Structural relaxations and relaxation times, tau, of lactose in freeze-dried amorphous lactose and lactose-WPI mixture systems were studied with glass transition data. The results indicated that T-g, enthalpy relaxation and alpha-relaxation temperature, T-alpha, of amorphous lactose was affected by the presence of lactose crystals and WPI at 0.33 a(w) (25 degrees C) due to water migration. The tau and T-alpha-T-g relationship of the materials was successfully modeled by the WLF equation with the material-special constants (-C-1 and C-2). A strength parameter, S, indicating decrease of tau from 10(2) to 10(-2) s, could be used to measure the structural stability of lactose systems above their glass transition. The strength of amorphous lactose decreased with aw increasing up to 0.44 and enhanced by a lactose crystals and WPI in storage at 0.33 a(w). Hence, the strength parameter provided a simple and convenient means to measure compositional effects on structure formation in food processing. (C) 2015 Elsevier Ltd. All rights reserved.