Skim milk powder agglomeration was investigated by spraying water in a conical batch fluidized bed at lab scale. Evolution of powder water content and particle size distribution along a "long" agglomeration process (33 min) was followed by taking samples (similar to 4 g) at different times (every similar to 5 min). Then "short" agglomeration experiments, with the same operating conditions, were stopped after the same times. The water content and the particle size distribution of the whole powder were compared to that of the sample taken at the same times during the "long" process. Samples were found to be representative of the whole powder for the size and particle size distribution, but not for the water content, due to non homogeneous distribution of humidity in the fluid bed. From the evolution of the particle size distribution, agglomerates formation and growth were found to occur in stages with the appearance of intermediate population till reaching a maximum size depending on conditions, especially the fluidizing air flow rate. (C) 2012 Elsevier B.V. All rights reserved.