Aqueous starch–oil dispersions prepared by steam jet cooking. Starch films at the oil–water interface

Abstract

Starch–oil composites were prepared by passing aqueous mixtures of starch and soybean oil (100 : 40, by weight) through a steam jet cooker operating under excess steam conditions. Dilution of jet cooked dispersions with a 20-fold excess of water reduced the viscosity and caused a lipophilic fraction with low specific gravity to separate from the dispersion. This fraction could be collected and washed with water without coalescence of oil droplets. Microscopy showed that this fraction was comprised of oil droplets surrounded by thin films of starch at the oil–water interface. The most detailed view of these spherical starch films was obtained by scanning electron microscopy, after isolating films by ethanol precipitation and critical point drying. Films prepared from normal food grade cornstarch, waxy cornstarch and high amylose cornstarch were compared. Interfacial starch films were observed not only when aqueous mixtures of starch and soybean oil were co-jet cooked, but also when starch solutions were first jet cooked and then blended with soybean oil in a separate step. Starch films were also observed with lipophilic materials other than soybean oil, for example, mineral oil, paraffin wax and α-tocopherol. We have considered the question of why these starch films are spontaneously formed at the droplet interface, despite the fact that starch is not surface active and no surface active materials are used during the preparation. A reasonable explanation is provided by the known thermodynamic properties of aqueous polymer solutions at interfaces. Formation of a layer of polymer at an oil–water interface (prewetting) occurs when adsorption of polymer leads to a reduction in interfacial tension and when the solvent for the polymer is relatively poor.