The morphological characteristics and surface structure of waxy, low-amylose and high-amylose rice starches before and after extraction of granule-associated proteins (SGAPs), especially surface-proteins and channel-proteins, were systematically investigated. Removal of SGAPs on surfaces and in channels of granules without significant change in morphological structure was evidenced by microscope images of granules and confocal laser scanning microscope images of fluorescamine-stained and 8-amino-1,3,6-pyrenetrisulfonic acid-stained granules. For nano-scale surface features, atomic force microscopy showed that a more roughened surface occurred on granules after removal of SGAPs, meanwhile, specific surface area, total pore volume, surface and average diameter of pores increased in granules. The results gave direct evidence that extraction of SGAPs induced larger surface area and increased size of channels (pores) in starch granules. Once SGAPs have been removed, dextran-probes could quickly diffuse into channels and cavities, thus probe-diffusion of the granular matrix was accelerated. Although waxy rice starch had the lowest SGAPs content, the change of dextran diffusion into its granule was the greatest of three tested rice starches when SGAPs were removed. The surface texture and porosity of the starch granules before and after removal of SGAPs were significantly correlated to the pasting properties of starch, providing a new insight into the SGAPs-surface microstructure-function relationship of starch granules.
- Atomic force microscopy
- Low-temperature nitrogen adsorption
- Pasting properties
- Rice starch
- Starch granule-associated proteins