Drying uniformity and technology of sea buckthorn with infrared combined hot air via temperature and humidity control

Sea buckthorn is one type of the most favorite fruits rich in minerals. However, the short shelf life has limited food and medicinal applications. Among them, drying has been the most popular processing to prepare sea buckthorn. However, the existing hot-air drying cannot fully meet the large-scale production of sea buckthorn, particularly because of the long drying cycle, low quality, and high energy consumption. In this study, an infrared combined hot-air drying system was developed via temperature and humidity control. A five-layer material tray was also designed to increase the amount of sea buckthorn to be dried. An electric heating of infrared radiation was adopted with the carbon-fiber plate along a finned single-head electric heating tube. Radiation drying was then achieved in the sea buckthorn. An axial fan was used as the circulating fan of the dryer, and a steam generator was used as a humidifying device. A systematic optimization was made on the structure of the airflow distribution chamber in the drying device. A cylindrical spoiler and a square wind baffle were designed to simulate the velocity, temperature, and humidity fields inside the drying device using the numerical simulation program COMSOL. There was a great variation in the uneven temperature and humidity within the drying layers. The results demonstrate that the drying inhomogeneity was alleviated by altering the airflow between the drying layers or the size of the air outlet of each drying layer. A spoiler was added to increase the air velocity, in order to prevent uneven drying in various locations of the drying layer. The drying capacity was determined for the high-temperature and low-humidity drying medium in the free flow area. The maximum velocity deviation ratio between the five drying layers decreased to 0.88% in the modified structure of the drying chamber. Taking the sea buckthorn as the research subject, an experimental test was conducted on the infrared combined hot air-drying process using temperature and humidity control. The drying moisture ratio decreased exponentially with an increase in drying time. Drying temperatures shared a substantial influence on the drying duration of sea buckthorn. A systematic evaluation was utilized to acquire the highest comprehensive score at 75 ℃ under various drying temperatures using the hierarchical analysis. The weight of each drying characteristic and quality was then determined. Specifically, the maximum brightness, rehydration ratio, Vc retention rate, and total flavonoid content were obtained at 40 min under different conditions of medium humidity. The lowest drying energy consumption, the highest rehydration ratio, the highest Vc retention rate, and the total flavonoid content were obtained at 10% medium humidity. The highest overall score was obtained at 40% medium humidity under different drying durations. The optimal combination was achieved for sea buckthorn in the process of infrared combined hot air-drying using temperature and humidity control. The finding can provide a strong reference to improve the level of mechanization in the primary processing of sea buckthorn for the healthy and sustainable development of the food industry. © 2024 Chinese Society of Agricultural Engineering. All rights reserved.