Atmospheric Freeze Drying (AFD): Fundamentals and Innovative Approaches

Atmospheric freeze drying (AFD) is a promising alternative to conventional vacuum freeze-drying (VFD), operating under atmospheric conditions with lower energy consumption, continuous processing, and cost-effectiveness, especially in cold climates. However, AFD faces challenges such as prolonged drying time, product shrinkage, and ice thawing. These issues are addressed through hybrid techniques incorporating thermal or mechanical energy to enhance drying efficiency. This review paper presentes recent advancements in AFD by examining its fundamental principles underlying the process and innovative approaches designed to improve its efficiency. The application of differential scanning calorimetry (DSC) and the development of state diagrams have been discussed as tools for analyzing thermal characteristics and designing efficient drying regimes. The review also explores the influence of process parameters such as drying temperature, air velocity, and sample characteristics on drying kinetics and product attributes, offering insights into optimal conditions. Hybrid approaches, including heat pumps, vortex tubes, expanders, ultrasonic and microwave assistance, adsorbent usage, and fluidization, have shown significant energy savings and product quality improvements. Finally, the predominant modeling approaches employed in AFD have been explored to provide a comprehensive understanding of drying kinetics. Despite advancements, ongoing research is needed to overcome technical barriers and extend AFD's applicability across various industries.