Water sorption-induced crystallization, alpha-relaxations and relaxation times of freeze-dried lactose/whey protein isolate (WPI) systems were studied using dynamic dewpoint isotherms (DDI) method and dielectric analysis (DEA), respectively. The fractional water sorption behavior of lactose/WPI mixtures shown at a(w) <= 0.44 and the critical aw for water sorption-related crystallization (a(w)((Cr))) of lactose were strongly affected by protein content based on DDI data. DEA results showed that the alpha-relaxation temperatures of amorphous lactose at various relaxation times were affected by the presence of water and WPI. The alpha-relaxation-derived strength parameter (S) of amorphous lactose decreased with aw up to 0.44 aw but the presence of WPI increased S. The linear relationship for 40 and S for lactose/WPI mixtures was also established with R-2 > 0.98. Therefore, DDI offers another structural investigation of water sorption-related crystallization as governed by a(w)((cr)) and S may be used to describe real time effects of structural relaxations in noncrystalline multicomponent solids. (C) 2016 Elsevier Ltd. All rights reserved.