(32f) Porous Frozen Material Approach to Freeze-Drying of Instant Coffee

Instant coffee is commercially prepared by either spray drying or freeze-drying. Although spray drying provides most instant coffee production due to its high capacities, it often brings possibilities of losing some heat-sensitive ingredients as a typical thermal drying. Freeze-drying is a unique form of drying that removes moisture mainly by sublimation. It can lead to the best quality of dried products among almost all drying techniques. Freeze-drying yields a better quality product of instant coffee than spray-drying because it can preserve aroma and antioxidant ingredients as much as possible. However, freeze-drying is an energy intensive and expensive process due to relatively long drying time and poor overall energy efficiency. Reducing freeze-drying time so as for lowering energy consumption and raising productivity therefore becomes a worldwide challenge during the past decades. Conventional freeze-drying of aqueous solutions is to freeze a liquid material into a solid frozen material without initial porosity. Mass transfer within this frozen material is limited largely during drying. If an aqueous solution to be dried is first frozen into a porous material with a certain initial porosity like ice-cream and then freeze-dried, mass transfer in such a material is expected to promote significantly. This innovative technique has the advantages of better product quality, faster drying rate and easy operation.

To reduce the drying time and increase the energy utilization efficiency, an experimental investigation is presented aimed at verifying the proposed idea. Initially porous frozen materials with prefabricated porosity were prepared to explore its influence on freeze-drying experimentally. Coffee powder was selected as the solute in aqueous solution. Liquid nitrogen ice-cream making method was employed to prepare frozen materials with different initial porosities. Results showed that freeze-drying can be remarkably enhanced with the initially porous frozen material, and more than 30% of drying time can be saved compared with the traditionally solid frozen material. SEM images of dried products revealed that the initially porous frozen material has the spherical and uniform porous structure, which is favorable to migration of sublimated/desorbed vapor and improvement of the drying rate. Chamber pressure has little effect on the freeze-drying process and increasing ambient temperature can be beneficial to the process. Combining radiation with conduction heating can reduce the drying time further. When the temperature was above 45^oC, the solid frozen material was found to collapse leading to the failure of freeze-drying. In contrast, the porous one can bear higher operation temperature resulting in a relatively high drying efficiency. The conventionally solid material experienced the raising rate stage, constant rate stage and falling rate stage during freeze-drying, while the porous material was always in the falling rate stage throughout the process.

Acknowledgement

The authors are grateful to the financial supports from National Natural Science Foundation of China (21676042) and Natural Science Foundation of Liaoning Province (201602167).

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