The variety in foamed products found in the food, cosmetic and pharmaceutical domain has been steadily growing for years. Related research in the field of process engineering is directed at optimizing foam microstructure since mean bubble size and size distribution strongly influence the texture and stability of the product. Within this study, a model system containing milk protein as foaming agent and guar gum as thickener was foamed on a rotor-stator gas dispersing device. A wide range in gas volume fraction was achieved (0.2 <= phi(v) <= 0.9) and the resulting foam microstructure analyzed. The work focuses on the impact of static pressure, volumetric energy input and gas volume fraction on the resulting foam microstructure. The static pressure applied during the whipping process was shown to have a governing impact on foam microstructure. The dispersing characteristics revealed a minimum mean bubble size found for increasing volumetric input. The gas volume fraction had no impact on the mean bubble size in a lower range (0 < phi <= 0.65), while a steady, linear increase in mean bubble diameter with increasing gas volume fraction was found in a higher range (phi(v) > 0.65). (c) 2006 Elsevier Ltd. All rights reserved.