Dynamic analysis of moisture, dielectric property and microstructure of ginger slices during microwave hot-air flow rolling drying

This study investigates the drying process of ginger slices using the microwave hot air rolling bed drying (MHARD) method, in terms of measuring drying kinetic change, moisture state and migration, dielectric characteristics, and color changes. The low-field nuclear magnetic resonance (LF-NMR) signal of free, immobilized, and bound water analysis indicates that during the drying of the sample, the drying rate followed a typical “rapid rise-steady-decline” trend in the “preliminary-constant rate-falling rate” stages, respectively. The LF-NMR result shows the content of free water (A23) in the ginger slices decreased gradually during the drying. Meanwhile, bound water (A21) content and immobilized water (A22) content firstly increased and subsequently decreased, because of the microwave and hot air, the movement of water molecules accelerated and the binding force weakened, and then bound water (A21) and immobilized water (A22) decreased. On the other hand, both dielectric constant (ε′) and loss factor (ε′′) of the sample decreased gradually with the decrease in moisture content and logarithmic models were established to describe this correlation. In the preliminary drying stage, although increased microwave power led to a higher drying rate, it resulted in significant damage to the microstructure and compromised the color profile of the final dehydrated ginger slices. This research aims to discuss the mechanism of ginger slides dehydration using the MHARD technique to produce a final product with high quality.