Comparative Study of Convective and Intermittent Microwave Drying of Red Water Lily (Nymphaea × rubra) Rhizomes: Drying Behavior, Microstructural Changes and Drying Effects on Thermal, Antioxidant and Adsorption Properties

Red water lily rhizome is consumed as food due to its nutritional composition, making it a potential alternative food resource. However, its high moisture content makes it highly susceptible to microbiological and biochemical spoilage. Moisture removal is hence necessary to extend its shelf-life. In this study, three different drying methods were employed: tray drying (TD) at 50°C, 60°C, and 70°C; intermittent microwave drying (MWD) at 450, 720, and 900 W; and freeze drying (FD), which served as a positive control due to its ability to preserve the quality and integrity of heat-sensitive materials. Mathematical models were applied to describe drying and adsorption behaviors of the rhizomes. Quality attributes, including color values, total phenolics, total flavonoids, and antioxidant capacity of dried samples were analyzed. Microstructural properties were evaluated using synchrotron radiation x-ray tomographic microscopy, while thermal properties were determined via differential scanning calorimetry. Moisture adsorption characteristics were studied using dynamic vapor sorption. Additionally, multivariate analyses including principal component analysis (PCA) and hierarchical cluster analysis (HCA) were utilized to assess the correlation between quality parameters and classify drying conditions. The results showed that Midilli and Kucuk and Double log polynomial models were the most suitable models to describe drying and moisture adsorption behaviors. Freeze drying and tray drying better preserved color, microstructure (more porous and uniform structure), thermal properties (lower gelatinization degree), and moisture adsorption compared to microwave drying. However, microwave drying resulted in higher total phenolics, total flavonoids, and antioxidant capacity than tray drying. The application of PCA and HCA provided useful information for food processors and researchers in designing drying conditions to achieve the desired quality attributes.