Effect of different sizes and percentages of dockage on physical properties of stored wheat mixtures

Abstract

To answer which dockage size and its percentage mostly influence grain physical properties during grain handling and storage, the effects of dockage size and percentage on the physical properties of wheat mixtures were studied. Wheat mixtures including three different dockage sizes (≤1.0 mm, 1.1–2.0 mm, and >3.35 mm) and four varying percentages (0, 2.5, 5, and 10 %) at moisture content between 13.2 % and 13.4 % (wet basis) were tested. The measured physical properties were bulk density, kernel density, porosity, angles of repose (both filling and emptying), internal friction coefficient, cohesion, and coefficients of friction on surfaces of wood-floated concrete, plastic, galvanized steel, and plywood. Both bulk and kernel densities decreased exponentially with increasing dockage percentages, while porosity exhibited an exponential increase, reaching up to 65.1 %. Small and large size dockage significantly raised the filling and emptying angles of repose as dockage percentages increased, and this trend was not observed on the medium size dockage. Cohesion increased and internal friction decreased with the increase of dockage percentage, particularly in the mixtures with small size dockage. The coefficient of wall friction for clean wheat (0 % dockage) was highest against wood-floated concrete and lowest against galvanized steel. The roughness of the tested surfaces and the particle density of the mixtures explained the complex trend of coefficient of friction on different surfaces. These conclusions provided the basic information and guideline for grain industry on grain cleaning during handling and storage.