson says. He began applying for grants to fund further research. The U.S. Department of Agriculture (USDA) nominated him for a Presidential Early Career Award to study the Maillard reaction, giving him a research grant of $420,000 on top of $280,000 he received from industry. In 2005 he shook then President George W. Bush's hand during a visit to the White House with other awardees.

The model chemistry reactions could only take him so far, however, so he dug into real products. First, his team looked at improving the flavor of ultra high temperature (UHT) milk, which is common in Europe and heated to a higher temperature than U.S. pasteurized milk to effectively eliminate bacteria. Because it is sterilized, UHT milk does not require refrigeration until it is opened, so it can be kept on store and kitchen shelves, but Peterson says the Maillard reaction gives it a cooked flavor that U.S. consumers won't accept. He and a graduate student took phenolic compounds from grapes and cocoa and added them to the UHT milk before processing it. The two versions—the UHT and pasteurized—were indistinguishable. Penn State has since applied for a patent on the process.