Process intensification of microwave-assisted freeze-drying by frequency adjustment in real-time using solid-state technology

In microwave-assisted processes, solid-state microwave technology enables control of the electromagnetic field, which can be used for process intensification. This work investigates the impact of real-time frequency adjustment on microwave-assisted freeze-drying (MFD). Frequency-based control concepts were applied in experiments with tylose gel as a food model product in a laboratory-scale plant. By applying energy-efficient resonant frequencies based on feedback related to product state, the drying time was reduced by 24.2% compared to conventional freeze-drying (CFD) due to increases in energy efficiency. Comparable temperature profiles, residual moistures, and ascorbic acid retentions to CFD were achieved. However, structural degradation occurred with MFD in up to 10.4% of the samples. The degradation could be counteracted by more refined frequency and power application. In summary, this work demonstrates the high potential of frequency-based control for MFD process intensification.