Twin-screw extrusion cooking of starches: Flow behaviour of starch pastes,expansion and mechanical properties of extrudates

Several starches, corn and durum wheat semolina, and wheat flour were extruded with a twin-screw, pilot-scale machine (Creusot-Loire BC 45). After grinding, pastes of the extruded samples were made and flow curves were obtained at 60°C with a coaxial-cylinder viscometer: a power law can be used to describe them from about 20 to 2600 s^?1. Apparent Young's moduli and rupture strengths of extrudates were evaluated with an Instron, and their diametral and longitudinal expansion measured. The following variables were studied: corn semolina water content, barrel temperature and the corn starch amylose/amylopectin ratio. When water content and barrel temperature vary within certain limits, the values of the constants in the power law equation describe a characteristic line which depends on the amylose and lipid contents of the starch. The formation of an amylose-lipid complex during extrusion is suggested as a key factor in the flow properties of pastes and probably also in the rupture strength of extrudates. There is generally a negative correlation between longitudinal and diametral expansion. It is considered that volume expansion phenomena are mainly dependent on viscous and elastic properties of melted starch.