Determination of the influence of primary drying rates on the microscale structural attributes and physicochemical properties of protein containing lyophilized products.

This work investigated the impact of primary drying conditions on the microstructure and protein stability of bovine serum albumin (BSA) containing lyophilized cakes. Two primary drying conditions were employed (termed 'conservative', slower drying rate and 'aggressive', higher drying rate) at two protein loadings (5 and 50 mg mL(-1)). The cake attributes were characterized using micro-X-ray computed tomography (micro-CT), scanning electron microscopy, manometric temperature measurements, Fourier transform infrared spectroscopy (FTIR) and size exclusion chromatography (SEC). The combination of analytical techniques revealed that although the aggressive drying conditions produced intact cakes which retained their macrostructure, they had undergone various degrees of microcollapse. The study demonstrates the applicability of micro-CT for resolving microstructural attributes of the freeze-dried cakes such as residual porosity, pore size distribution and connectivity. Micro-CT data showed significant differences in residual porosity and matrix connectivity between aggressively and conservatively dried cakes. The FTIR data show that at each protein loading, there is no observable difference in the secondary structure of the protein and the SEC data show comparable stability in the cakes produced under different primary drying conditions. Good content uniformity was observed with respect to BSA and sucrose distribution in the cakes.