Ultrasound pretreatment enhances drying efficiency and phenolic retention in raspberries during heat pump drying by...

Prolonged drying time and substantial loss of bioactive compounds remain major challenges in raspberry drying. This study investigated the effects of ultrasound pretreatment (45 kHz, 500 W, 45 °C, 5-20 min) on cell wall structure and physicochemical properties, water status, drying behavior, and phenolic content during heat pump drying. Ultrasound disrupted raspberry tissue structure, increased accessible surface area, reshaped pore size distribution, promoted pectin solubilization and depolymerization, reduced pectin molecular weight, and enhanced water mobility. These structural changes likely reduced internal resistance to water transport, which in turn shortened the drying time from 8.87 h in the control group to 5.53 h after 15 min treatment, representing a 37.7% reduction. Fifteen phenolics were identified, and US-15 showed the best preservation of anthocyanins, whose contents were 8.46%-25.52% higher than those in control group. Overall, ultrasound pretreatment improved drying efficiency and phenolic retention by remodeling cell wall architecture and promoting water redistribution.