Aqueous starch-oil dispersions prepared by steam jet cooking. Starch filmsat the oil-water interface

Starch-oil composites were prepared by passing aqueous mixtures of starch a nd soybean oil (100: 40, by weight) through a steam jet cooker operating un der excess steam conditions. Dilution of jet cooked dispersions with a 20-f old excess of water reduced the viscosity and caused a lipophilic fraction with low specific gravity to separate from the dispersion. This fraction co uld be collected and washed with water without coalescence of oil droplets. Microscopy showed that this fraction was comprised of oil droplets surroun ded by thin films of starch at the oil-water interface. The most detailed v iew of these spherical starch films was obtained by scanning electron micro scopy, after isolating films by ethanol precipitation and critical point dr ying. Films prepared from normal food grade cornstarch, waxy cornstarch and high amylose cornstarch were compared, interfacial starch films were obser ved not only when aqueous mixtures of starch and soybean oil were co-jet co oked, 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, para ffin wax and alpha-tocopherol. We have considered the question of why these starch films are spontaneously formed at the droplet interface, despite th e fact that starch is not surface active and no surface active materials ar e 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) occu rs when adsorption of polymer leads to a reduction in interfacial tension a nd when the solvent for the polymer is relatively poor. (C) 1999 Elsevier S cience Ltd. All rights reserved.