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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