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On the gelatinisation and gelation of aqueous starch suspensions: Molecular, nanomorphological, and mechanistic aspects
B. GODERIS (1), S. Gomand (1), J. Putseys (1), C. J. Gommes (1), P. Van Puyvelde (1), J. A. Delcour (2). (1) Katholieke Universiteit Leuven, Leuven, Belgium; (2) Katholieke Universiteit Leuven, Heverlee, Belgium

Starch consists of quasi linear amylose (AM) and the highly branched amylopectin (AP). The molecules are packed into micrometer-sized, semicrystalline granules containing nanometer-sized lamellar crystallites. The importance of the granule molecular composition and thermal treatment on the suspension viscosity during gelatinisation and pasting in the presence of shear was studied using calorimetric and rheological methods in conjunction with Small Angle X-ray Scattering (SAXS). Annealing tends to increase the crystallite thickness for AM-rich starch, whereas the crystal surface energy decreases for AM-free starch. In starches with intermediate AP/AM ratio, both mechanisms occur. These crystal stabilization mechanisms increase the gelatinisation temperatures and affect the pasting-related rheological behaviour. Usually, the viscosity starts to increase during heating at a given temperature and collapses again during further heating beyond the peak viscosity. Deviations from this classical behaviour are grounded in the presence of very stable AM crystals that survive the suspension heating-shearing treatment. Cooling of a sheared hot starch paste causes the viscosity to increase again and produces a gel. <i>In situ</i> SAXS experiments clearly point to the creation of finely dispersed, elongated nano-objects that are interpreted as aggregated AM double helices that provide the system with physical, intermolecular crosslinks. The gel storage modulus is proportional to the aggregate size and concentration. When shearing is stopped, the aggregates promptly grow and agglomerate into larger fractal objects, resulting in an increased gel stiffness.

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