Are you asking how bread get's it's crusty flavor, or how to get a bready/crusty flavor in your beer
In bread it is from the Maillard reaction; a chemical reaction between an amino acid and a reducing sugar, usually requiring heat. It is vitally important in the preparation or presentation of many types of food, and, like caramelization, it is a form of non-enzymatic browning. Basically the sugars on the crust begin to carmelize in the heat.
In beer there are certain grains that impart a bready flavor, such as Victory Malt
SOme yeast can also produce those esters that remind people of bread as well...