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

A pre-shearing rheometer, the Rheoplast(R), was used to perform thermomecha nical processing and simulate extrusion-cooking Under various well-controll ed conditions of thermal and mechanical (60-370 J g(-1)) inputs, native pea starch (moisture content MC = 25 and 30%) was processed at temperatures be low and above the melting endotherm determined by differential scanning cal orimetry (DSC). The extent of starch transformation was studied at macrosco pic, 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 th e hilum without swelling. No mechanical effects of shearing were observed o n granular or crystalline structures, although conversion of mechanical ene rgy into heat apparently occurred either by solid friction or viscous dissi pation. (C) 2000 Elsevier Science Ltd. All rights reserved.