Effect of soybean hull, screw speed and temperature on expanded maize extrudates

Abstract

Summary Brazil is one of the biggest producers and exporters of soybean grains and also produces large amounts of soy for animal feed. However the amount of soy based products for human consumption is surprisingly low in Brazil, due, in part, to the limited preference for soy based products. During the processing of soybeans, a considerable quantity of soybean hull is discarded, but this could be used as an source of insoluble fibre in food products, for example, in extruded expanded products. The objective of this work was to study the effects of soybean hull, moisture content and temperature on the production of expanded maize extrudates. Mixtures of maize grits and soybean hull were extruded in a single screw extruder to produce directly expanded extrudates, and the effects of three independent variables, screw speed (100-200 rpm), temperature (100-200 °C) and soybean hull content (0-40%) were investigated using a central composite rotatable design and response surface methodology. The response variables used in this study were: specific mechanical energy (SME), sectional expansion index (SEI) and the paste viscosity readings (cold viscosity, peak value, breakdown and setback). All the response variables were affected significantly (p < 0.05) by the addition of soybean hull. The interaction between soybean hull and temperature increased breakdown and reduced setback. SME and SEI decreased as the addition of soybean hull and the temperature increased. No significant effect of increasing screw speed on SME and SEI was observed in this study. The sensory evaluation showed that no statistical difference was found between extrudates containing 10, 20 and 30% soybean hull, indicating that maize extrudates containing added soybean hull could be used as a rich source of insoluble fibre.