Analysis of desorption and diffusion during secondary drying in vacuum freeze-drying of hydroxyethyl starch

Freeze-drying is a common process for the long-term preservation of biochemicals derived from aqueous solutions. One of the important factors involved in a successful freeze-drying procedure is the secondary drying step. The effect of temperature (254–293 K) and pressure (5–44 Pa) on water mass transport during the secondary drying stage has been examined for concentrated, aqueous solutions of hydroxyethyl starch, a stabilizing agent common in the freeze-drying of biological material. Desorption equilibria were reached after evacuation for 5–8 d. The equilibrium water content W showed a significant variation with water activity aw. Based on a Freundlich Sorption model, the equilibrium water content may be described by the equation W = 0.578 • aw0.515. Water diffusion coefficients were derived from the equilibration behaviour according to Fickʹs diffusion law and were found to vary over the range 1.0–2.8 × 10−11m2s−1. The mass transport properties and their dependence on temperature, pressure and composition are discussed in detail. The importance of these results for a comprehensive understanding of the freeze-drying process, efficient process design and final product quality are emphasized