Experimental study on the microwave freeze-drying of foamed blueberry puree assisted by wave-absorbing material

Abstract: Berry fruits are very popular food products for their high nutritional value and unique flavor. But, the juiciness and thin skin of berries cannot be suitable for long-term storage and transportation. Therefore, berry fruits are often processed into dried products or solid powders, in order to minimize the transportation cost for the long shelf life. Among them, freeze-drying (FD) can be used for high-quality products at low temperatures under a nearly oxygen-free environment. However, the long drying time and high energy consumption of FD cannot fully meet the large-scale application in recent years. It is necessary to simultaneously consider the typical coupling mass and heat transfer for the enhancement of FD. In the present work, the technical idea of "freeze-drying of unsaturated porous media with microwave heating assisted by wave-absorbing material" was proposed to apply the berry processing. Much effort was also made to establish a high-efficient and low-cost FD. The blueberry fruits were taken as the research objects of berry fruits during experiments. The eutectic temperature of blueberry puree was measured (-20.42 ℃) as the reference of pre-freezing temperature. Quartz and silicon carbide (SiC) were also used as wave-absorbing materials in the supporting samples for comparison. Unfoamed and foamed frozen samples were prepared with the same initial mass and moisture content. Whey protein isolates and pectin were used as the forming agent and stabilizer. Conventional and wave-absorbing material-assisted microwave FD experiments of blueberry puree were conducted in a lab-scale multifunctional microwave freeze-dryer. Color, total monomeric anthocyanin (TMA), and total phenolic content (TPC) were selected as the indexes to evaluate the quality of freeze-dried products. Results showed that the FD process was enhanced by the single factor of either forming treatment or wave-absorbing material-assisted microwave heating. The foamed sample saved 39.1% freeze-drying time at the radiation temperature of 30 ℃ and 15 Pa chamber pressure with the quartz pad, compared with the unfoamed one. In the case of the same operating conditions with the SiC pad, the microwave FD time of unfoamed sample was shortened by 23.9% at the microwave power of 2 W, and by 39.1% at 4 W, compared with the conventional. The performances of SiC assisted microwave heating were enhanced by 14.3% at 2 W and 25.0% at 4 W for the foamed material, respectively. The microwave FD of foamed material assisted by wave-absorbing material was realized by the simultaneous enhancement of mass and heat transfer, indicating the greatly accelerated FD rate. The microwave FD time of foamed sample was reduced by 47.8% at 2 W and by 54.3% at 4 W, compared with the conventional FD of unfoamed sample. There were no significant differences in colors. TMA and TPC contents of microwave freeze-dried products were comparable with those of conventional ones. The retentions of TMA and TPC in all freeze-dried products were more than 80% and 75%, respectively. The SEM images of dried products revealed that the substrate of the foamed material was porous and tenuous, which was conducive to water vapor migration and bound water desorption. There was no influence of microwave heating on the pore structure of either unfoamed or foamed material. Wave-absorbing material-assisted microwave FD of foamed material can largely decrease the drying time for the process economy. There was almost the same quality of products between the microwave and conventional FD. The finding can provide promising guidance for efficient FD in the food processing industry.