Fluidization of solid particles has become the state of the art technique, facilitating powder handling and processing. The case of non-stationary fluidization during a Vacuum-Steam-Vacuum (VSV) treatment of conventional milk powder was investigated. Thereby, the food material is fluidized in a free fall under partial vacuum conditions. Microbiological decontamination is achieved through a short injection of saturated steam in counterfiow. Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA) were used for quantification and characterization of solid motion in the highly dynamic system. The non-invasive analysis revealed three distinct flow domains, which were the free fall in partial vacuum, a transient and intermediate floating state showing turbulent mixing and the final collapse of floating state due to progressed particle agglomeration. Important insight of the process were obtained. Qualitative behavior of the underlying physics of the VSV process was well characterized using the results from PIV and DIA. The gained knowledge is applicable for various powder fluidization operations involving non-stationary conditions as well as non-uniform spatial distribution of solid particles. (C) 2016 Elsevier B.V. All rights reserved.