Annealing induced gelation of xanthan/water systems

Abstract

The effect of annealing at a temperature higher than the gel-sol transition temperature on the junction formation in the gelation process of xanthan/water solutions was investigated by the falling ball method (FBM) and differential scanning calorimetry (d.s.c.). Aqueous solutions of xanthan dissolved at room temperature were inhomogeneous. Although gel-like solutions were formed after subsequent cooling, the solutions could not be categorised as a gel. In contrast, when the solutions were annealed at 40°C, firm gels were formed by subsequent cooling. The gel-sol transition temperature ( T g-s) was clearly observed by FBM. It was found that T g-s gradually increased from 27°C with increasing annealing time and approached a constant value, i.e. 33°C for 1 wt% sample and 40°C for 2 wt% sample. The annealing time, where T g-s levelled off, depended on sample concentration and annealing temperature. During annealing from 0 to 12 h, it was observed that the amount of non-freezing water estimated by d.s.c. fluctuated, although a steady increase of T g-s was observed by FBM. This phenomenon is interpreted as a homogenisation of the solution, i.e. xanthan molecules become free from their molecular assembly through desorption of non-freezing water by annealing. The above results indicate that xanthan/water solutions form the gel state when they are fully annealed in the sol state.