Qualitative, energy and environmental aspects of microwave drying of pre-treated apple slices

In the present research, response parameters such as specific energy consumption ( SEC ), thermal efficiency ( TE ), energy efficiency ( EF ), drying time ( DT ), greenhouse gas (GHG) emission (such as CO 2 and NO x ), and quality features (color variation and shrinkage) were modeled by response surface methodology ( RSM ) for apple slices dried in a microwave dryer under ultrasonication (30 ℃—10 min) and blanching (80 °C—2 min) pretreatments. Also, RSM was applied to optimize two independent parameters including microwave power and sample thickness in the levels 100, 200, and 300 W and 2, 4, and 6 mm, respectively. The results indicated the significant influence ( P < 0.01) of the independent parameters on the response parameters. The vales of SEC , DT , GHG emission, shrinkage, and color difference were linearly decreased with the declining sample thickness and increasing microwave power, while the energy and thermal efficiencies were increased by a quadratic equation. The use of ultrasonication and blanching pretreatments decreased the SEC , GHG emissions, and DT ; while improving the quality of the samples as compared to the non-treated slices. The optimization results showed the optimal drying times (31.55, 82.19, and 50.55 min), SEC (3.42, 10.07, and 4.37 MJ/kg), CO 2 with natural gas (1539.75, 1518.75, and 4585 g), CO 2 with gas oil (3662.53, 2099.25, 2721.25 g), NO x with natural gas (10.094, 9.956, and 12.906 g), and NO x with gas oil (12.934, 12.758, and 16.538 g) at a microwave power of 300 W and sample thickness of 2 mm with desirability of 0.921, 0.935, and 0.916 for control samples, ultrasonicated, and blanched, respectively.