Physical and morphological properties of whole milk powder produced by dynamic freeze-drying

Freeze-drying is widely used to preserve heat-sensitive products, but conventional static processes suffer from long drying times and limited heat transfer. Dynamic freeze-drying, which involves mixing of frozen material during sublimation, can enhance heat and mass transfer efficiency. In this study, milk powders obtained by spray drying (commercial sample), static freeze-drying, and dynamic freeze-drying were compared. Dynamic drying was carried out in an oscillating drum system using frozen beads of 1 mm and 5 mm diameter. The powders were characterized in terms of flowability, moisture content, water activity, specific surface area, microstructure and thermal properties. Results showed that dynamic freeze-dried powders exhibited superior flowability (lowest Carr Index), stable moisture content over 70 days of storage, and specific surface areas approximately ten times higher than those of static freeze-dried or spray-dried powders. SEM analysis confirmed a fine and homogeneous porous microstructure in dynamically dried samples. The glass transition temperatures were also markedly higher and DVS analysis revealed no recrystallization up to 90% RH for 5 mm beads, demonstrating exceptional stability of the amorphous phase. The freezing process is the critical point for most of the differences found between static and dynamic freeze-drying. These findings demonstrate that dynamic freeze-drying can produce free-flowing, stable, and structurally enhanced milk powders, representing a promising alternative to conventional drying methods.