Experimental and computational evaluation of the degree of micro-collapse formations in freeze-dried cakes

The quality of freeze-dried products is intensely related to the occurrence of collapse. In this regard, the present study was aimed at developing a method to quantitatively evaluate and predict the degree of collapse of freeze-dried products, which has not been established to date, based on the operating conditions of the freeze-drying process. Freeze-dried specimens were prepared using dextrins with different glass transition temperatures and analyzed by X-ray computed tomography. The deformation of the freeze-dried-cake microstructure was evaluated using a shape factor (i.e., the mean circularity of the fragmented domain) as an index of the occurrence of collapse. The obtained shape factors increased with the degree of collapse and varied between 0.5 and 1.0; moreover, they adequately correlated with the values obtained by tapped density experiments. The deformation of the fragmented domain during freeze-drying was analyzed using computational fluid dynamics simulations, assuming surface-tension-induced deformation during dehydration under non-isothermal conditions. The results confirmed that the microstructural changes in the freeze-dried cakes were determined by the kinetic balance between viscous flow and water removal from the matrix, and the validity of employing the proposed shape factor as an indicator of micro-collapse was demonstrated.