The retrogradation of concentrated wheat starch systems

The ageing of non-expanded wheat starch extrudates containing 37% and 51% water on a dry solids basis (d.s.b.) at 257C was studied using Differential Scanning Calorimetry (DSC), Wide Angle X-ray Diffraction (XRD), proton Nuclear Magnetic Resonance (NMR) relaxometry and Dynamic Mechanical Thermal Analysis (DMTA). The retrogradation rate increased with water content (,0.02 h21 at 37% water (d.s.b.) compared to ,0.06 h21 at 51%). While a good correlation was found between the DSC, XRD and NMR data, the kinetics of retrogradation measured by DMTA was delayed. The findings were interpreted in terms of the different molecular processes probed by the different techniques. In addition to the kinetics, information on the physical structure of the partially crystalline retrograded materials were obtained. DSC suggested a broad bimodal melting behaviour, which was attributed to the melting of the crystalline structure followed by the dissociation of the double helices. XRD suggested that at both water contents, the recrystallisation of amylopectin led principally to the A-polymorph. DMTA suggested a significant interaction between the amorphous and crystalline phases, with a requirement of a minimum relative crystallinity index of ,0.8 (e.g. ,80% of the crystallinity index of the fully retrograded material), before any increase in the elastic modulus (at 257C) was measured.