Rheology of moist food powders as affected by moisture content

Dynamic testing to determine rheological characteristics of moist food powders (semolina, coarse wheat flour, potato starch) was carried out using a powder rheometer of a new construction. The unique feature of the rheometer is that scale of shearing was confined to the thickness of shearing band of powder bed only. It was found that flow pattern of moistened samples was noticeably and diversely affected by both moisture content (varying in the range of 0–15% w/w) and shear rate. The observed changes showed statistical significance p < 0.01 in all trials carried out. What is noteworthy about the conducted research is that at some shear rate values, the shear stress of the bed reached the maximum for specific moisture content levels, irrespective of particle size of the bed. Such behavior may provide an indication of complex interference of different powder shearing mechanisms in the presence of moisture. For beds consisted of larger particles, shear stress values decreased considerably with increasing moisture content. To explain this, modeling of the shearing process with Discrete Element Method (DEM) was performed. The results obtained supported the idea that friction coefficients of particulate material were significantly reduced at higher moisture content of the powder bed in the whole range of shear rates applied.