Relations between rheological properties of molten starches and their expansion behaviour in extrusion
1997
Della Valle, G. | Vergnes, B. | Colonna, P. | Patria, A.
Starches with various amylose contents were expanded by extrusion through a specific slit die rheometer. At a given moisture content (MC) and temperature (T(p)), volumetric expansion increased as melt viscosity decreased. At moisture contents higher than 0.205, marked shrinkage was observed. A limit temperature was found (160 degrees C < T(p) < 180 degrees C), above which longitudinal and sectional expansion indices are negatively correlated, which is explained by the start of bubble growth inside the die. At any temperature and moisture content, volumetric expansion increased with amylose content. Under low-temperature conditions (T(p) approximately 100 degrees C), no expansion but only a slight die swell was observed. No difference in elastic properties between starches with different amylose contents could be measured, either by hole pressure error or exit pressure methods. Non-expanded pellets exhibited similar glass transition temperatures T(g), but their temperature of expansion in oil bath T(bo) was 10 to 70 degrees C higher than T(g), and was strongly influenced by shear rate and amylose content. An overall mechanism is proposed, based on the cessation of bubble growth at this temperature T(bo).
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