Physico-chemical, nutritional and infrared spectroscopy evaluation of an optimized soybean/corn flour extrudate

A central composite design using RMS (Response Surface Methodology) successfully described the effect of independent variables (feed moisture, die temperature and soybean proportion) on the specific parameters of product quality as expansion index (EI), water absorption index (WAI), water solubility index (WSI) and total color difference (Delta E) studied. The regression model indicated that EI, WAI, WSI and Delta E were significant (p<0.05) with coefficients of determination (R-2) of 0.7371, 0.7588, 0.7622, 0.8150, respectively. The optimized processing conditions were obtained with 25.8 % feed moisture, 160 degrees C die temperature and 58 %/42 % soybean/corn proportion. It was not found statistically changes in amino acid profile due to extrusion process. The electrophoretic profile of extruded soybean/corn mix presented low intensity molecular weight bands, compared to the unprocessed sample. The generation of low molecular weight polypeptides was associated to an increased in In vitro protein digestibility (IVPD) of the extrudate. The FTIR spectra of the soybean/corn mix before and after extrusion showed that the alpha-helix structure remained unchanged after extrusion. However, the band associated with beta-sheet structure showed to be split into two bands at 1624 and 1640 cm(-1). The changes in the beta-sheet structures may be also associated to the increased in IVPD in the extruded sample.