This study investigates the effect of relative humidity (RH) and milk composition on the changes in solubility, lactose crystallinity and protein structural form on a commercial infant formula, by identifying the interactions between water, lactose and protein at different storage conditions. Two spray dried model milk emulsions were studied for comparison, by storing these powders under varying relative humidity (RH) levels between 11 and 94% at room temperature. The properties of powders were evaluated from the degree of insolubility, the rate of browning, and the extent of protein denaturation. The model samples were found to be more stable than the commercial powder, even at high relative humidity (>50% RH). The difference was attributed to the presence of casein, which did not denature to the same extent as whey protein. The results also suggested that minerals present in the commercial powder could be responsible in enhancing protein denaturation, thus accelerating the rate of browning and decreasing solubility. (C) 2015 Elsevier Ltd. All rights reserved.