Microwave pre-treatment for vacuum drying of orange slices: Drying characteristics, rehydration capacity and quality properties

Microwave pre-treatment for vacuum drying is a promising method in order to enhance heat and mass transfer rate. In this study, the influences of microwave pre-treatment (90 W 30 min) on vacuum drying conducted at different combinations of drying temperature and absolute pressure (60, 70, and 80 degrees C with 15 and 30 kPa) were investigated by evaluating the drying characteristics, rehydration capacity, and some quality attributes of orange slices. The vacuum drying treatments were carried out both with (MWVD) and without the microwave-pretreatment (VD) at the same temperature and absolute pressure combinations. It was revealed that application of MWVD shortened the drying time of orange slices while increasing drying rate, effective moisture diffusivity and rehydration capacity. Six different semi-theoretical mathematical models were applied, and Page, Modified Page, Logarithmic, and Lewis models were best fitted to experimental data of orange slices. It was observed that, among the different drying conditions applied in the scope of the study, VD at 80 degrees C-15 kPa caused lowest color change (Delta E) that indicates the difference between the dried and fresh orange slices. All drying treatments caused a reduction in ascorbic acid content (AAC) (56.24-59.34%), total phenolic content (TPC) (15.92-55.79%) and total antioxidant capacity (TAC) (6.05-35.18%, 21.03-42.28%, and 54.76-69.07% for DPPH, FRAP, and CUPRAC assays, respectively) of orange slices. When MWVD and VD were compared, it was observed that MWVD caused less decrease in TPC and more in AAC and TAC. In conclusion, MWVD method is suggested as the more effective method providing lower drying time, faster drying rate, and better quality parameters of the product. Practical applications Orange including various nutrients with health beneficial effects is a perishable fruit. Therefore it is processed in to frozen, dried, canned, jam and fruit juice forms in order to increase its shelf life. Especially in seasons with overproduction, orange can be processed into dried form, and it can be turned into an alternative healthy snack with increased commercial value. In this research, drying characteristics of orange slices by microwave pre-treated for vacuum drying were assessed with mathematical modeling. Drying conditions affected rehydration capacity, total phenolic content (TPC), total antioxidant capacity (TAC), ascorbic acid content (AAC) and color values of orange slices. Based on those outcomes, the optimum process conditions for an industrial drying application can be designed to enhance heat and mass transfer rate and to improve quality attributes of the product.