Fracture properties and microstructure of low-moisture starch probes

Abstract

Different microstructures were induced in thin rectangular sheets of a pregelatinized starch gel by drying in an air oven at 5 different temperatures (range 65-105 degrees C) and by freeze drying. As drying temperature increased, air-dried probes (ADP) varied from thin, dense, and transparent to thick, cellular, and translucent, and the presence of larger air cells became more evident. Freeze-dried probes (FDP) were porous, white, and exhibited minimal shrinkage. Most samples exhibited a brittle-type fracture pattern in the range 0.0 < a(w) < 0.75. FDP had much higher fracture force (FF, three-point bending test) than ADP. FF increased sharply between a, 0.44 and 0.65 for ADP but not for FDP. Light and environmental electron scanning microscopy (ESEM) were used to study the morphology and microstructure of dried probes (thickness and air cells). This work suggests that drying conditions and the microstructural features derived thereof have a larger influence on fracture properties of the starchy material than the glass transition temperature.