Optimization of the extrusion process through response surface methodology for improvement in functional and nutritional properties of soybean hull

Soybean hull as a rich and inexpensive source of dietary fiber and because of its health properties, is a good option for supplying dietary fiber to various food industries. Optimization of the extrusion process and the effects of its variables [Feed moisture (35–45%), screw speed (160–200 rpm), and temperature (75–95 °C)] were investigated on the structural, nutritional, and functional properties of soybean hull by response surface methodology. Based on response surface analysis, the maximum soluble dietary fiber content and water absorption index in addition to the minimum specific mechanical energy and anti-nutritional composition of tannin were obtained at a feed moisture content of 42.58%, screw speed of 182.46 rpm, and temperature of 87.43 °C. The extrusion process also increased the rate of swelling capacity, solubility index, yellowness, and redness of the samples. It also reduced the oil absorption index and lightness in comparison with the control. The FTIR spectroscopy did not show a new functional group and the urease test (trypsin inhibitor activity) was negative in the extruded samples.