Ageing of rubbery thermoplastic barley and oat starches

Ageing of thermoplastic barley and oat starches prepared by the extrusion t echnique was investigated. Glycerol-plasticized thermoplastic starches were stored in the rubbery state at 20 degrees C and at a relative humidity of 50% for 8 months. The ageing was followed by measurements of the tensile fa ilure stress and strain and by characterizing thermal transitions using dif ferential scanning calorimetry, crystallization using X-ray diffraction and small-strain viscoelastic properties using dynamic mechanical thermal anal ysis. The initial crystal structures and the crystallization kinetics depen ded on the starch origin but the ultimate crystallinities and endothermic m elting transitions were the same for both starches. Mechanical relaxations observed by dynamic mechanical analysis correlated well with calorimetric g rass transitions. The main mechanism underlying the changes in failure prop erties is suggested to be slow amylopectin crystallization. (C) 1999 Elsevi er Science Ltd. All rights reserved.