具有初始孔隙的速溶咖啡冷冻干燥试验

Experiment on Freeze-drying of Instant Coffee with Initial Porosity

为了缩短冷冻干燥时间,降低过程能耗,研究了初始非饱和物料干燥技术在速溶咖啡的冷冻干燥工艺中的应用。以速溶咖啡为溶质,制备了具有不同初始饱和度的冷冻咖啡物料样品,并进行了冷冻干燥试验。研究结果表明,相对于传统的咖啡冷冻干燥,初始非饱和咖啡物料能够显著强化冷冻干燥过程。干燥产品SEM形貌显示,初始非饱和物料的干燥产品具有疏松的球状孔隙空间,孔隙之间连通性好,有利于水蒸气的迁移。在干燥室压力11~33 Pa条件下,压力对冷冻干燥过程几乎没有影响,提高环境温度有利于缩短冷冻干燥时间,组合加热能够进一步强化冷冻干燥过程。当温度超过45℃时,饱和物料会发生崩塌,而非饱和物料则能够保持稳定的结构。饱和咖啡物料的冷冻干燥过程经历升速、恒速和降速3个阶段,非饱和物料则始终处于降速阶段。

In order to decrease the freeze-drying time and reduce the process energy consumption, frozen coffee materials with prefabricated porosity were prepared and freeze-dried to explore its influence on freeze-drying experimentally. Instant coffee was selected as the solute in aqueous solution. Results showed that freeze-drying can be remarkably enhanced with the initially unsaturated frozen material compared with the traditionally saturated one. SEM images of dried products revealed that the initial unsaturated material had the spherical and uniformed porous structure, which was favorable to migration of sublimated vapor and improvement of the drying rate. Chamber pressure, which was ranged from 11 Pa to 33 Pa, had little effect on the freeze-drying process and increasing ambient temperature can be beneficial to the freeze-drying process. Combined beating can significantly reduce the drying time further. When the temperature was above 45℃ , the saturated material was found to collapse causing the failure of freeze-drying. In contrast, the unsaturated one can bear higher operation temperature resulting in a relatively high drying efficiency. The conventionally saturated material experiences the raising rate stage, constant rate stage and falling rate stage during freeze-drying, while the unsaturated material was always in the falling rate stage throughout the process. Freeze-drying of initially unsaturated material would impose significant effect on the traditional freeze-drying process of agricultural products.