Structural modifications of low hydrated pea starch subjected to high thermomechanical processing

A pre-shearing rheometer, the Rheoplast(R), was used to perform thermomechanical processing and simulate extrusion-cooking Under various well-controlled conditions of thermal and mechanical (60-370 J g(-1)) inputs, native pea starch (moisture content MC = 25 and 30%) was processed at temperatures below and above the melting endotherm determined by differential scanning calorimetry (DSC). The extent of starch transformation was studied at macroscopic, granular, crystalline and macromolecular levels by light microscopy, X-ray diffractometry, DSC and intrinsic viscosity, respectively. Incomplete transformations were obtained for lower temperatures. Crystalline structure was greatly modified, showing partial melting with a higher sensitivity of the B-type crystalline structure. Melting of starch granules began near the hilum without swelling. No mechanical effects of shearing were observed on granular or crystalline structures, although conversion of mechanical energy into heat apparently occurred either by solid friction or viscous dissipation. (C) 2000 Elsevier Science Ltd. All rights reserved.