Mathias Hilmer, Emilia Jackermaier and Petra Först
The gentle yet cost-effective drying of sensitive products in the food and pharmaceutical industries is gaining increasing importance. An example is the drying of living starter or probiotic cultures. To maintain viability and make the cultures storable, they are often freeze-dried. For this purpose, the bacterial suspension is dripped into liquid nitrogen after fermentation and processed into frozen pellets. However, due to poor heat and mass transfer, the freeze-drying process is very time and cost-intensive. The heat transfer problem can be addressed by volumetric energy input using microwaves as heat source instead of heated shelves. Since the product is pelletized, it can be mixed in a drum system instead of a conventional microwave turntable to ensure temperature homogeneity and overcome the outer mass transfer resistance of the bulk. This work compares dynamic microwave freeze drying in a drum system with the established turntable method. The results show that the process takes 20% less time when mixing particles during the drying process, demonstrating that the outer mass transfer resistance of the particle bed can be overcome. Regarding microwave energy efficiency, this study proves a reduction of up to 35% when using a lower specific microwave energy input. Furthermore, the investigations show that mixing can improve the moisture homogeneity in the particle bed by 82% compared to a stationary bed on a turntable.
https://doi.org/10.1080/07373937.2025.2549723