Comparative drying of pomelo peel and optimization using ultrasound-assisted near-infrared drying

Pomelo peel is a major by-product of pomelo processing, yet its high moisture content and complex structure pose challenges for efficient and high-quality drying. In this study, hot air drying (HAD), near-infrared drying (NID), and microwave drying (MD) were systematically compared in terms of drying kinetics, effective moisture diffusivity, specific energy consumption (SEC), CO₂ emissions, and key quality attributes of pomelo peel. MD achieved the fastest drying, reducing drying time by 87.5% and SEC by 52.7% compared with HAD, whereas NID exhibited superior color retention with the lowest total color difference (ΔE = 9.54) and browning index (BI = 121.30). HAD at 50°C yielded the highest rehydration ratio (287%). Based on the balanced performance of NID in drying efficiency and product quality, ultrasonic pretreatment was further introduced to intensify the NID process. Ultrasonic pretreatment significantly enhanced moisture migration during NID, reducing drying time by up to 28.6% under the investigated conditions. The optimal ultrasound-assisted NID conditions were identified as 300 W ultrasonic power, 5 min treatment time, and 70°C drying temperature. The Modified Page model provided the best fit for drying kinetics across all methods. These results demonstrate that ultrasonic pretreatment effectively strengthens NID by promoting internal mass transfer while preserving product quality, offering a promising strategy for high-value pomelo peel drying.