Advanced topics in the application of the WLF/free volume theory to high sugar/biopolymer mixtures: a review

The vitrification properties of mixtures of several biopolymers with sugars have been investigated in this laboratory using the technique of small deformation dynamic oscillation. It was demonstrated that the free volume followed a reciprocal relationship with the number average molecular weight of the biopolymer, which should be associated with imperfect intermolecular associations around the ends of the molecules. The effect of free volume on the kinetics of vitrification is quantified by the friction coefficient (ζo) per monomer unit of the biopolymer. The magnitude of the coefficient ζo increases with the extent of intermolecular associations and enhanced network cohesion diminishes the free volume thus elevating the rheological glass transition temperature (Tg). The approach was also applied to high solids starch systems where moisture loss owing to heating above 100°C results in viscoelastic properties characteristic of the glass transition region. These can be described by the Williams-Landel-Ferry equation modified with a ‘moisture term’. For all samples, the temperature dependence of shear moduli was described by the method of reduced variables. The generated shift factors (aT) were used to engineer a state of ‘iso-free-volume’ thus demonstrating that, within the glass transition region, viscoelastic rate processes are solely governed by free volume. Practical applications include the construction of a state diagram for food materials, which utilizes the rheological Tg to follow the concentration dependence of the metastable vitrification events.