We report on the physicochemical changes of infant formula (IF) powder and its macronutrients (lactose, fat, and proteins) under given storage conditions. Colloidal (particle size distribution, emulsion stability and sedimentation), morphological (scanning electron microscopy), thermal (differential scanning calorimetry), structural (synchrotron X-ray diffraction) as well as surface and chemical (X-ray photoelectron and Fourier transform infrared spectroscopies) data were used to elucidate the main cause-effect relationships for microstructural, functional, and other properties of the IF powder. The wetting behavior of the powder was found to be significantly affected by water activity (a(w)) during storage (a(w) in the range between 0.24 and 0.42). At the highest a(w) (a(w) = 0.42), lactose crystallization and fat migration took place, leading to changes on the surface of the particles that reduced powder wettability. We propose possible mechanisms to explain the observations, associated with changes in protein conformation. Interestingly, no major changes in the pH and colloidal characteristics, including particle size and distribution, stability, and sedimentation were observed in the reconstituted IF powder upon storage for 6 weeks. The results indicated a negligible contribution from possible Maillard reactions. We propose leading microstructural and wetting characterization to troubleshoot changes in the quality of IF powder, most relevant from the perspective of reconstitution after storage. (C) 2020 Elsevier Inc. All rights reserved.