Integrated Analytical Approach to Identify Whey Permeate Powder Caking: Revealing Internal Structure Using X-Ray...

Caking represents a critical stability challenge for whey permeate powders (WPPs), frequently developing during storage and handling due to moisture-driven structural transformations within the powder bed. This study investigated the physical, morphological, and microstructural characteristics associated with caking in a limited set of industrial WPPs. Five commercial WPP samples differing in production date and storage conditions were characterized in terms of dry matter content, water activity (a_w), particle size distribution (PSD), bulk density, porosity, color, and X-ray micro-computed tomography (micro-CT). Dry matter contents were similar among samples (97.74-98.20% w.b.); however, significant differences were observed in a_w, bulk density, porosity, and PSD between the caked sample (WPP2) and the free-flowing powders. WPP2 exhibited the highest a_w (0.261), the lowest bulk density (676 kg/m³), the highest porosity (0.569), and a distinctly coarser PSD. In addition, WPP2 showed the highest yellowness index (44.45), suggesting altered light-scattering behavior associated with structural changes. Micro-CT analysis revealed the presence of enlarged particle clusters and extensive particle-particle solid bridging in WPP2, accompanied by a heterogeneous pore distribution and reduced void connectivity, indicating consolidation of the powder bed. The integrated analytical approach demonstrates the potential of combining conventional measurements with micro-CT to provide detailed insight into the relationships between moisture-related properties and internal powder structure.