Vortex formation in a stirred-tank improves the sinkability of powders in liquids and consequently allows the mixing of powders in liquids to form higher solids content dispersions and solutions. This work investigates the effect of vortex formation on the sinkability of milk powders in water, and how vortex depth, impeller position and solution viscosity influences sinkability and impeller power requirement. When adding milk powder into solution, there was a surprising sudden improvement in powder sinkability at higher solution viscosities. At higher viscosity, more power is required by the impeller to form the vortex, and this provides much greater suction in the vortex, which may explain the rapid powder sink-rates obtained. In addition, impeller position below the liquid surface has a major influence on sink-rate and impeller power requirement at higher viscosities. An impeller position close to the bottom of the desired depth of the vortex greatly reduces the power required to maintain this vortex and sink the powder. Sinking of powder into a liquid should not limit the dispersion or solubilization of powder provided the impeller has sufficient power input to maintain the vortex providing the suction to sink the powder. It should be possible to sink powder up until a % powder addition is attained at which the dispersion or solution has become so viscous that it behaves more like a solid than a liquid. (c) 2004 Elsevier Ltd. All rights reserved.