DEPOLYMERIZATION OF BARLEY STARCH DURING EXTRUSION IN WATER GLYCEROL MIXTURES

For the purpose of producing thermoplastic starch, barley starch was e xtruded in mixtures of water and glycerol in a twin-screw extruder equ ipped with a specially constructed slit die. The process parameters - water feed (15-25 g/min), glycerol feed (15-25 g/min), barrel temperat ure (150-200 degrees C) and screw speed (272-400 rpm) - were varied ac cording to a statistical experimental design consisting of 28 trials. The values of the process parameters were selected to lie within range s technically feasible with the equipment. Starch amylopectin was clea rly depolymerized, and starch retained on average molar mass at the le vel of (13-37) x 10(6) g/mol. Intrinsic viscosities decreased from 230 mL/g to about 150 mWg. The tensile strength depended only on glycerol content in the experimental area, varying from 4.5 to 16 MPa. The spe cific mechanical energy consumption was 85-240 Wh/kg, In all trials, w ater had only a slightly greater effect than did glycerol on the depen dent variables. All the models of dependent variables were statistical ly very significant. In the experimental range used it was possible to produce thermoplastic starch with relatively high molar mass and mech anical strength.