ANNEALING-INDUCED GELATION OF XANTHAN WATER SYSTEMS/

The effect of annealing at a temperature higher than the gel-sol trans ition 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 solution s could not be categorised as a gel. In contrast, when the solutions w ere annealed at 40 degrees C, firm gels were formed by subsequent cool ing. The gel-sol transition temperature (Tg-s) was clearly observed by FBM. It was found that Tg-s gradually increased from 27 degrees C wit h increasing annealing time and approached a constant value, i.e. 33 d egrees C for 1 wt% sample and 40 degrees C for 2 wt% sample. The annea ling time, where Tg-s levelled off, depended on sample concentration a nd annealing temperature. During annealing from 0 to 12 h, it was obse rved that the amount of non-freezing water estimated by d.s.c. fluctua ted, although a steady increase of Tg-s was observed by FBM. This phen omenon is interpreted as a homogenisation of the solution, i.e. xantha n molecules become free from their molecular assembly through desorpti on of non-freezing water by annealing. The above results indicate that xanthan/water solutions form the gel state when they are fully anneal ed in the sol state. (C) 1997 Elsevier Science Ltd. All rights reserve d.