Non-Equilibrium Phase Transitions of Aqueous Starch Systems

Abstract

Experimental data on phase transitions of aqueous starch systems, obtained by thermal analysis (TA) methods, are often indicative of irreversible (non-equilibrium) processes involving various metastable states. The thermal responses usually reflect composite effects from contributions of several opposing processes [e.g. annealing, melting, and (re)crystallization] taking place concurrently during TA. It is important, therefore, to recognize the temperature- and time-dependence of the structure of starch materials, if non-isothermal techniques are used for their characterization. Identifying the pertinent morphological features (supermolecular structure) of each particular system, as well as recognizing the role of water as a plasticizer which depresses the Tg of the amorphous domains, is essential to predict heat/moisture-mediated transformations of this biopolymer. The phase transition behaviour of granular starch and amylose-lipid complexes, as revealed by Differential Scanning Calorimetry and Thermomechanical Analysis, and the metastability of these materials are considered herein with respect to the effects of water and low molecular weight solutes.