Depolymerization of barley starch during extrusion in water glycerol mixtures

For the purpose of producing thermoplastic starch, barley starch was extruded in mixtures of water and glycerol in a twin-screw extruder equipped with a specially constructed slit die. The process parameters — water feed (15–25 g/min), glycerol feed (15–25 g/min), barrel temperature (150–200 °C) and screw speed (272–400 rpm) — were varied according to a statistical experimental design consisting of 28 trials. The values of the process parameters were selected to lie within ranges technically feasible with the equipment. Starch amylopectin was clearly depolymerized, and starch retained on average molar mass at the level of (13–37) × 10⁶g/mol. Intrinsic viscosities decreased from 230 mL/g to about 150 mL/g. The tensile strength depended only on glycerol content in the experimental area, varying from 4.5 to 16 MPa. The specific mechanical energy consumption was 85–240 W.h/kg. In all trials, water had only a slightly greater effect than did glycerol on the dependent variables. All the models of dependent variables were statistically very significant. In the experimental range used it was possible to produce thermoplastic starch with relatively high molar mass and mechanical strength.