Strength analysis for understanding structural relaxations in food materials

Fundamental understanding of structural relaxation time is important in characterization of food materials. Various relationships, such as Arrhenius, VogelTammann-Fulcher (VTF), Williams-Landel-Ferry (WLF) and Angell’s fragility have been used to model the temperature dependence of structural relaxation times of amorphous materials. However, the complexity of real systems reduces the validity of such models, although a simple approach to describe material characteristics is often required. A new approach uses a simple structural “strength” concept. Strength uses modeling of structural relaxation times for single and multicomponent amorphous solid systems around measured glass transition to translate experimental relaxation times data to a practically meaningful critical temperature difference value of food and pharmaceutical materials where liquid properties are exceeded for successful processing and storage performance. The present study focused on systems with various ratios (0-100% at 20% intervals) of trehalose-whey protein isolate (WPI) and lactose-WPI as carbohydrate-protein model; trehalose-maltodextrin represented miscible carbohydrates and partially crystalline with trehalose was a pre-crystallised system. The experimental part of structural strength analysis involved two main sections: (I) sample preparation and preliminary analyses (initial water content, water sorption, freeze-drying); (II) thermal (DSC), mechanical (DMA), dielectric (DEA) analyses and volume rheology of high water content systems (60%, 70%, 80% and 90% of water). The Guggenheim-Anderson-de Boer (GAB) equation was used as a tool to model water sorption isotherms including high water activities (0 - 0.85 aw) for all systems. The calorimetric onset glass transition temperature (Tg) was detected by differential scanning calorimeter (DSC). The Gordon-Taylor (GT) equation was fitted to experimental Tg data and used for extrapolation of Tg values of high water content systems. Dynamic mechanical (DMA) and dielectric (DEA) analyses in multi-frequency mode allowed determination of α- relaxation temperatures (Tα) over a wide range of structural relaxation times. Volume rheology provided structural relaxation time – temperature dependence for high water content systems. The presence of water significantly decreased Tg and Tα in all systems. 6 In the present study, the structural strength was defined and established using trehalose-WPI systems as a basic carbohydrate-protein model. Lactose-WPI systems were used to create a relationship, which allowed determination of strength values over a range of water contents. Structural strength of pure water (6.0°C) and WPI at different water contents were also obtained. Trehalose-maltodextrin systems showed that new strength concept can explain and predict variation of structural relaxation times at different ratios of miscible components. Moreover, structural strength analysis showed good consistency in measuring the amorphous or crystalline content in partially crystalline systems. A known strength value for pure substances allows calculation of a strength estimate for mixtures of such components at various ratios. The structural strength approach was proposed and validated in a wide area of practical applications for adaptation to processing characterisation of food materials as well as for quality and stability control during production and storage