Effect of heterogeneous protein distribution on in situ pasting properties of black rice starch

Abstract

In situ scanning electron microscopy/energy-dispersive spectroscopy (SEM-EDS) was used for depth profiling of black rice and the visualisation and quantification of the heterogeneous protein distribution. The SEM-EDS images of black rice grains revealed high protein content in the aleuronic layer, cell membrane and amyloplasts, especially in the spheroidal particles among the amyloplasts. This phenomenon has not been directly observed by other apparatuses because successive abrasion of brown rice by milling destroys the integrity of the cell walls and amyloplasts. Consistent with the SEM-EDS findings, confocal laser scanning microscopy and the Kjeldahl method preformed after successive milling indicated that the protein content of black rice decreases from the surface to the inner layer. Pasting curves obtained with a viscometer indicate that the protein distribution significantly affects the peak viscosity, setback viscosity, and pasting temperature of rice starch. Protein extraction weakened the structural stability of starch granules, possibly because the protective effect of protein is lost during extraction. This study improves the understanding of the protein distribution in rice grains and its effect on the pasting properties of rice starch; the findings are useful in food processing for managing rice nutrient compositions.