Fracture Mechanics of Vitreous and Mealy Wheat Endosperm

Abstract

The concepts of fracture mechanics have been applied to quantify the fracture behaviour of vitreous and mealy wheat endosperm in a single wheat cultivar. Two new techniques were developed and used to measure fracture toughness (energy per unit area of fracture) of individual grains: (1) load cycling of a notched grain; and (2) instrumented microtome cutting. The load cycling method gave average fracture toughness values for vitreous endosperm of 130 ± 42 J/m2 and 50 ± 12 J/m2 for mealy endosperm. Fracture toughness measured using the instrumented microtome gave values of 159 ± 7·4 J/m2 for vitreous endosperm and 44 ± 4·6 J/m2 for mealy endosperm. The results are consistent with the hypothesis that vitreous endosperm has stronger starch-protein matrix bonding than mealy endosperm. The effect of changing grain moisture on cutting fracture properties was investigated. As moisture decreased, values of fracture toughness increased for both mealy and vitreous endosperms at the same rate down to 11% moisture content, below which fracture toughness increased more rapidly for vitreous than for mealy endosperm. Intra-grain fracture toughness was also investigated by cutting successive sections across individual wheat grains. These showed a decrease in cutting force from the outside of the grain towards the centre, and then an increase near the crease. The critical particle size at which a transition from brittle to ductile failure occurs was calculated, giving predicted values of 1·2 mm for vitreous endosperm and 0·9 mm for mealy endosperm at 15% moisture content. This shows that vitreous endosperm undergoes more ductile deformation during deformation than does mealy endosperm, and that larger particle sizes are predicted for vitreous endosperm as a result of milling.