When you pop a slice of bread into the toaster, you're setting off a complicated set of chemical reactions. When you bake bread, the addition of heat causes carbon found in the carbohydrates of the bread to combine with the amino acids of the proteins, resulting in a browning of the surface of the bread. This process, known as the Maillard reaction, discovered by Louis-Camille Maillard in the early 1900s, was long credited by scientists for producing different flavor components and the brown color on the surface of baked breads. However, in recent years, researchers have credited the Maillard reaction with producing antioxidants that are beneficial to those who consume bread crust.
The antioxidant pronyl-lysine forms as a result of the Maillard reaction when starch and reducing sugars react with the protein-bound amino acid L-lysine. In a study published in the European Journal of Cancer Prevention, researchers found that rats that ingested pronyl-lysine experienced increased enzymic antioxidant activities [source: European Journal of Cancer Prevention].
Pronyl-lysine is more prevalent when bread is broken down into smaller pieces; that is, smaller loaves make for a bigger percentage of crust per slice. While pronyl-lysine is produced by both yeast-based and yeast-free bread, darker breads like wheat and pumpernickel contain higher levels of antioxidants than lighter breads like white.
But be careful of too much browning. Burning or overly browning bread can actually lower the level of antioxidants. In fact, burning bread flips from cancer-preventing to cancer-creating -- burning your bread can produce carcinogens [source: Health and Natural Lifestyles].