Application of different drying methods for evaluation of phytochemical content and physical properties of broccoli, kale, and spinach

In this study, quality characteristics of broccoli, kale, and spinach dried with hot air drying (AD), freeze drying (FD) and Refractance Window drying (RWD) were investigated, including phenolics, flavonoids, vitamins, glucosinolates, color, T_g and microstructure. Broccoli, kale and spinach samples dried using RWD had the highest total phenolics content (8.50, 18.77 and 12.40 mg/g, respectively). The highest retention of flavonoids was found in FD broccoli (8.98 mg/g), RWD kale (19.51 mg/g), and RWD spinach (9.31 mg/g). The FD-samples had the highest glucosinolates, followed by RWD. The FD and RWD samples also showed less color changes compared to the AD samples, which had darker color indicating chlorophyll degradation. T_g was in the range of 55–60 °C for all RWD, AD and FD vegetable samples, apart from AD broccoli which showed a slightly higher T_g. Among the 3 drying methods, the AD vegetables had a rough surface while that of the FD and RWD samples were relatively smooth. Overall, this study concludes that the quality retention in the dried vegetables is product and drying method specific and, therefore, selection of the best drying method for a product needs to be done considering both, the quality retention and energy consumption.