This work presents an evaluation of the effect of storage conditions (temperature, relative humidity and storage time) on wall friction characteristics and hopper design for flour, tea and whey permeate powders. Jenike's shear cell technique was applied to evaluate the wall friction, in particular, the wall yield locus and angle of wall friction, and to evaluate the minimum hopper angle required for mass flow. Wall friction increased with temperature for flour and tea powders while it decreased for whey permeate powder. The effect of relative humidity on wall friction and hopper design was significant. All powders sorbed moisture when exposed to air with relative humidity in the range of 22-66%, which resulted in an increase in wall friction and hopper angle requirement with increasing relative humidity. A consolidating bench was used in the investigation of storage time. Seven day storage time did not affect significantly the wall friction of flour, but did affect the wall friction of both tea and whey permeate powders. This work demonstrates the importance of performing wall friction tests for a number of possible variations in storage conditions in order to assess how these affect hopper design or how they may affect the flow pattern in an existing hopper. (c) 2005 Elsevier Ltd. All rights reserved.