Losses of essential mineral nutrients by polishing of rice differ among genotypes due to contrasting grain hardness and mineral distribution

The effect of different polishing techniques on loss of mineral elements from rice grains was quantified using a panel of indica and tropical japonica genotypes, previously classified as differing in ease of polishing. Gradients in mineral elements across the bran-endosperm interface were quantified using micro-scaled precision abrasive polishing in combination with inductively coupled plasma mass spectrometry and synchrotron X-ray fluorescence microscopy. Frictional polishing, similar to that of commercial mills, i.e. 8–10% loss of grain weight, reduced the concentration of Fe, Mg, P, K and Mn by 60–80% in all genotypes. Following gentler polishing (3–5% weight loss), genotypes classified as difficult to polish showed smaller decreases in Fe, Mg, P, K and Mn compared to genotypes classified as easy to polish. The concentration of other elements, e.g. Zn, S, Ca, Cu, Mo and Cd, showed comparable reductions (<30%) irrespective of polishing technique or ease of polishing. The different patterns of polishing losses of minerals reflected their distribution within the grain. Five-fold differences in the reduction of Zn concentration during polishing were observed for different genotypes which started with similar Zn concentrations in the unpolished grain, thus showing clear potential for selecting genotypes with reduced polishing losses of Zn.