Influence of operating conditions on the extent of enzymatic conversion of rice starch in wet extrusion

Rice starch mixtures with 550-650g water per kg rice flour were extruded in a co-rotating twin-screw extruder with thermostable α-amylase mixed in the feed slurry in concentrations between 0.1-1.0g enzyme per kg rice flour, at temperatures between 70 and 100 °C. The extent of starch hydrolysis promoted by the resulting combination of extrusion and enzymatic activity was monitored by measuring the DE (dextrose equivalent) values of the extruded material. DE values between 20 and 90 were observed, with the control of operating conditions providing for a good control of starch hydrolysis. Nonenzymatic extrusion yielded DE values around 5. The experimental design followed a full three-level factorial plan, allowing a conclusion to be made that interactive effects between the factors studied (temperature, enzyme concentration and water content) were not relevant, while a quadratic effect of water content was evident, corresponding to a maximum starch hydrolysis at 600 g water per kg rice flour. The results were adequately fitted to a multi-linear response-surface model, with a correlation coefficient of 0.89.