THE DEFORMATION AND FAILURE BEHAVIOR OF WHEAT-STARCH PLASTICIZED WITH WATER AND POLYOLS

The mechanical properties of starch plasticized with water and glycerol or xylitol have been studied in flexure at room temperature. The addition of a polyol lowers the glassy modulus and the glass transition temperature of the starch. With the addition of polyols, the fall in modulus with increasing water content at the glass transition becomes less sharp and shifts to lower water contents. The high strain deformation shows that all samples fracture at a water content below 10% (wet weight basis), a proportion of which tears at water contents in the range 9%-10% and no fracture occurs in samples above about 10% water. Microscopy shows that there is evidence of permanent plastic deformation in fractured samples as the water content is increased to 10% water, although there is a negligible change in fracture stress and strain.