Crystallization and structural relaxation times in structural strength analysis of amorphous sugar/whey protein systems

Water sorption, crystallization, calorimetric glass transition temperature (T-g), structural relaxations and relaxation times (tau) of freeze-dried sugars (lactose and trehalose) and whey protein isolates (WPI) in mixed systems were measured at various water activities (a(w) <= 0.76, and 25 degrees C). The results indicated that sugar/WPI mixtures showed fractional water sorption behavior. The crystallization and crystalline forms of lactose were affected by trehalose and WPI based on XRD analysis. DMA analysis showed that the alpha-relaxation temperatures (T-alpha) at loss modulus peak at frequency of 0.5 Hz for sugar/protein systems were affected by the presence of water and WPI. The tau and T-alpha-T-g relationships in sugar/WPI systems were successfully modeled by using the WLF model. The WLF-type structural strength (S), indicating decrease of tau from 10(2) to 10(-2) s, of trehalose decreased with a(w) up to 0.44 (from 15.8 to 12.1 degrees C) but WPI increased S. Moreover, a relationship for S and water content for glass forming sugar systems at a(w) >= 0.56 was established. The S concept gave a quantitative measure to estimate compositional effects on tau and could be used to estimate crystallization of food solids components. (C) 2016 Elsevier Ltd. All rights reserved.