Use of ultrasound pre‐treatment before microwave drying of kiwifruits – An optimization approach with response surface methodology

In the present study, the thermal properties (drying time, effective moisture diffusion (Deff), specific energy consumption (SEC)), and quality (color and shrinkage) of kiwi slices were optimized using the response surface methodology (RSM). The optimization process was done using central composite design (CCD) in Design Expert software with three independent variables including microwave power (100, 200, and 300 W), sample thickness (4, 6, and 8 mm), and ultrasound pretreatment time (10, 20, and 30 min). The results showed that by increasing the microwave power and ultrasound pretreatment time and decreasing the thickness, the SEC, and drying time decreased in quadratic form, while the Deff linearly increased. Moreover, decreasing microwave power and sample thickness as well as increasing the treatment time diminished the shrinkage and color. The obtained optimal results (4 mm thickness, ultrasound for 30 min, 257.2 W of microwave power) were: drying time for 13.779 min, Deff equal 6.67 × 10−9 m2/s, SEC equal 6.343 MJ/kg, the total color difference of 17.94, and shrinkage of 46%. Novelty impact statement Numerous studies have addressed the drying of food products using ultrasound pre-treatment. However, the optimization process is required for different methods of drying, parameters of treatment as well as type of the material. Thus, the response surface methodology (RSM) was applied for optimization and modeling the drying process and chosen physical properties of ultrasound-treated kiwifruits slices prior to microwave drying.