Retrogradation of concentrated starch systems : mechanism and consequences for product properties

The mechanical properties of concentrated starch + water systems were studied during heating, cooling and storage. Methods used were a small-amplitude dynamic rheological test and compression between parallel plates. The mechanical properties were related to the structure of the gels. Information about the structure of the gels was obtained by electron and light microscopy and DSC. Starches used were from wheat and potato.During heating of starch suspensions at rest, storage moduli first increased and subsequently decreased. This result is related to swelling of starch granules, melting of crystallites, separation of amylose and amylopectin, and loss of entanglements between starch molecules. Concentrated starch gels formed during heating at rest consist of partly swollen, irregularly shaped granules, which are tightly packed, with a thin amylose gel layer in between. The mechanical properties of these gels at large deformations are determined by the stiffness of the swollen granules, their shape and the mechanical properties of the thin amylose gel layer. Observed changes in Young modulus, and in the stress and strain at fracture during ageing are primarily ascribed to the increase in stiffness of the swollen granules. This increase is due to the formation of (semi) crystalline domains consisting of clusters of ordered double helices of short branches of amylopectin molecules.The mechanical properties of starch breads were measured in two successive compression/decompression cycles and the results were discussed by applying a theory developed for cellular solids. The mechanical properties of starch bread are determined by the mechanical properties and the dimensions of the condensed lamellae and beams, which have a structure comparable with that of concentrated starch gels, as well as the size and size distribution of the gas cells. GMS and SSL affect the mechanical properties of starch bread in two opposite ways; by affecting the properties of the lamellae and beams forming the bread structure and by making the crumb structure finer (gas cell size distribution) and more even. They hardly affect amylopectin recrystallization in starch bread