Structural relaxations of amorphous lactose and lactose-whey protein mixtures

•Structural relaxation and relaxation time studied in lactose and lactose-WPI mixtures.•Glass transition and structural relaxation affected by lactose crystals and WPI.•WLF with the material-special constants successfully modeled τ and Tα−Tg.•Strength parameter could measure compositional effects and control stability of food.

Structural relaxations and relaxation times, τ, of lactose in freeze-dried amorphous lactose and lactose-WPI mixture systems were studied with glass transition data. The results indicated that Tg, enthalpy relaxation and α-relaxation temperature, Tα, of amorphous lactose was affected by the presence of lactose crystals and WPI at 0.33 aw (25 °C) due to water migration. The τ and Tα−Tg relationship of the materials was successfully modeled by the WLF equation with the material-special constants (−C1 and C2). A strength parameter, S, indicating decrease of τ from 102 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 α lactose crystals and WPI in storage at 0.33 aw. Hence, the strength parameter provided a simple and convenient means to measure compositional effects on structure formation in food processing.