Glass transitions and the fracture behaviour of gluten and starches within the glassy state

Abstract

Granular particles (crumbs) of gluten, wheat starch, and waxy maize starch were obtained by extrusion in a twin-screw cooker extruder and conditioned to a range of moisture contents (4–18%). The textures of the granules were assessed by a small sensory panel and the sounds produced during crushing (acoustic emission) measured. Solid, essentially amorphous, bars of gluten, wheat starch and waxy maize starch were produced by heat setting under pressure. The glass transition temperatures were measured by differential scanning calorimetry and dynamic mechanical thermal analysis, crystallinity by X-ray diffraction and the fracture behaviour examined using a three-point bend test. Heat-set gluten failed via elastic (brittle) fracture below its glass transition temperature, whereas, as the moisture content was increased, the glassy gelatinised starches underwent a brittle-ductile transition while still in the glassy state. This change in fracture mechanism explains why extruded gluten granules exhibited crispness at higher moisture contents than extruded starch granules, despite gluten having a lower glass transition temperature. The results suggest that the prediction of brittle textures (crisp, crunchy) from a knowledge of the glass transition temperature alone is not possible because bipolymers can exhibit different fracture mechanisms when in the glassy state.