Mechanical spectra and calorimetric evaluation of gelatin–xanthan gum systems with high levels of co-solutes in the glassy state

Abstract

Mechanical properties of gelatin–xanthan gum (XG) mixtures with high levels of co-solutes were examined by dynamic mechanical analysis (DMA). The mechanical spectra of the samples were modeled according to the Williams–Landel–Ferry (WLF) equation/free-volume theory, which requires an entropic lightly cross-linked network. For the α dispersion, E′ and E′′ superposed with the horizontal shift factor aT, which was temperature-dependent according to the WLF equation; no other secondary dispersion mechanism was detected. The addition of XG to gelatin networks with high levels of co-solutes changed the glass transition temperature (Tg) and kinetics of glass transition and glassy states. In the glassy state, the WLF equation was unable to follow progress in the mechanical properties, which were better described by the Andrade equation. The calorimetric measurements of the gelatin–XG systems were made using a modulated temperature differential scanning calorimetry (MTDSC) to improve the determination of Tg. The samples were exposed to two cooling and heating cycles to provide a controlled recent thermal history in the temperature range of 40 °C to −70 °C. The Tg values of the samples were determined from the second heating cycle in the reversing heat signal. The calorimetric Tg values increased with increasing glucose syrup:sucrose ratio due to increased crosslinking, whereas mechanical Tg decreased with increased XG content due to network formation.