ABSTRACT
Dry waxy wheat starch granules were heat-treated at 120°C for 5 hr, and then shaken vigorously in a biphasic system of oil and water. Non-heat- treated starch remained in the aqueous phase, whereas the heat-treated starch granules showed a strong oil-binding ability that was lost by trypsin treatment. This result showed that the starch granule surface protein changed from hydrophilic to hydrophobic due to the heat treatment. The presence of starch granule surface protein was ascertained by staining with fluorescamine and fluorescence microscopic observation. Heat-treated waxy wheat starch granules were incubated with a 25% KI/10% I2 (w/v) solution, which produced “ghosts” (exterior and interior) structures. The exteriors stained red-brown, whereas the interiors stained black-brown. Sonication (20 kHz for 255 sec) followed by centrifugation separated the structures, which were then shaken vigorously in an oil and water system. Only the exterior ghosts exhibited a remarkable emulsification property, which disappeared after trypsin treatment. The ghosts from unheated control granules did not show emulsification. The presence of protein in the exterior ghost fraction was further substantiated by fluorescamine treatment. No protein was detectable in the interior fraction with this dye. From these results, we suggest that the ghost fraction of the waxy wheat starch contained the starch granule surface protein that was made hydrophobic by heat treatment. Also, the nature of the induced emulsification property of the exterior fraction (ghosts) and the oil-binding ability of the heat-treated waxy wheat starch granules coincided. Both were due to the hydrophobic nature of the same starch granule surface protein, which showed that the ghosts were the swollen form of the outer region of the waxy starch granule.
