Preparation and quality evaluation of red dragon fruit (Hylocereus polyrhizus) powder by foam-mat microwave freeze-drying

Red dragon fruit (Hylocereus polyrhizus) is one type of berry fruit rich in nutrients and trace elements, particularly with the high food, health and medicine values. However, the dragon fruit can easily dewater and soften during transportation, resulting in quality degradation, or even decay and spoilage. Fresh dragon fruit can often be processed into dried cakes or powders, in order to extend the shelf life. Freeze-drying (FD) and Spray drying (SD) are commonly used to process berry fruits into powder products. This study aims to reduce the energy consumption of drying and retain the nutrients of the product. Fresh red dragon fruit pulp was taken as the raw material with (87.03±1.26) % of mean moisture content. Red dragon fruit powder was prepared using FD and SD. Product qualities were also evaluated, including residual moisture content, color, betacyanin content, rutin content, and total phenolic content retentions. With 3%(w/w) (mass ratio, the same below) of whey protein isolate (WPI) and 2.5%(w/w) of pectin (P) as the additives, the unfoamed and foamed frozen samples were prepared with the same initial mass and moisture content. Quartz and silicon carbide (SiC) were utilized as the supporting pads of samples for comparison. Traditional and microwave FD experiments of dragon fruit pulp were conducted in a lab-scale microwave FD apparatus. Results showed that the foamed sample was reduced by 39.06% of drying time, compared with the unfoamed one at 30 ℃ of radiation temperature and 20 Pa of chamber pressure with the quartz pad. When 1 W of microwave power was applied under the same operating conditions, drying times of unfoamed and foamed samples decreased by 18.75% and 12.82%, respectively, compared with the traditional FD without microwave heating. With the SiC pad instead of the quartz pad under 30℃, 20 Pa and 1 W, the microwave FD time of the foamed sample was shortened by 65.62% and 43.59%, respectively, compared with traditional FD. SiC-assisted microwave FD of foamed material was achieved the simultaneous enhancement of mass and heat transfers. With 5.5%(w/w) of WPI or 15.6%(w/w) of maltodextrin (MD) as the carrier agent, SD experiments were conducted in a pilot-scale SD apparatus. Operating conditions were 150℃ of inlet air temperature, 85℃ of outlet air temperature and 28 mL/min of feed speed. The quality evaluation revealed that the FD products basically had no significant color differences, except for the product from microwave FD of the unfoamed material with the quartz pad. SD products exhibited light colors, compared with FD products. Betacyanin, rutin and total phenolic content retentions of FD products were all above 72%, 83% and 75%, but those of SD products were much lower. SEM images showed that the porous and tenuous skeletons were observed in the FD products from the foamed materials, which was beneficial to the vapor migration and bound water desorption. There was no impact of microwave heating on the pore structure of both unfoamed and foamed materials. SD products appeared as spherical particles with smooth surfaces. The drying time was significantly reduced to produce high-quality products. The FD can provide an industrial solution to prepare berry powders. © 2024 Chinese Society of Agricultural Engineering. All rights reserved.