Effects of extrusion cooking on some functional properties of soy-sweet potato mixtures--a response surface analysis.

Mixtures of defatted soy flour and sweet potatoes were extruded at 0:100, 25:75, 50:50, 75:25 and 100:0 ratios in an Insta-Pro 600 single screw extruder operated at 180, 200, 220 rpm. Oil (3-5%) was added into the mix before extrusion. A 5 x 32 fractional factorial central composite response surface design was adopted for investigating the variables: feed composition, % oil, and screw rotation speed (RPM). Percent oil and screw speed were randomly investigated at three levels, while feed composition was randomly investigated at five levels. Effect of extrusion cooking on functional properties: bulk density, expansion ratio, water absorption and solubility indices and trypsin inhibitor were assessed. Results showed that feed composition and screw speed had strong influences on the process. The effect of adding oil, into defatted soybean significantly (p < 0.05) affected only trypsin inhibitor content. High extrusion temperature (> 130 degrees C), and accompanying high shear, were the result of the non-pasting nature of the extrudate. Therefore the products may have limited uses.