Freeze-drying of porous frozen material with prefabricated porosity

The porous frozen materials with prefabricated porosity were prepared to verify the effect on freeze-drying of liquid materials experimentally. Ceftriaxone sodium, a kind of commonly used antibiotics for injection, was selected as the primary solute in aqueous solution. Liquid nitrogen ice-cream making method was adopted to fabricate frozen materials with certain initial porosity. Freeze-drying experiments were conducted at the same operating conditions for two kinds of frozen materials, the initially unsaturated and saturated ones for comparison. The results showed that freeze-drying could be enhanced significantly using the porous frozen materials with prefabricated porosity. The drying time for the initially unsaturated frozen sample (S0=0.3 or 0.67 of initial porosity) can be 21.3% shorter than that required for the saturated one (S0=1.00 or zero porosity). SEM images of dried products displayed that the unsaturated materials had more tenuous solid matrix and continuous void space than those of the conventional one. This would be beneficial to migration of sublimated vapor and desorption of adsorbed moisture, favoring reduction of mass transfer resistance. Freezing rate had a little effect on freeze-drying of two kinds of frozen materials. Annealing could decrease the drying times, which were 16.2% saved for the initially unsaturated sample and 14.8% for the saturated one. Appropriately increasing ambient temperature had a positive impact on the freeze-drying process, and the change in chamber pressure had little contribution to the process.