This study examined the effects of storage conditions on physicochemical and microstructural characteristics of whole milk powder (WMP) and low-, medium- and high-heat skim milk powder (SMP). Powders were stored at three temperatures (25 degrees C, 35 degrees C and 45 degrees C) and three humidity levels (11%, 44% and 85%) for 21 days. WMP exhibited lower equilibrium moisture content (EMC) than SMP at these storage conditions. SMP and WMP stored at 45 degrees C had lower EMC and monolayer moisture content than those stored at 25 degrees C and 35 degrees C. WMP had lower T-g, crystallinity and caking strength than SMP. At 85% RH, the particles became irregularly shaped, caused by lactose crystallization. Crystallization of lactose and caking of powders became prominent when T-g decreased below the storage temperature. Crystallization of lactose in these powders increased caking strength, Maillard reaction and surface free fat, particularly at high RH (85%) and high temperature (45 degrees C) conditions. (C) 2020 Elsevier B.V. All rights reserved.