Effects of Hot-Air Drying Temperature on Biochemical Quality, Drying Kinetics, and Predictive Modeling of Ziziphus...

Jujube (Ziziphus jujuba Mill.) is an important functional fruit with high levels of bioactive compounds. However, its high moisture content limits its postharvest shelf life. The biochemical properties of jujube fruits influenced by hot-air drying at 50, 60, and 70 °C were studied and predictive models were evaluated to optimize drying. The drying kinetics were modeled using the six classical thin-layer models and artificial neural network (ANN) approach. The Midilli-Kucuk model was the best classical model, and the ANN model showed the best predictive performance, with the RMSE values improved up to 19.52% in the independent validation dataset. The drying temperature significantly influenced the color, composition of sugars, organic acids, total phenolic content, and antioxidant activity. Color difference (ΔE = 5.88) and browning index of the 70 °C treatment showed less color change, indicating better color retention. Sugar and organic acid concentrations were significantly higher in dried fruits than in fresh fruits. The highest total phenolic content (5393.43 mg GAE 100 g^-1) and DPPH radical scavenging activity (73.56%) were found at 50 °C and progressively decreased with increasing drying temperature. Multivariate analyses further showed clear biochemical differentiation between fresh and dried samples according to drying temperature. Generally, the drying temperature had a significant effect on the nutritional and functional quality of jujube fruits. Although 50 °C was the most effective for maintaining phenolic compounds and antioxidant activity, 70 °C offered better color retention and shorter drying time. The ANN method was a useful tool to predict the drying behavior and optimize processing conditions in the production of jujube fruit.