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

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 (aw), 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 aw, bulk density, porosity, and PSD between the caked sample (WPP2) and the free-flowing powders. WPP2 exhibited the highest aw (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.