VACUUM FREEZE-DRYING OF A LIQUID IN A VIAL - DETERMINATION OF HEAT AND MASS-TRANSFER COEFFICIENTS AND OPTIMIZATION OF OPERATING PRESSURE

The kinetics of freeze-drying and temperature changes in the product c ore have been established for milk and water frozen in vials. The infl uence of heating plate temperature, working pressure and initial water content on the sublimation kinetics have been studied. The operating pressure is paramount since it causes dehydration speeds to vary by a factor of 1-2. Through the use of a simple model of the 'uniformly ret reating ice front' type, it has been possible to identify the transfer parameters of the model. Correlations with the operating pressure hav e been established. The results show a working pressure optimum at 200 Pa.