Microwave freeze-drying of coffee solution frozen with initial pores assisted by wave-absorbing medium

Abstract

Wave-absorbing medium (WAM) assisted microwave freeze-drying (FD) of initially unsaturated frozen material was experimentally investigated in an attempt to simultaneously enhance heat and mass transfer of conventional FD. Silicon carbide (SiC) was utilized as the WAM and coffee solution was selected as the raw material. The results showed that microwave FD of the unsaturated sample at 22 Pa, 35 °C and 1 W with quartz pad saved 30.8% of drying time compared with conventional FD of the saturated one, which achieved a simultaneous enhancement of heat and mass transfer. SEM images revealed that the unsaturated sample skeleton was porous and tenuous, which profited the vapor migration and bound moisture desorption. WAM assisted microwave FD of the unsaturated sample with SiC pad shortened 41.0% of time compared with conventional FD of the saturated one, which realized a thorough enhancement of heat and mass transfer. The retentions of chlorogenic acid in all dried products were more than 92%. WAM assisted microwave FD of initially unsaturated material realized a simultaneous enhancement of heat and mass transfer in the conventional FD process while maintaining a high product quality.