Relationship between structure and viscoelastic behavior of plasticized starch

The linear viscoelastic behavior of starches from various origins, blended with different amounts of a plasticizer using a twin screw extruder, has been investigated. Conditions of stability for reliable measurements and linearity domain have been determined. At low strain, the plasticized starch is found to behave as a viscoelastic gel-like material. This behavior is partially explained by its semicrystalline structure, as evidenced by x-ray diffraction patterns. Crystallites are assumed to participate in the formation of an elastic network, embedded in a viscoelastic amorphous phase. Additional thermomechanical treatment reduces crystallinity and leads to a decrease of the storage and loss moduli. The plasticizing role of water is found to be important. The higher molecular weight of the potato starch is responsible for its larger moduli. In the case of maize starch, the highly branched amorphous amylopectin softens the structured behavior, resulting in lower moduli compared to those of rich-content amylose. (C) 1998 The Society of Rheology.