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 bi ochemicals derived from aqueous solutions. One of the important factor s involved in a successful freeze-drying procedure is the secondary dr ying step. The effect of temperature (254-293 K) and pressure (5-44Pa) on water mass transport during the secondary drying stage has been ex amined for concentrated, aqueous solutions of hydroxyethyl starch, a s tabilizing agent common in the freeze-drying of biological material. D esorption equilibria were reached after evacuation for 5-8 d. The equi librium water content W showed a significant variation with water acti vity a(w). Based on a Freundlich sorption model, the equilibrium water content may be described by the equation W = 0.578 a(w)0.515 Water di ffusion coefficients were derived from the equilibration behaviour acc ording to Fick's diffusion law and were found to vary over the range 1 .0-2.8 x 10(-11) m(2)s(-1). The mass transport properties and their de pendence on temperature, pressure and composition are discussed in det ail. The importance of these results for a comprehensive understanding of the freeze-drying process, efficient process design and final prod uct quality are emphasized.