Freeze-drying of aqueous solution frozen with prebuilt pores

To save drying time and increase productivity, a novel idea was proposed for freeze-drying of liquid materials by creating an initially unsaturated frozen structure. An experimental investigation was carried out aiming at verifying the idea using a multifunctional freeze-drying apparatus. Mannitol was selected as the primary solute in aqueous solution. Liquid nitrogen ice-cream making method was used to prepare the frozen materials with different initial porosities. Results show that freeze-drying can be significantly enhanced with the initially unsaturated frozen material, and substantial drying time can be saved compared with conventional freeze-drying of the initially saturated one. Drying time was found to decrease with the decrease in the initial saturation. The drying time for the initially unsaturated frozen sample (S₀=0.28 or 0.69 of initial porosity) can be at best 32% shorter than that required for the saturated one (S₀=1.00 or zero porosity). This unique technique is easy to implement and improves the freeze-drying performance of liquid materials.