Stress-strain analysis and visual observation of wheat kernel breakage during roller milling using fluted rolls

The endosperm and bran of a wheat grain have different mechanical properties and break differently under the same stresses. Stress-strain analysis was used to model the factors affecting wheat kernel breakage during milling using fluted rolls. The planes of principal compressive and tensile stress and the maximum shear stresses, along which the kernel is most likely to be broken, were calculated for a sharp-to-sharp roll disposition. With the occurrence of compressive stress in the horizontal direction and shear stress in the vertical direction, a kernel tends to break along a principal tensile stress plane because the tensile strength of the endosperm is much smaller than its compressive strength. The model presented quantifies the mathematical relationship of three design and operational factors affecting the principal stresses and the maximum shear stresses: roll gap, differential, and roll diameter. High-speed video was used to observe wheat breakage events during milling; the results show consistency with the theoretical analysis.