In situ SAXS under shear unveils the gelation of aqueous starch suspensions and the impact of added amylose-lipid complexes

The gelation of heat-treated, aqueous starch suspensions has been explained either in terms of a nanoscopic amylose aggregation or a fractal-like phase separation. These concepts, as well as the supporting experimental evidence, at first sight seem incompatible. The present study exploits rheometry in combination with in situ small angle X-ray scattering and clearly points to creation of finely dispersed, elongated nano-objects when a starch gel is formed in the presence of shear forces. These objects are interpreted as aggregated amylose double helices that provide the system with physical, intermolecular crosslinks. Increasing the aggregate size and concentration increases the gel storage modulus. The gelation process is enhanced in the presence of amylose-lipid complexes as, during heating, short amylose fragments are liberated from these complexes that, upon cooling, promote amylose aggregation and thus gel formation. When shearing is stopped, the aggregates promptly grow and agglomerate into larger fractal objects resulting in an increased gel stiffness. The present work thus suggests that small scale amylose aggregation and the formation of fractal phases have to be considered as subsequent events rather than as mutually exclusive concepts. Finally, it is argued that size-induced polymorphic changes may also contribute to the starch gel stiffness. (C) 2011 Elsevier Ltd. All rights reserved.