Instant properties of agglomerated powders are linked to the size and structure of agglomerates. In this study, agglomerates structure was investigated through the measurement of their porosity. An image analysis method for 3D re-construction of particle shape from silhouette was developed to measure the envelope volume of dehydrated milk agglomerates (1-2 mm). With known true density of the powder, the intra-particular porosity of agglomerates could be estimated. It was found that intra-particular porosity increases with agglomerates diameter indicating a fractal structure with a fractal dimension of 2.64. This allowed understanding the different behavior of large agglomerates settling and dissolving rapidly in water whilst small ones settle and dissolve slowly and must be avoided in a good instant powder.