POROSITY AND PORE-SIZE DISTRIBUTION OF STARCH MATERIALS

Bulk porosity and pore size distribution were measured for granular, heat-gelatinized and extruded starch materials, using helium stereopycnometry and mercury porosimetry. Most of the pores of granular materials had a radius of 1-3 mum and bulk porosity about 0.6. About 20% of the total porosity is due to the 6-8-mum voids between starch agglomerates, about 60% is due to the voids around starch granules (1-3.5 mum) and 10% is due to the very small pores. The remaining 10% of the total porosity might be due to pores larger than 150 mum. Gelatinized starches had average pore sizes of 0.8-1.5 mum and bulk porosity lower than 0.1. Extruded starches had pores varying from 1 to 120 mum and bulk porosity 0.1-0.95, depending on the experimental extrusion conditions. High porosity and large pores resulted when starch was extruded at high temperatures and/or low moisture contents. Mechanical compression reduced the bulk porosity and the size of the pores. The porosity and the mean pore size of starches increased during the drying process. Porosity and pore size distribution are important parameters in the transport (diffusion) of water in starch materials and starch-based foods.