Integrated optimization of ultrasound-assisted osmotic dehydration and convective drying of litchi: Linking microstructure, physicochemical attributes and functional properties

This study optimized ultrasound-assisted osmotic dehydration (USOD) followed by convective drying of litchi ( Litchi chinensis ) using Response Surface Methodology based on a Box–Behnken design. Four processing variables ultrasound amplitude (20–40%), sonication time (10–30 min), sucrose concentration (20–60%, w/w), and drying temperature (50–70°C) were evaluated for their effects on moisture content, water activity, colour change, hardness, shrinkage, rehydration ratio, total phenolic content, and antioxidant activity. A second-order polynomial model adequately described the responses, with high coefficients of determination (R 2 > 0.92) and non-significant lack-of-fit, indicating strong model reliability. Numerical optimization identified the optimum conditions as 30% ultrasound amplitude, 20 min sonication, 40% sucrose concentration, and 58°C drying temperature, with an overall desirability of 0.82. Under these conditions, predicted values were 10.76% moisture content, 0.41 water activity, 15.02 colour difference, 21.53 N hardness, 74.29% shrinkage, 2.67 rehydration ratio, 10.79 mg GAE/g DW total phenolic content, and 53.76% antioxidant activity. Experimental validation showed relative errors below 5%. Microstructural analysis confirmed enhanced porosity, while FTIR indicated improved antioxidant preservation and sensory evaluation further validated these benefits, with optimized USOD-treated samples scoring higher in overall acceptability compared to control samples, demonstrating the effectiveness of integrated USOD and convective drying optimization for quality retention in dried litchi.