Ageing of rubbery thermoplastic barley and oat starches

Ageing of thermoplastic barley and oat starches prepared by the extrusion technique was investigated. Glycerol-plasticized thermoplastic starches were stored in the rubbery state at 20°C and at a relative humidity of 50% for 8 months. The ageing was followed by measurements of the tensile failure stress and strain and by characterizing thermal transitions using differential scanning calorimetry, crystallization using X-ray diffraction and small-strain viscoelastic properties using dynamic mechanical thermal analysis. The initial crystal structures and the crystallization kinetics depended on the starch origin but the ultimate crystallinities and endothermic melting transitions were the same for both starches. Mechanical relaxations observed by dynamic mechanical analysis correlated well with calorimetric glass transitions. The main mechanism underlying the changes in failure properties is suggested to be slow amylopectin crystallization.