Extrusion of a hard-to-cook bean (Phaseolus vulgaris L.) and quality protein maize (Zea mays L.) flour blend

Heated extrusion was tested as an alternative process for incorporating hard-to-cook beans into food products. A 3 2 factorial design was used to evaluate extrusion conditions for a 40/60 (w/w) blend of hard-to-cook beans and quality protein maize. Tested extrusion variables were temperature (155, 170 and 185 degrees C) and moisture content (15.5, 17.5 and 19.5 g/100 g). Screw speed was fixed at 130 rpm. The extrudates obtained at 155 and 170 degrees C with 15.5% moisture had the best physical characteristics and were chosen for comparative analysis of nutritional changes between the unprocessed hard-to-cook bean/quality protein maize flour blend and the resulting extrudates. In vitro protein digestibility was higher in the extrudates (80%) than in the flour blend (76%). In vitro starch digestibility was higher at 155 degrees C (89%) and 170 degrees C (92%) than in the flour blend (12%). Processing conditions decreased dietary fibre content by 38% at 155 degrees C and 44% at 170 degrees C. (C) 2008 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.