具有初始孔隙的速溶咖啡冷冻干燥试验

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 heating 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.