GLASS TRANSITIONS AND THE FRACTURE-BEHAVIOR OF GLUTEN AND STARCHES WITHIN THE GLASSY STATE

Granular particles (crumbs) of gluten, wheat starch, and waxy maize st arch were obtained by extrusion in a twin-screw cooker extruder and co nditioned to a range of moisture contents (4-18%). The textures of the granules were assessed by a small sensory panel and the sounds produc ed during crushing (acoustic emission) measured. Solid, essentially am orphous, bars of gluten, wheat starch and waxy maize starch were produ ced by heat setting under pressure. The glass transition temperatures were measured by differential scanning calorimetry and dynamic mechani cal thermal analysis, crystallinity by X-ray diffraction and the fract ure behaviour examined using a three-point bend test. Heat-set gluten failed via elastic (brittle) fracture below its glass transition tempe rature, whereas, as the moisture content was increased, the glassy gel atinised starches underwent a brittle-ductile transition while still i n the glassy state. This change in fracture mechanism explains why ext ruded gluten granules exhibited crispness at higher moisture contents than extruded starch granules, despite gluten having a lower glass tra nsition temperature. The results suggest that the prediction of brittl e textures (crisp, crunchy) from a knowledge of the glass transition t emperature alone is not possible because bipolymers can exhibit differ ent fracture mechanisms when in the glassy state.