The present study investigated the effect of flow conditions (temperature, agitators and stirring speed) on particle size reduction during the reconstitution process of milk powders. Images acquired with a granulomorphometer during reconstitution showed that the mixing system design had a strong influence on the course of the rehydration process. Laser light scattering data obtained for each temperature of reconstitution and agitator revealed that whatever the powder tested, the number of agitator revolutions required to achieve a given level of rehydration was constant and independent of the stirring speed. Kinetics of rehydration were shown to be much more sensitive to temperature than to stirring speed. Inversely, as expected, it was shown that granulation did not always improve rehydration time, depending on surface composition. This result revealed that both granulation/composition and flow should be considered in controlling the reconstitution process.