Measurement of the viscosity of freeze dried amorphous lactose near the glass transition temperature

In the literature, the value of the viscosity of amorphous lactose at its glass transition point (Tg) has been assumed to be 1012 Pa s, which is the assumed value for glasses at their transition point, but no one has actually measured it. This paper gives the details of a method, based on parallel plate rheometry, used to determine the viscosity of amorphous lactose near its glass transition point. The measured value for freeze dried amorphous lactose, prepared from a 20% (w/w) lactose solution, is 1.1 × 1014 Pa s, which is two orders of magnitude above the assumed value. The results were compared with the predictions of the Williams-Landel-Ferry (WLF) equations, assumed in the literature to show the dependence of viscosity on the particle rigidity (T - Tg). It was found that the Peleg constants were slightly better at predicting the trend in the data.