Skim milk powders produced by spray drying (mostly amorphous) are commonly used in downstream food industries and have to be stored for long times after production before final use. However, their physicochemical qualities and properties decline during long-term storage due to the hygroscopicity of amorphous lactose. Surface modification of particles by crystallisation of the outer layer, giving so called "egg-shell" particles, has shown significant improvements in the physical properties of the powder in their fresh state. In this study, the raw powder and processed powder (with modified surface) were stored at around 33% relative humidity and 25-30 degrees C for 30 weeks to investigate the effect of ageing on these two types of powders. Agglomeration, large lactose crystal formation on the surface, surface composition changes and protein modifications were studied. The changes between the raw powder and process powder were compared after ageing. The non-hygroscopic crystalline surface layer showed significant benefits in maintaining the physicochemical qualities of the powders over long storage times. The aged raw powder showed a 24% change in the crystallinity, a 3% change in the lactose/protein ratio on the surface and 6% protein denaturation compared with the aged processed powder with a 4% change in the crystallinity, a 1.5% change in the lactose/protein ratio on the surface and 2% protein denaturation, respectively, after 30 weeks storage. (C) 2012 Elsevier Ltd. All rights reserved.