 The increased protein binds to the flour to entrap carbon dioxide released by the yeast fermentation process, resulting in a stronger rise and more chewy crumb.Bread flour may be made with a hard spring wheat.Hard is a general term for flours with high gluten protein content, commonly refers to extra strong flour, with 13.5-16% (or 14-15% from some sources) protein (16% is a theoretically possible protein content).