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

•Crystallization and structural strength studied in trehalose and trehalose mixtures.•Fractional water sorption found in binary sugar and sugar/WPI systems.•Crystallization and structural strength affected by water and components.•Strength parameter could be calculated by water content and control crystallization.

Water sorption, crystallization, calorimetric glass transition temperature (Tg), structural relaxations and relaxation times (τ) of freeze-dried sugars (lactose and trehalose) and whey protein isolates (WPI) in mixed systems were measured at various water activities (aw ≤ 0.76, and 25 °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 α-relaxation temperatures (Tα) at loss modulus peak at frequency of 0.5 Hz for sugar/protein systems were affected by the presence of water and WPI. The τ and Tα−Tg relationships in sugar/WPI systems were successfully modeled by using the WLF model. The WLF-type structural strength (S), indicating decrease of τ from 102 to 10−2 s, of trehalose decreased with aw up to 0.44 (from 15.8 to 12.1 °C) but WPI increased S. Moreover, a relationship for S and water content for glass forming sugar systems at aw ≥ 0.56 was established. The S concept gave a quantitative measure to estimate compositional effects on τ and could be used to estimate crystallization of food solids components.

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