Physical properties and glycemic index of organic and conventional black rice grown in Bali, Indonesia

Abstract

Similar variety of black rice was grown by organic and conventional methods in Jatiluweh and Penatih districts of West Bali, respectively, separated by 46 km distance. Each rice is very similar, having black pericarp and white rice inside. In this study, we found that the degree of crystallinity of the two rice is about 24%. The Bragg parameters of the conventional rice are very close to dehydrating α-amylose, whereas the organic rice exists in polymorphic form. The crystallites in the endosperm area are dominated by polyhedral shapes in contact with each other at the edges and formed a cyclic geometric environment. The size of the crystallites was between three and six microns. Overall, the crystallites are packed in rectangular blocks connected radially from the center towards the pericarp. XRF experiments show each rice contain P, K, S, Si, Mg, Cl, Al, Zn, Fe, Cu, and Rb. However, ruthenium and manganese were detected in organic rice. Both organic and conventional black rice possesses a low glycemic index of about 31±5.0 compared to 50±5.8 for commercial Basmati rice. Thermogravimetric investigation showed that the black rice was stable up to about 300°C, and the gradual mass loss between 80 and 210°C is due to water molecules and additional components present in the rice. After the major decomposition at 310°C, the mass gradually decreased and reached 18% residual at 600°C.