Study of three-dimensional structure of wheat starch granules stained with Remazolbrilliant blue dye and extracted with aqueous sodium dodecyl sulfate and mercaptoethanol solution

Wheat starch granules were obtained from soft wheat flour by acetic acid fractionation (pH 3.5), and the starch was stained by reaction with Remazolbrilliant blue (RBB) dye. RBB-stained starch was extracted with 1% sodium dodecyl sulfate (SDS) and 1% 2-mercaptoethanol (ME) for 14.5 hr at room temperature. This extraction step was repeated five times (extracts 1-5). SDS-ME extracts were subjected to size-exclusion column chromatography, and comparisons of their profiles for specific absorbance at 650 nm (A(650)) and carbohydrates were made. After high molecular weight (HMW) carbohydrates on the starch granule surface were extracted, HMW carbohydrates inside the granule appeared to be extracted. Finally, low molecular weight (LMW) carbohydrates near the granule surface were extracted. Phase-contrast tight microscopy of the treated starch granules showed that all granules became transparent. Two different interior structures were observed. Scanning electron microscopy indicated that the granule was split into two parts at the equatorial groove. The interior of the granule showed two different areas: a central hole area and the surrounding stratified area. Extraction beyond five times with the same solvent dissolved the weak part of the granule structure and left two types of skeletal structures. The appearance of the skeletal structure of the granule surface was different from the appearance of interior structures.