Porosity measurement of fragile agglomerates

Agglomeration processes in the food and pharmaceutical industries frequently produce porous, brittle agglomerates intended for redispersion in liquids (a.k.a. "instant powders"). Typical products have a porosity ranging from epsilon = 0.4 to epsilon = 0.8 and a particle size between 0.2 and 2 nun. The agglomerate porosity is an important product property, as it has a major influence on the wetting behaviour. Due to their high porosity and comparatively weak internal bondings, instant agglomerates are fragile. Compared to the wide range of currently available particle sizing methods, few porosity-measuring methods exist, most of which are not applicable to instant agglomerates. For controlling the porosity of instant agglomerates produced in our laboratory, we used individual particle weighing and sizing. A balance with 1 mug of precision allowed reliable weighing of individual agglomerates as small as 300 mum diameter. The volume of the particles was determined using a long-distance microscope with CCD camera attached to a computer running public domain image analysis software. Porosity measurements of agglomerated skim and whole milk powder, agglomerated pectin, agglomerated flour/maltodextrin mixture, and agglomerated rice starch/maltodextrin mixtures were carried out. The results of our measurements show, for the majority of the samples, an approximately constant porosity over a wide particle size range, with a tendency towards lower porosity for smaller particles. (C) 2002 Elsevier Science B.V. All rights reserved.