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

Abstract

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°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°C–15 kPa caused lowest color change (ΔE) 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.

Microwave and convective drying kinetics and thermal properties of orange slices and effect of drying on some phytochemical parameters

“…Bozkir [13] dehydrated the Valencia variety with the convective drying method at 50 °C, 60 °C, and 70 °C and emphasized that the color change (ΔE) decreased due to the increase in temperature. We noticed that the color parameters of fresh orange slices measured in a study conducted by Karabacak et al [58] were in parallel with our results. Deng et al [2] dehydrated the orange peels with five ambient temperatures between 50-70 °C and, unlike our finding in our study, found that the most color loss occurred at 65 °C.…”

“…Bozkir [ 13 ] analyzed the vitamin C content of orange slices dried at 50, 60, and 70 °C along with fresh ones and determined that slices dried at 50 °C and 70 °C had 56% and 79% less vitamin C concentrations, respectively, than fresh ones. Karabacak et al [ 58 ] found that the ascorbic acid content of orange slices dried by vacuum drying was approximately 2–2.5 times reduced compared to fresh ones. Deng et al [ 2 ] measured the vitamin C content of convective dried orange peels at different temperatures from 50 to 70 °C.…”

Improvement of Pacific White Shrimp (Litopenaeus vannamei) Drying Characteristics and Quality Attributes by a Combination of Salting Pretreatment and Microwave

“…As a novel drying method, microwave (MW) drying converts electromagnetic energy into heat energy to heat materials volumetrically [ 14 ]. Due to its fast drying rate and being easy to operate, it has been widely used in drying and processing of cereals, fruits, vegetables and aquatic products [ 15 , 16 ]. Kipcak and İsmail [ 17 ] studied the effects of MW drying power on the drying kinetics of fish and observed that drying time decreased as MW power increased, and the MW power level was the main factor affecting the color change in the treated material.…”

Drying Kinetics and Quality of Whole, Halved, and Pulverized Tiger Nut Tubers (Cyperus esculentus)

“…The D eff amounts recorded in this study for tiger nut is within the broad range of 10 −8 –10 −12 m 2 s −1 to dry food materials [ 24 ]. D eff recorded in this study is relatively higher than hot-air drying of dika kernels and nuts [ 23 ] but lower than ultrasonic hot-air drying of walnuts [ 25 ].…”

“…At this stage, the sample's moisture diffuses rapidly through the pieces' pores; thus, there is a high loss factor at higher moisture content. This movement increases the water vapour pressure inside the pores and results in pressure-inducing pore openings in the tiger nut's tissues [ 23 ]. The D eff amounts recorded in this study for tiger nut is within the broad range of 10 −8 –10 −12 m 2 s −1 to dry food materials [ 24 ].…”