The Relevance Of Product Resistance In Primary Drying

Abstract:
Product resistance (R_p) has been recently suggested as a critical product parameter (CPtP) in freeze drying because it instantaneously draws a picture of the (already dried) inner cake morphology at the point of measurement.² From a physical point of view, R_p of the dried product layer impacts the resistance to water vapor flow and thus indirectly determines the maximally allowable shelf temperature and primary drying time. With higher product resistance the product temperature increases and mass flow decreases. ¹ Product resistance data allow the assessment of structural changes in the cake (e.g. microcollapse, shrinkage or collapse) as a function of temperature during primary drying. Moreover, product resistance data obtained during primary drying can reveal changes in the freezing situation of a batch (e.g. supercooling, nucleation) or can simply confirm consistency of the freezing behavior between batches during scale-up. ⁵