Light microscopy and viscoelastic measurements were used to study the heat-induced changes in low-sheared native and acid-modified oat starch dispersions. A significant structural change occurred in the native oat starch dispersion between 90 and 95°C: the granule structure broke down and, at 95°C, the amylopectin formed a continuous phase in which amylose was dispersed. During cooling, oat starch dispersions, pre-heated to 95 and 98°C, underwent two transitions in viscoelastic behaviour: one below 85°C, the other below 40°C. The former is related to phase separation of amylose and amylopectin, the latter to aggregation of amylose. Both cooling and storage at 4°C enhanced separation into amylose- and amylopectin-rich domains.
Most of the acid-modified oat starch was solubilized when pre-heating was to 95°C, and the viscosity of the hot paste was substantially lower than that of native oat starch dispersion. During cooling, the acid-modified oat starch underwent a transition in viscoelastic behaviour; below 40°C storage modulus increased and phase angle decreased. This partly hydrolysed gel, although less rigid, was more elastic than the corresponding native starch gel. Microstructural studies showed that amylose was responsible for the gel structure of the acid-modified oat starch.