Structural relaxations and crystallization in amorphous food model formulations
University College Cork
Formulation and design of amorphous food solids structures for improved dehydration properties, stabilization of active components, and controlled structure deterioration require materials science characterization of components and mixtures. However, understanding phase transitions associated with structural relaxations and their coupling with properties of food materials is still a challenging and developing area of food materials science. Our laboratory has developed a new approach for the use of α-relaxation times (τ) and introduced a “Strength, S” concept, which is the temperature difference between the material temperature and its calorimetric onset glass transition temperature (Tg) that gives rise to a 104 reduction in τ (or viscosity), for characterization of noncrystalline food materials, their properties and performance in typical processing and storage conditions where a component or miscible components within food structure may undergo phase and state transitions. Besides the S concept of amorphous food solids, the Deborah number (De) was applied to provide a useful translation of measured τ to real experimental timescales. Our systems were composed of crystalline and amorphous lactose, trehalose, and whey protein isolates (WPI) at various water activities (0 ≤ aw ≤ 0.76) at 25 °C to 45 °C. Differential scanning calorimetry (DSC), dynamic-mechanical (DMA) and dielectric (DEA) analysis, static sorption measurement (SSM) and dynamic dewpoint isotherm (DDI) analysis, and powder X-ray diffractometry (XRD) were used in material characterization and determination of glass transition temperatures (Tg), α-relaxation temperatures (Tα) and corresponding τ (aw ≤ 0.44), water sorption (0.11 ≤ aw ≤ 0.76), time-dependent crystallization (0.65 ≤ aw ≤ 0.76) and crystalline forms of components. The static water sorption data and calorimetric Tg were modeled by using Guggenheim-Anderson-de Boer (GAB) and Gordon-Taylor (GT) relationships, respectively. Moreover, the William-Landel-Ferry (WLF) relationship was used to model the τ and Tα−Tg relationships of amorphous solids systems, where vi material-specific constants C1 and C2 were used to obtain S values. Both SSM and DDI measurements were part of fractional water sorption analysis of sugars/WPI mixtures at aw ≤ 0.44 which allowed quantification of water in specific components within solids mixtures from 25 °C to 45 °C. Such quantification of water in component fractions was required for the use of Tg data in solids characterization. DDI data also showed that the critical aw for water sorption-related crystallization of lactose (aw (cr)) was strongly affected by protein content at 25 °C. Crystallization of amorphous sugars in sugar/WPI mixtures was delayed by protein due to reduced nucleation and retarded diffusion during crystal-growth. XRD patterns showed that the crystalline forms of lactose and trehalose could be affected by the presence of water and WPI. The crystallization and crystalline forms of amorphous lactose were affected by trehalose and WPI based on XRD analysis. XRD showed no anhydrous β-lactose and mixed α-/β-lactose with molar ratios of 4:1 crystals in crystallized lactose/WPI systems (7:3 and 1:1 solids ratios). The enthalpy relaxation (H-relaxation) of amorphous lactose was affected by the presence of lactose crystals and WPI at 0.33 aw (25 °C) due to water migration. The Tα at loss modulus (E’’) and dielectric loss (ε’’) peak at each frequency for sugars was affected by the presence of crystals and protein. Tα shifted to higher temperatures with increasing frequency but it decreased steeply as water content increased. The WLF-derived S values of sugars decreased with aw increasing up to 0.44 aw but the presence of WPI and α-type of lactose crystals increased S. An instant crystallization of amorphous lactose in heating was more rapid in systems with a smaller S, which also had more extensive water plasticization and lower instant crystallization temperature. We also found that the S values could be estimated from water contents of glass forming sugar systems. S gives a quantitative tool to estimate compositional effects on τ and for the control of time-dependent solids transformations, such as crystallization. Amorphous food solids, such as sugars and proteins, are common ingredients in the food and pharmaceutical industries. Thus, understanding the physicochemical vii properties of glass forming food solids has a great importance in the development of processing and shelf life control procedures for such ingredients and relevant products. Relaxation times refer to the time factor corresponding to material response to a change in internal or external thermodynamic conditions such as temperature and aw, which can provide a new approach for the description of process-structure-function relationships to foods and food constituents. While the “Fragility” concept developed by Angell may predict some properties of single glass-forming food components, our WLF-type S concept, which was set to show a decrease of τ from 102 to 10-2 s above glass transition, was superior for the description of relaxation times and solids characterization, and therefore, stability and industrial applicability of relevant foods and pharmaceutical materials. For example, the S parameter as a new food property can be used to formulate materials in order to improve material performance during drying and powder handling. Also, it can be used to control and predict the physicochemical properties and performance of food solids during storage.
Crystallization , Water , Structural relaxations , Structural strength , Amorphous food solids
Fanghui, F. 2017. Structural relaxations and crystallization in amorphous food model formulations. PhD Thesis, University College Cork.