Effect of initially unsaturated porous frozen material on freeze-drying

To overcome disadvantages of freeze-drying that is usually lengthy and expensive, a novel idea, freeze-drying of initially unsaturated porous frozen material, was proposed. An experimental investigation was performed to verify the effect of unsaturated frozen materials with a certain initial porosity on freeze-drying of liquid materials. A multifunctional freeze-drying apparatus on the lab scale was designed, made and assembled. Mannitol was selected as the primary solute in the solution. Liquid nitrogen ice-cream making method was used to prepare frozen materials with different initial saturations. Results show that the unsaturated frozen material can enhance the freeze-drying process significantly and the drying time is reduced substantially compared to conventional freeze-drying for the same material quantity and initial moisture content under the same operating conditions. The drying time with the unsaturated material (initial saturation=0.28) was 36.4% shorter than that with the conventional one(initial saturation=1.00). The smaller the initial saturation is, the shorter the drying time is and the lower the moisture content of dried products is.