Freeze-drying of porous frozen material with initial porosity was experimentally investigated. Mannitol was selected as the primary solute in aqueous solution to be dried. The liquid nitrogen ice-cream making method was used to prepare frozen materials with initial porosity. Freeze-drying experiments were performed using two kinds of frozen materials

the initially unsaturated one and the conventionally saturated one for comparison. Freeze-drying could be significantly enhanced with the initially unsaturated frozen material. SEM images of dried products showed that such prepared initially unsaturated material had larger void space and more tenuous solid matrix that was continuous and uniform than those with the conventionally saturated one. This would be beneficial to migration of sublimed vapor and reduction of mass transfer resistance. There was indeed an overall sublimation for the initially unsaturated frozen material

and a primary sublimation region still existed through examining temperature variations at different locations inside the material. Predominant drying rate-controlling factor was heat transfer for the initially unsaturated frozen material

and mass transfer for the conventionally saturated one. Operating pressure had little influence on the freeze-drying process. Combination of radiation heating with conduction heating was able to improve heat transfer in freeze-drying of initially unsaturated porous materials

further shortening drying time.