The aim was to compare the effect of intact or hydrolysed whey protein in spray-dried lactose/protein powders on water diffusion properties and microstructure. Dispersions of protein/lactose (0.21:1) containing either intact or hydrolysed whey protein were spray-dried at pilot scale, and physical properties were determined. Lactose/hydrolysed whey protein powders had significantly increased (P < 0.05) particle density, resulting in lower bulk density and occluded air, and higher interstitial air. Moisture sorption analysis at 25 degrees C showed that dispersions containing intact whey protein exhibited lactose crystallisation at a lower relative humidity (RH) compared to the dispersions containing hydrolysed whey protein. Hydrolysed whey protein dispersions had a lower monolayer moisture value (mm) than intact whey protein dispersions, as calculated using the Guggenheim-Anderson-de Boer (GAB) equation. Water diffusivity, determined at 25 degrees C from water sorption kinetics and the application of a mathematical model based on Fick's 2nd law, was significantly different (P < 0.05) with respect to the presence of intact or hydrolysed whey protein over the RH range examined (0-60% RH), except at 40% RH. The presence of hydrolysed whey protein resulted in a significantly higher (P < 0.05) water diffusivity in powders, with potential implications for hygroscopicity, caking, stickiness and flowability in humid environments. (C) 2015 Elsevier Ltd. All rights reserved.The aim was to compare the effect of intact or hydrolysed whey protein in spray-dried lactose/protein powders on water diffusion properties and microstructure. Dispersions of protein/lactose (0.21:1) containing either intact or hydrolysed whey protein were spray-dried at pilot scale, and physical properties were determined. Lactose/hydrolysed whey protein powders had significantly increased (P < 0.05) particle density, resulting in lower bulk density and occluded air, and higher interstitial air. Moisture sorption analysis at 25 degrees C showed that dispersions containing intact whey protein exhibited lactose crystallisation at a lower relative humidity (RH) compared to the dispersions containing hydrolysed whey protein. Hydrolysed whey protein dispersions had a lower monolayer moisture value (mm) than intact whey protein dispersions, as calculated using the Guggenheim-Anderson-de Boer (GAB) equation. Water diffusivity, determined at 25 degrees C from water sorption kinetics and the application of a mathematical model based on Fick's 2nd law, was significantly different (P < 0.05) with respect to the presence of intact or hydrolysed whey protein over the RH range examined (0-60% RH), except at 40% RH. The presence of hydrolysed whey protein resulted in a significantly higher (P < 0.05) water diffusivity in powders, with potential implications for hygroscopicity, caking, stickiness and flowability in humid environments. (C) 2015 Elsevier Ltd. All rights reserved.