The mechanical properties of pressed processed wheat material

Breakfast wheatflake materials, produced by two methods, were milled and different sieve fractions reconstituted by hot pressing into bar-shaped test pieces, to reduce the geometry and structure effects of flakes. The stiffness and fracture properties of these pressed bars of different particle size in ranges <0.5 mm, 0.5–1 mm, 1–1.4 mm, 1.4–2 mm and of different water content were compared. Dynamic mechanical thermal analysis showed that the bending modulus, E, superimposed as a function of temperature in the range -40 to 140 °C. The value of E at 20 °C decreased with increasing water content corresponding to depression of the glass transition temperature. Microscopy of the test pieces revealed that starch was the continuous phase. The stiffness properties were similar in many respects to data published for pressed starch specimens. However, the energy to break samples at 7% water content (wet weight basis) was greater when a range of particle sizes was used compared to the results of narrow particle size ranges. This is consistent with published results on fracture toughness of particulate compacts. The energy to break samples increased with increasing water content.